1 概述
我們在編寫Android程序時，常常會用到廣播（Broadcast）機制。從易用性的角度來說，使用廣播是非常簡單的。不過，這個不是本文關心的重點，我們希望探索得再深入一點兒。我想，許多人也不想僅僅停留在使用廣播的階段，而是希望瞭解一些廣播機制的內部機理。如果是這樣的話，請容我斟一杯紅茶，慢慢道來。
簡單地說，Android廣播機制的主要工作是爲了實現一處發生事情，多處得到通知的效果。這種通知工作常常要牽涉跨進程通訊，所以需要由AMS（Activity Manager Service）集中管理。
在Android系統中，接收廣播的組件叫作receiver，而且receiver還分爲動態和靜態的。動態receiver是在運行期通過調用registerReceiver()註冊的，而靜態receiver則是在AndroidManifest.xml中聲明的。動態receiver比較簡單，靜態的就麻煩一些了，因爲在廣播遞送之時，靜態receiver所從屬的進程可能還沒有啓動呢，這就需要先啓動新的進程，費時費力。另一方面，有些時候用戶希望廣播能夠按照一定順序遞送，爲此，Android又搞出了ordered broadcast的概念。
細節如此繁雜，非一言可以說清。我們先從receiver這一側入手吧。
2 兩種receiver
Android中的receiver，分爲“動態receiver”和“靜態receiver”。
2.1 動態receiver
動態receiver必須在運行期動態註冊，其實際的註冊動作由ContextImpl對象完成：
- <code class="hljs less" style=""><span class="hljs-variable" style="">@Override</span>
- public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter)
- {
- return registerReceiver(receiver, filter, null, null);
- }
- <span class="hljs-variable" style="">@Override</span>
- public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter,
- String broadcastPermission, Handler scheduler)
- {
- return registerReceiverInternal(receiver, filter, broadcastPermission,
- scheduler, getOuterContext());
- }</code>
註冊之時，用戶會把一個自定義的receiver對象作爲第一個參數傳入。當然，用戶的receiver都是繼承於BroadcastReceiver的。使用過廣播機制的程序員，對這個BroadcastReceiver應該都不陌生，這裏就不多說了。我們需要關心的是，這個registerReceiverInternal()內部還包含了什麼重要的細節。
registerReceiverInternal()代碼的截選如下：
- <code class="hljs kotlin" style=""><span class="hljs-keyword" style="">private</span> Intent registerReceiverInternal(BroadcastReceiver receiver,
- IntentFilter filter, String broadcastPermission,
- Handler scheduler, Context context)
- {
- IIntentReceiver rd = <span class="hljs-literal" style="">null</span>;
- <span class="hljs-keyword" style="">if</span> (receiver != <span class="hljs-literal" style="">null</span>)
- {
- <span class="hljs-keyword" style="">if</span> (mPackageInfo != <span class="hljs-literal" style="">null</span> && context != <span class="hljs-literal" style="">null</span>)
- {
- <span class="hljs-keyword" style="">if</span> (scheduler == <span class="hljs-literal" style="">null</span>)
- {
- scheduler = mMainThread.getHandler();
- }
- <span class="hljs-comment" style="">// 查找和context對應的“子哈希表”裏的ReceiverDispatcher，如果找不到，就重新new一個</span>
- rd = mPackageInfo.getReceiverDispatcher(receiver, context, scheduler,
- mMainThread.getInstrumentation(), <span class="hljs-literal" style="">true</span>);
- }
- . . . . . .
- }
- <span class="hljs-keyword" style="">try</span>
- {
- <span class="hljs-keyword" style="">return</span> ActivityManagerNative.getDefault().registerReceiver(
- mMainThread.getApplicationThread(), mBasePackageName,
- rd, filter, broadcastPermission);
- }
- <span class="hljs-keyword" style="">catch</span> (RemoteException e)
- {
- <span class="hljs-keyword" style="">return</span> <span class="hljs-literal" style="">null</span>;
- }
- }</code>
請大家注意那個rd對象（IIntentReceiver rd）。我們知道，在Android架構中，廣播動作最終其實都是由AMS遞送出來的。AMS利用binder機制，將語義傳遞給各個應用進程，應用進程再輾轉調用到receiver的onReceive()，完成這次廣播。而此處的rd對象正是承擔“語義傳遞工作“的binder實體。
爲了管理這個重要的binder實體，Android搞出了一個叫做ReceiveDispatcher的類。該類的定義截選如下：
【frameworks/base/core/Java/android/app/LoadedApk.java】
- <code class="hljs java" style=""><span class="hljs-keyword" style="">static</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">ReceiverDispatcher</span></span></span><span class="hljs-class" style="">
- </span></span>{
- <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">static</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">InnerReceiver</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">extends</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">IIntentReceiver</span></span></span><span class="hljs-class" style="">.</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">Stub</span></span></span><span class="hljs-class" style=""> </span></span>{
- . . . . . .
- . . . . . .
- }
- <span class="hljs-keyword" style="">final</span> IIntentReceiver.Stub mIIntentReceiver; <span class="hljs-comment" style="">// 請注意這個域！它就是傳到外面的rd。</span>
- <span class="hljs-keyword" style="">final</span> BroadcastReceiver mReceiver;
- <span class="hljs-keyword" style="">final</span> Context mContext;
- <span class="hljs-keyword" style="">final</span> Handler mActivityThread;
- <span class="hljs-keyword" style="">final</span> Instrumentation mInstrumentation;
- <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">boolean</span> mRegistered;
- <span class="hljs-keyword" style="">final</span> IntentReceiverLeaked mLocation;
- RuntimeException mUnregisterLocation;
- <span class="hljs-keyword" style="">boolean</span> mForgotten;
- . . . . . .</code>
這樣看來，“動態註冊的BroadcastReceiver”和“ReceiverDispatcher節點”具有一一對應的關係。示意圖如下：
一個應用裏可能會註冊多個動態receiver，所以這種一一對應關係最好整理成表，這個表就位於LoadedApk中。前文mPackageInfo.getReceiverDispatcher()一句中的mPackageInfo就是LoadedApk對象。
在Android的架構裏，應用進程裏是用LoadedApk來對應一個apk的，進程里加載了多少個apk，就會有多少LoadedApk。每個LoadedApk裏會有一張“關於本apk動態註冊的所有receiver”的哈希表（mReceivers）。當然，在LoadedApk初創之時，這張表只是個空表。
mReceivers表的定義如下：
- <code class="hljs ruby" style="">private final
- HashMap<Context, HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher<span class="hljs-meta" style="">>> </span>mReceivers
- = new HashMap<Context, HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher<span class="hljs-meta" style="">>></span>();</code>
該表的key項是我們比較熟悉的Context，也就是說可以是Activity、Service或Application。而value項則是另一張“子哈希表”。這是個“表中表”的形式。言下之意就是，每個Context（比如一個activity），是可以註冊多個receiver的，這個很好理解。mReceivers裏的“子哈希表”的key值爲BroadcastReceiver，value項爲ReceiverDispatcher，示意圖如下：
圖：客戶進程中的mReceivers表
接下來我們繼續看registerReceiverInternal()，它最終調用到
- <code class="hljs css" style=""><span class="hljs-selector-tag" style="">ActivityManagerNative</span><span class="hljs-selector-class" style="">.getDefault</span>()<span class="hljs-selector-class" style="">.registerReceiver</span>(
- <span class="hljs-selector-tag" style="">mMainThread</span><span class="hljs-selector-class" style="">.getApplicationThread</span>(), <span class="hljs-selector-tag" style="">mBasePackageName</span>,
- <span class="hljs-selector-tag" style="">rd</span>, <span class="hljs-selector-tag" style="">filter</span>, <span class="hljs-selector-tag" style="">broadcastPermission</span>);</code>
registerReceiver()函數的filter參數指明瞭用戶對哪些intent感興趣。對同一個BroadcastReceiver對象來說，可以註冊多個感興趣的filter，就好像聲明靜態receiver時，也可以爲一個receiver編寫多個<intent-filter>一樣。這些IntentFilter信息會彙總到AMS的mRegisteredReceivers表中。在AMS端，我們可以這樣訪問相應的彙總表：
- <code class="hljs nginx" style=""><span class="hljs-attribute" style="">ReceiverList</span> rl = (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());</code>
其中的receiver參數爲IIntentReceiver型，正對應着ReceiverDispatcher中那個binder實體。也就是說，每個客戶端的ReceiverDispatcher，會對應AMS端的一個ReceiverList。
ReceiverList的定義截選如下：
- <code class="hljs java" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">ReceiverList</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">extends</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">ArrayList</span></span></span><span class="hljs-class" style=""><</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">BroadcastFilter</span></span></span><span class="hljs-class" style="">>
- </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">implements</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">IBinder</span></span></span><span class="hljs-class" style="">.</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">DeathRecipient</span></span></span><span class="hljs-class" style="">
- </span></span>{
- <span class="hljs-keyword" style="">final</span> ActivityManagerService owner;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">final</span> IIntentReceiver receiver;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">final</span> ProcessRecord app;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">int</span> pid;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">int</span> uid;
- BroadcastRecord curBroadcast = <span class="hljs-keyword" style="">null</span>;
- <span class="hljs-keyword" style="">boolean</span> linkedToDeath = <span class="hljs-keyword" style="">false</span>;
- String stringName;
- . . . . . .</code>
ReceiverList繼承於ArrayList<BroadcastFilter>，而BroadcastFilter又繼承於IntentFilter，所以ReceiverList可以被理解爲一個IntentFilter數組列表。
- <code class="hljs actionscript" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">BroadcastFilter</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">extends</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">IntentFilter</span></span></span><span class="hljs-class" style=""> </span></span>{
- <span class="hljs-keyword" style="">final</span> ReceiverList receiverList;
- <span class="hljs-keyword" style="">final</span> String packageName;
- <span class="hljs-keyword" style="">final</span> String requiredPermission;
- . . . . . .</code>
現在，我們可以繪製一張完整一點兒的圖：
這張圖只畫了一個用戶進程，在實際的系統裏當然會有很多用戶進程了，不過其關係是大致統一的，所以我們不再重複繪製。關於動態receiver的註冊，我們就先說這麼多。至於激發廣播時，又會做什麼動作，我們會在後文闡述，現在我們先接着說明和動態receiver相對的靜態receiver。
2.2 靜態receiver
靜態receiver是指那些在AndroidManifest.xml文件中聲明的receiver，它們的信息會在系統啓動時，由Package Manager Service（PKMS）解析並記錄下來。以後，當AMS調用PKMS的接口來查詢“和intent匹配的組件”時，PKMS內部就會去查詢當初記錄下來的數據，並把結果返回AMS。有的同學認爲靜態receiver是常駐內存的，這種說法並不準確。因爲常駐內存的只是靜態receiver的描述性信息，並不是receiver實體本身。
在PKMS內部，會有一個針對receiver而設置的Resolver（決策器），其示意圖如下：
關於PKMS的查詢動作的細節，可參考PKMS的相關文檔。目前我們只需知道，PKMS向外界提供了queryIntentReceivers()函數，該函數可以返回一個List<ResolveInfo>列表。
我們舉個實際的例子：
- <code class="hljs php" style="">Intent intent = <span class="hljs-keyword" style="">new</span> Intent(Intent.ACTION_PRE_BOOT_COMPLETED);
- <span class="hljs-keyword" style="">List</span><ResolveInfo> ris = <span class="hljs-keyword" style="">null</span>;<span class="hljs-keyword" style="">try</span> {
- ris = AppGlobals.getPackageManager().queryIntentReceivers(intent, <span class="hljs-keyword" style="">null</span>, <span class="hljs-number" style="">0</span>, <span class="hljs-number" style="">0</span>);
- } <span class="hljs-keyword" style="">catch</span> (RemoteException e) {}</code>
這是AMS的systemReady()函數裏的一段代碼，意思是查找有多少receiver對ACTION_PRE_BOOT_COMPLETED感興趣。
ResolveInfo的定義截選如下：
- <code class="hljs java" style=""><span class="hljs-keyword" style="">public</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">ResolveInfo</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">implements</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">Parcelable</span></span></span><span class="hljs-class" style="">
- </span></span>{
- <span class="hljs-keyword" style="">public</span> ActivityInfo activityInfo;
- <span class="hljs-keyword" style="">public</span> ServiceInfo serviceInfo;
- <span class="hljs-keyword" style="">public</span> IntentFilter filter;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">int</span> priority;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">int</span> preferredOrder;
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">int</span> match;
- . . . . . .
- . . . . . .</code>
總之，當系統希望發出一個廣播時，PKMS必須能夠決策出，有多少靜態receiver對這個廣播感興趣，而且這些receiver的信息分別又是什麼。
關於receiver的註冊動作，我們就先說這麼多。下面我們來看看激發廣播時的動作。
3 激發廣播
大家常見的激發廣播的函數有哪些呢？從ContextImpl.java文件中，我們可以看到一系列發送廣播的接口，列舉如下：
-
public void sendBroadcast(Intent intent)
-
public void sendBroadcast(Intent intent, int userId)
-
public void sendBroadcast(Intent intent, String receiverPermission)
-
public void sendOrderedBroadcast(Intent intent, String receiverPermission)
-
public void sendOrderedBroadcast(Intent intent, String receiverPermission, BroadcastReceiver resultReceiver, Handler scheduler, int initialCode, String initialData, Bundle initialExtras)
-
public void sendStickyBroadcast(Intent intent)
-
public void sendStickyOrderedBroadcast(Intent intent, BroadcastReceiver resultReceiver, Handler scheduler, int initialCode, String initialData, Bundle initialExtras)
其中sendBroadcast()是最簡單的發送廣播的動作。而sendOrderedBroadcast()，則是用來向系統發出有序廣播(Ordered broadcast)的。這種有序廣播對應的所有接收器只能按照一定的優先級順序，依次接收intent。這些優先級一般記錄在AndroidManifest.xml文件中，具體位置在<intent-filter>元素的android:priority屬性中，其數值越大表示優先級越高，取值範圍爲-1000到1000。另外，有時候我們也可以調用IntentFilter對象的setPriority()方法來設置優先級。
對於有序廣播而言，前面的接收者可以對接收到的廣播intent進行處理，並將處理結果放置到廣播intent中，然後傳遞給下一個接收者。需要注意的是，前面的接收者有權終止廣播的進一步傳播。也就是說，如果廣播被前面的接收者終止了，那麼後面的接收器就再也無法接收到廣播了。
還有一個怪東西，叫做sticky廣播，它又是什麼呢？簡單地說，sticky廣播可以保證“在廣播遞送時尚未註冊的receiver”，一旦日後註冊進系統，就能夠馬上接到“錯過”的sticky廣播。有關它的細節，我們在後文再說。
在發送方，我們熟悉的調用sendBroadcast()的代碼片段如下：
- <code class="hljs actionscript" style="">mContext = getApplicationContext();
- Intent intent = <span class="hljs-keyword" style="">new</span> Intent();
- intent.setAction(<span class="hljs-string" style="">"com.android.xxxxx"</span>);
- intent.setFlags(<span class="hljs-number" style="">1</span>);
- mContext.sendBroadcast(intent);</code>
上面的mContext的內部其實是在調用一個ContextImpl對象的同名函數，所以我們繼續查看ContextImpl.java文件。
【frameworks/base/core/java/android/app/ContextImpl.java】
- <code class="hljs java" style=""><span class="hljs-meta" style="">@Override</span>
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">sendBroadcast</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Intent intent)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
- <span class="hljs-keyword" style="">try</span>
- {
- intent.setAllowFds(<span class="hljs-keyword" style="">false</span>);
- ActivityManagerNative.getDefault().broadcastIntent(
- mMainThread.getApplicationThread(), intent, resolvedType, <span class="hljs-keyword" style="">null</span>,
- Activity.RESULT_OK, <span class="hljs-keyword" style="">null</span>, <span class="hljs-keyword" style="">null</span>, <span class="hljs-keyword" style="">null</span>, <span class="hljs-keyword" style="">false</span>, <span class="hljs-keyword" style="">false</span>,
- Binder.getOrigCallingUser());
- } <span class="hljs-keyword" style="">catch</span> (RemoteException e) {
- }
- }</code>
簡單地調用broadcastIntent()向AMS發出請求了。
3.1 AMS一側的broadcastIntentLocked()
用戶進程把發送廣播的語義傳遞到AMS之後，最終會由AMS的broadcastIntentLocked()處理。其原型如下：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">private</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">int</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">broadcastIntentLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(ProcessRecord callerApp,
- String callerPackage,
- Intent intent, String resolvedType,
- IIntentReceiver resultTo, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode,
- String resultData,
- Bundle map, String requiredPermission,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> callingPid, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> callingUid,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> userId)</span></span></span></span></code>
broadcastIntentLocked()需要考慮以下方面的技術細節。
首先，有些廣播intent只能由具有特定權限的進程發送，而有些廣播intent在發送之前需要做一些其他動作。當然，如果發送方進程是系統進程、phone進程、shell進程，或者具有root權限的進程，那麼必然有權發出廣播。
另外，有時候用戶希望發送sticky廣播，以便日後註冊的receiver可以收到“錯過”的sticky廣播。要達到這個目的，系統必須在內部維護一張sticky廣播表，在具體的實現中，AMS會把廣播intent加入mStickyBroadcasts映射表中。mStickyBroadcasts是一張哈希映射表，其key值爲intent的action字符串，value值爲“與這個action對應的intent數組列表”的引用。當我們發送sticky廣播時，新的廣播intent要麼替換掉intent數組列表中的某項，要麼作爲一個新項被添加進數組列表，以備日後使用。
發送廣播時，還需要考慮所發送的廣播是否需要有序（ordered）遞送。而且，receiver本身又分爲動態註冊和靜態聲明的，這讓我們面對的情況更加複雜。從目前的代碼來看，靜態receiver一直是按照有序方式遞送的，而動態receiver則需要根據ordered參數的值，做不同的處理。當我們需要有序遞送時，AMS會把動態receivers和靜態receivers合併到一張表中，這樣才能依照receiver的優先級，做出正確的處理，此時動態receivers和靜態receivers可能呈現一種交錯順序。
另一方面，有些廣播是需要發給特定目標組件的，這個也要加以考慮。
現在我們來分析broadcastIntentLocked()函數。說得難聽點兒，這個函數的實現代碼頗有些裹腳布的味道，我們必須耐下性子解讀這部分代碼。經過一番努力，我們可以將其邏輯大致整理成以下幾步：
1） 爲intent添加FLAG_EXCLUDE_STOPPED_PACKAGES標記；
2） 處理和package相關的廣播；
3） 處理其他一些系統廣播；
4） 判斷當前是否有權力發出廣播；
5） 如果要發出sticky廣播，那麼要更新一下系統中的sticky廣播列表；
6） 查詢和intent匹配的靜態receivers；
7） 查詢和intent匹配的動態receivers；
8） 嘗試向並行receivers遞送廣播；
9） 整合（剩下的）並行receivers，以及靜態receivers，形成一個串行receivers表；
10） 嘗試逐個向串行receivers遞送廣播。
下面我們來詳細說這幾個部分。
3.1.1 爲intent添加FLAG_EXCLUDE_STOPPED_PACKAGES標記
對應的代碼爲：
- <code class="hljs haskell" style=""><span class="hljs-title" style="">intent</span> = new <span class="hljs-type" style="">Intent</span>(intent);// <span class="hljs-type" style="">By</span> <span class="hljs-keyword" style="">default</span> broadcasts do not go to stopped apps.intent.addFlags(<span class="hljs-type" style="">Intent</span>.<span class="hljs-type" style="">FLAG_EXCLUDE_STOPPED_PACKAGES</span>);</code>
爲什麼intent要添加FLAG_EXCLUDE_STOPPED_PACKAGES標記呢？原因是這樣的，在Android 3.1之後，PKMS加強了對“處於停止狀態的”應用的管理。如果一個應用在安裝後從來沒有啓動過，或者已經被用戶強制停止了，那麼這個應用就處於停止狀態（stopped state）。爲了達到精細調整的目的，Android增加了2個flag：FLAG_INCLUDE_STOPPED_PACKAGES和FLAG_EXCLUDE_STOPPED_PACKAGES，以此來表示intent是否要激活“處於停止狀態的”應用。
- <code class="hljs gradle" style=""><span class="hljs-comment" style="">/**
- * If set, this intent will not match any components in packages that
- * are currently stopped. If this is not set, then the default behavior
- * is to include such applications in the result.
- */</span>
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">static</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">int</span> FLAG_EXCLUDE_STOPPED_PACKAGES = <span class="hljs-number" style="">0</span>x00000010;
- <span class="hljs-comment" style="">/**
- * If set, this intent will always match any components in packages that
- * are currently stopped. This is the default behavior when
- * {@link #FLAG_EXCLUDE_STOPPED_PACKAGES} is not set. If both of these
- * flags are set, this one wins (it allows overriding of exclude for
- * places where the framework may automatically set the exclude flag).
- */</span>
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">static</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">int</span> FLAG_INCLUDE_STOPPED_PACKAGES = <span class="hljs-number" style="">0</span>x00000020;</code>
從上面的broadcastIntentLocked()函數可以看到，在默認情況下，AMS是不會把intent廣播發給“處於停止狀態的”應用的。據說Google這樣做是爲了防止一些流氓軟件或病毒幹壞事。當然，如果廣播的發起者認爲自己的確需要廣播到“處於停止狀態的”應用的話，它可以讓intent攜帶FLAG_INCLUDE_STOPPED_PACKAGES標記，從這個標記的註釋可以瞭解到，如果這兩個標記同時設置的話，那麼FLAG_INCLUDE_STOPPED_PACKAGES標記會“取勝”，它會覆蓋掉framework自動添加的FLAG_EXCLUDE_STOPPED_PACKAGES標記。
3.1.2 處理和package相關的廣播
接下來需要處理一些系統級的“Package廣播”，這些主要從PKMS（Package Manager Service）處發來。比如，當PKMS處理APK的添加、刪除或改動時，一般會發出類似下面的廣播：ACTION_PACKAGE_ADDED、ACTION_PACKAGE_REMOVED、ACTION_PACKAGE_CHANGED、ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE、ACTION_UID_REMOVED。
AMS必須確保發送“包廣播”的發起方具有BROADCAST_PACKAGE_REMOVED權限，如果沒有，那麼AMS會拋出異常（SecurityException）。接着，AMS判斷如果是某個用戶id被刪除了的話（Intent.ACTION_UID_REMOVED），那麼必須把這件事通知給“電池狀態服務”（Battery Stats Service）。另外，如果是SD卡等外部設備上的應用不可用了，這常常是因爲卡被unmount了，此時PKMS會發出Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE，而AMS則需要把SD卡上的所有包都強制停止（forceStopPackageLocked()），並立即發出另一個“Package廣播”——EXTERNAL_STORAGE_UNAVAILABLE。
如果只是某個外部包被刪除或改動了，則要進一步判斷intent裏是否攜帶了EXTRA_DONT_KILL_APP額外數據，如果沒有攜帶，說明需要立即強制結束package，否則，不強制結束package。看來有些應用即使在刪除或改動了包後，還會在系統（內存）中保留下來並繼續運行。另外，如果是刪除包的話，此時要發出PACKAGE_REMOVED廣播。
3.1.3 處理其他一些系統廣播
broadcastIntentLocked()不但要對“Package廣播”進行處理，還要關心其他一些系統廣播。比如ACTION_TIMEZONE_CHANGED、ACTION_CLEAR_DNS_CACHE、PROXY_CHANGE_ACTION等等，感興趣的同學可以自行研究這些廣播的意義。
3.1.4 判斷當前是否有權力發出廣播
接着，broadcastIntentLocked()會判斷當前是否有權力發出廣播，代碼截選如下：
- <code class="hljs lisp" style="">/*
- * Prevent non-system code (<span class="hljs-name" style="">defined</span> here to be non-persistent
- * processes) from sending protected broadcasts.
- */
- if (<span class="hljs-name" style="">callingUid</span> == Process.SYSTEM_UID || callingUid == Process.PHONE_UID
- || callingUid == Process.SHELL_UID || callingUid == <span class="hljs-number" style="">0</span>)
- {
- // Always okay.
- }
- else if (<span class="hljs-name" style="">callerApp</span> == null || !callerApp.persistent)
- {
- try
- {
- if (<span class="hljs-name" style="">AppGlobals</span>.getPackageManager().isProtectedBroadcast(<span class="hljs-name" style="">intent</span>.getAction()))
- {
- String msg = <span class="hljs-string" style="">"Permission Denial: not allowed to send broadcast "</span>
- + intent.getAction() + <span class="hljs-string" style="">" from pid="</span>
- + callingPid + <span class="hljs-string" style="">", uid="</span> + callingUid<span class="hljs-comment" style="">;</span>
- Slog.w(<span class="hljs-name" style="">TAG</span>, msg)<span class="hljs-comment" style="">;</span>
- throw new SecurityException(<span class="hljs-name" style="">msg</span>)<span class="hljs-comment" style="">;</span>
- }
- }
- catch (<span class="hljs-name" style="">RemoteException</span> e)
- {
- Slog.w(<span class="hljs-name" style="">TAG</span>, <span class="hljs-string" style="">"Remote exception"</span>, e)<span class="hljs-comment" style="">;</span>
- return ActivityManager.BROADCAST_SUCCESS<span class="hljs-comment" style="">;</span>
- }
- }</code>
如果發起方的Uid爲SYSTEM_UID、PHONE_UID或SHELL_UID，或者發起方具有root權限，那麼它一定有權力發送廣播。
另外，還有一個“保護性廣播”的概念，也要考慮進來。網上有一些人詢問AndroidManifest.xml中的一級標記<protected-broadcast>是什麼意思。簡單地說，Google認爲有一些廣播是隻能由系統發送的，如果某個系統級AndroidManifest.xml中寫了這個標記，那麼在PKMS解析該文件時，就會把“保護性廣播”標記中的名字（一般是Action字符串）記錄下來。在系統運作起來之後，如果某個不具有系統權限的應用試圖發送系統中的“保護性廣播”，那麼到AMS的broadcastIntentLocked()處就會被攔住，AMS會拋出異常，提示"Permission Denial: not allowed to send broadcast"。
我們在frameworks/base/core/res/AndroidManifest.xml文件中，可以看到<protected-broadcast>標記的具體寫法，截選如下：
3.1.5 必要時更新一下系統中的sticky廣播列表
接着，broadcastIntentLocked()中會判斷當前是否在發出sticky廣播，如果是的話，必須把廣播intent記錄下來。
一開始會判斷一下發起方是否具有發出sticky廣播的能力，比如說要擁有android.Manifest.permission.BROADCAST_STICKY權限等等。判斷合格後，broadcastIntentLocked()會更新AMS裏的一張表——mStickyBroadcasts，其大致代碼如下：
- <code class="hljs cpp" style=""> ArrayList<Intent> <span class="hljs-built_in" style="">list</span> = mStickyBroadcasts.get(intent.getAction());
- <span class="hljs-keyword" style="">if</span> (<span class="hljs-built_in" style="">list</span> == null)
- {
- <span class="hljs-built_in" style="">list</span> = <span class="hljs-keyword" style="">new</span> ArrayList<Intent>();
- mStickyBroadcasts.put(intent.getAction(), <span class="hljs-built_in" style="">list</span>);
- }
- <span class="hljs-keyword" style="">int</span> N = <span class="hljs-built_in" style="">list</span>.size();
- <span class="hljs-keyword" style="">int</span> i;
- <span class="hljs-keyword" style="">for</span> (i=<span class="hljs-number" style="">0</span>; i<N; i++)
- {
- <span class="hljs-keyword" style="">if</span> (intent.filterEquals(<span class="hljs-built_in" style="">list</span>.get(i)))
- {
- <span class="hljs-comment" style="">// This sticky already exists, replace it.</span>
- <span class="hljs-built_in" style="">list</span>.<span class="hljs-built_in" style="">set</span>(i, <span class="hljs-keyword" style="">new</span> Intent(intent));
- <span class="hljs-keyword" style="">break</span>;
- }
- }
- <span class="hljs-keyword" style="">if</span> (i >= N)
- {
- <span class="hljs-built_in" style="">list</span>.add(<span class="hljs-keyword" style="">new</span> Intent(intent));
- }</code>
mStickyBroadcasts的定義是這樣的：
- <code class="hljs javascript" style=""> final HashMap<<span class="hljs-built_in" style="">String</span>, ArrayList<Intent>> mStickyBroadcasts =
- <span class="hljs-keyword" style="">new</span> HashMap<<span class="hljs-built_in" style="">String</span>, ArrayList<Intent>>();</code>
上面代碼的filterEquals()函數會比較兩個intent的action、data、type、class以及categories等信息，但不會比較extra數據。如果兩個intent的action是一樣的，但其他信息不同，那麼它們在ArrayList<Intent>中會被記成兩個不同的intent。而如果發現新發送的intent在ArrayList中已經有個“相等的”舊intent時，則會用新的替掉舊的。
以後，每當註冊新的動態receiver時，註冊動作中都會遍歷一下mStickyBroadcast表，看哪些intent可以和新receiver的filter匹配，只有匹配的intent纔會遞送給新receiver，示意圖如下：
圖中新receiver的filter只對a1和a3這兩個action感興趣，所以遍歷時就不會考慮mStickyBroadcast表中的a2表項對應的子表，而a1、a3子表所對應的若干intent中又只有一部分可以和filter匹配，比如a1的intent1以及a3的intent2，所以圖中只選擇了這兩個intent遞送給新receiver。
除了記入mStickyBoradcast表的動作以外，sticky廣播和普通廣播在broadcastIntentLocked()中的代碼是一致的，並沒有其他什麼不同了。
3.1.6 嘗試向並行receivers遞送廣播
然後broadcastIntentLocked()會嘗試向並行receivers遞送廣播。此時會調用到queue.scheduleBroadcastsLocked()。相關代碼截選如下：
- <code class="hljs cpp" style=""><span class="hljs-keyword" style="">int</span> NR = registeredReceivers != null ? registeredReceivers.size() : <span class="hljs-number" style="">0</span>;
- <span class="hljs-keyword" style="">if</span> (!ordered && NR > <span class="hljs-number" style="">0</span>)
- {
- <span class="hljs-comment" style="">// If we are not serializing this broadcast, then send the</span>
- <span class="hljs-comment" style="">// registered receivers separately so they don't wait for the</span>
- <span class="hljs-comment" style="">// components to be launched.</span>
- final BroadcastQueue <span class="hljs-built_in" style="">queue</span> = broadcastQueueForIntent(intent);
- BroadcastRecord r = <span class="hljs-keyword" style="">new</span> BroadcastRecord(<span class="hljs-built_in" style="">queue</span>, intent, callerApp,
- callerPackage, callingPid, callingUid, requiredPermission,
- registeredReceivers, resultTo, resultCode, resultData, <span class="hljs-built_in" style="">map</span>,
- ordered, sticky, <span class="hljs-literal" style="">false</span>);
- <span class="hljs-keyword" style="">if</span> (DEBUG_BROADCAST) Slog.v(
- TAG, <span class="hljs-string" style="">"Enqueueing parallel broadcast "</span> + r);
- final boolean replaced = replacePending && <span class="hljs-built_in" style="">queue</span>.replaceParallelBroadcastLocked(r);
- <span class="hljs-keyword" style="">if</span> (!replaced) {
- <span class="hljs-built_in" style="">queue</span>.enqueueParallelBroadcastLocked(r);
- <span class="hljs-built_in" style="">queue</span>.scheduleBroadcastsLocked(); <span class="hljs-comment" style="">// 注意這句。。。</span>
- }
- registeredReceivers = null;
- NR = <span class="hljs-number" style="">0</span>;
- }</code>
簡單地說就是，new一個BroadcastRecord節點，並插入BroadcastQueue內的並行處理隊列，最後發起實際的廣播調度（scheduleBroadcastsLocked()）。關於上面代碼中的registeredReceivers列表，我們會在後文說明，這裏先跳過。
其實不光並行處理部分需要一個BroadcastRecord節點，串行處理部分也需要BroadcastRecord節點。也就是說，要激發一次廣播，AMS必須構造一個或兩個BroadcastRecord節點，並將之插入合適的廣播隊列（mFgBroadcastQueue或mBgBroadcastQueue）。插入成功後，再執行隊列的scheduleBroadcastsLocked()動作，進行實際的派發調度。示意圖如下：
請注意圖中BroadcastRecord節點所攜帶的節點鏈。在mParallelBroadcasts表中，每個BroadcastRecord只可能攜帶BroadcastFilter，因爲平行處理的節點只會對應動態receiver，而所有靜態receiver只能是串行處理的。另一方面，在mOrderedBroadcasts表中，BroadcastRecord中則既可能攜帶BroadcastFilter，也可能攜帶ResolveInfo。這個其實很容易理解，首先，ResolveInfo對應靜態receiver，放到這裏自不待言，其次，如果用戶在發送廣播時明確指定要按ordered方式發送的話，那麼即使目標方的receiver是動態註冊的，它對應的BroadcastFilter也會被強制放到這裏。
好，現在讓我們再整合一下思路。BroadcastRecord節點內部的receivers列表，記錄着和這個廣播動作相關的目標receiver信息，該列表內部的子節點可能是ResolveInfo類型的，也可能是BroadcastFilter類型的。ResolveInfo是從PKMS處查到的靜態receiver的描述信息，它的源頭是PKMS分析的那些AndroidManifest.xml文件。而BroadcastFilter事實上來自於本文一開始闡述動態receiver時，提到的AMS端的mRegisteredReceivers哈希映射表。現在，我們再畫一張示意圖：
因爲BroadcastRecord裏的BroadcastFilter，和AMS的mRegisteredReceivers表中（間接）所指的對應BroadcastFilter是同一個對象，所以我是用虛線將它們連起來的。
Ok，我們接着看scheduleBroadcastsLocked()動作。scheduleBroadcastsLocked()的代碼如下：
【frameworks/base/services/java/com/android/server/am/BroadcastQueue.java】
- <code class="hljs cpp" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">scheduleBroadcastsLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">()</span></span></span><span class="hljs-function" style="">
- </span></span>{
- . . . . . .
- <span class="hljs-keyword" style="">if</span> (mBroadcastsScheduled)
- {
- <span class="hljs-keyword" style="">return</span>;
- }
- mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, <span class="hljs-keyword" style="">this</span>));
- mBroadcastsScheduled = <span class="hljs-literal" style="">true</span>;
- }</code>
發出BROADCAST_INTENT_MSG消息。
上面用到的mHandler是這樣創建的：
- <code class="hljs groovy" style=""><span class="hljs-keyword" style="">final</span> Handler mHandler = <span class="hljs-keyword" style="">new</span> Handler()
- {
- <span class="hljs-keyword" style="">public</span> <span class="hljs-keyword" style="">void</span> handleMessage(Message msg)
- {
- <span class="hljs-keyword" style="">switch</span> (msg.what)
- {
- <span class="hljs-keyword" style="">case</span> <span class="hljs-string" style="">BROADCAST_INTENT_MSG:</span>
- {
- <span class="hljs-keyword" style="">if</span> (DEBUG_BROADCAST)
- Slog.v(TAG, <span class="hljs-string" style="">"Received BROADCAST_INTENT_MSG"</span>);
- processNextBroadcast(<span class="hljs-literal" style="">true</span>);
- }
- <span class="hljs-keyword" style="">break</span>;
- <span class="hljs-keyword" style="">case</span> <span class="hljs-string" style="">BROADCAST_TIMEOUT_MSG:</span>
- {
- <span class="hljs-keyword" style="">synchronized</span> (mService)
- {
- broadcastTimeoutLocked(<span class="hljs-literal" style="">true</span>);
- }
- }
- <span class="hljs-keyword" style="">break</span>;
- }
- }
- };</code>
也就是說，AMS端會在BroadcastQueue.java中的processNextBroadcast()具體處理廣播。
3.1.7 整理兩個receiver列表
我們前文已經說過，有些廣播是需要有序遞送的。爲了合理處理“有序遞送”和“平行遞送”，broadcastIntentLocked()函數內部搞出了兩個list：
- <code class="hljs php" style=""><span class="hljs-keyword" style="">List</span> receivers = <span class="hljs-keyword" style="">null</span>;
- <span class="hljs-keyword" style="">List</span><BroadcastFilter> registeredReceivers = <span class="hljs-keyword" style="">null</span>;</code>
其中，receivers主要用於記錄“有序遞送”的receiver，而registeredReceivers則用於記錄與intent相匹配的動態註冊的receiver。
關於這兩個list的大致運作是這樣的，我們先利用包管理器的queryIntentReceivers()接口，查詢出和intent匹配的所有靜態receivers，此時所返回的查詢結果本身已經排好序了，因此，該返回值被直接賦值給了receivers變量，代碼如下：
- <code class="hljs ini" style=""><span class="hljs-attr" style="">receivers</span> = AppGlobals.getPackageManager().queryIntentReceivers(intent, resolvedType, STOCK_PM_FLAGS, userId);</code>
而對於動態註冊的receiver信息，就不是從包管理器獲取了，這些信息本來就記錄在AMS之中，此時只需調用：
- <code class="hljs ini" style=""><span class="hljs-attr" style="">registeredReceivers</span> = mReceiverResolver.queryIntent(intent, resolvedType, <span class="hljs-literal" style="">false</span>, userId);</code>
就可以了。注意，此時返回的registeredReceivers中的子項是沒有經過排序的。而關於PKMS的queryIntentReceivers()，我們可以參考PKMS的專題文檔，此處不再贅述。
如果我們要“並行遞送”廣播， registeredReceivers中的各個receiver會在隨後的queue.scheduleBroadcastsLocked()動作中被並行處理掉。如果大家折回頭看看向並行receivers遞送廣播的代碼，會發現在調用完queue.scheduleBroadcastsLocked()後，registeredReceivers會被強制賦值成null值。
如果我們要“串行遞送”廣播，那麼必須考慮把registeredReceivers表合併到receivers表中去。我們知道，一開始receivers列表中只記錄了一些靜態receiver，這些receiver將會被“有序遞送”。現在我們只需再遍歷一下registeredReceivers列表，並將其中的每個子項插入到receivers列表的合適地方，就可以合併出一條順序列表了。當然，如果registeredReceivers已經被設爲null了，就無所謂合併了。
爲什麼靜態聲明的receiver只會“有序遞送”呢？我想也許和這種receiver的複雜性有關係，因爲在需要遞送廣播時，receiver所屬的進程可能還沒有啓動呢，所以也許會涉及到啓動進程的流程，這些都是比較複雜的流程。
當然，上面所說的是沒有明確指定目標組件的情況，如果intent裏含有明確的目標信息，那麼就不需要調用包管理器的queryIntentReceivers()了，只需new一個ArrayList，並賦值給receivers，然後把目標組件對應的ResolveInfo信息添加進receivers數組列表即可。
3.1.8 嘗試逐個向串行receivers遞送廣播
當receivers列表整理完畢之後，現在要開始嘗試逐個向串行receivers遞送廣播了。正如前文所說，這裏要重新new一個新的BroadcastRecord節點：
- <code class="hljs cpp" style=""><span class="hljs-keyword" style="">if</span> ((receivers != null && receivers.size() > <span class="hljs-number" style="">0</span>)
- || resultTo != null)
- {
- BroadcastQueue <span class="hljs-built_in" style="">queue</span> = broadcastQueueForIntent(intent);
- BroadcastRecord r = <span class="hljs-keyword" style="">new</span> BroadcastRecord(<span class="hljs-built_in" style="">queue</span>, intent, callerApp,
- callerPackage, callingPid, callingUid, requiredPermission,
- receivers, resultTo, resultCode, resultData, <span class="hljs-built_in" style="">map</span>, ordered,
- sticky, <span class="hljs-literal" style="">false</span>);
- . . . . . .
- boolean replaced = replacePending && <span class="hljs-built_in" style="">queue</span>.replaceOrderedBroadcastLocked(r);
- <span class="hljs-keyword" style="">if</span> (!replaced) {
- <span class="hljs-built_in" style="">queue</span>.enqueueOrderedBroadcastLocked(r);
- <span class="hljs-built_in" style="">queue</span>.scheduleBroadcastsLocked();
- }
- }</code>
而scheduleBroadcastsLocked()最終會間接導致走到 BroadcastQueue.java中的processNextBroadcast()。這一點和前文所說的“向並行receivers遞送廣播”的動作基本一致。
下面我們來看，遞送廣播動作中最重要的processNextBroadcast()。
3.2 最重要的processNextBroadcast()
從processNextBroadcast()的代碼，我們就可以看清楚前面說的“平行廣播”、“有序廣播”和“動態receiver”、“靜態receiver”之間的關係了。
我們在前文已經說過，所有的靜態receiver都是串行處理的，而動態receiver則會按照發廣播時指定的方式，進行“並行”或“串行”處理。能夠並行處理的廣播，其對應的若干receiver一定都已經存在了，不會牽扯到啓動新進程的操作，所以可以在一個while循環中，一次性全部deliver。而有序廣播，則需要一個一個地處理，其滾動處理的手段是發送事件，也就是說，在一個receiver處理完畢後，會利用廣播隊列（BroadcastQueue）的mHandler，發送一個BROADCAST_INTENT_MSG事件，從而執行下一次的processNextBroadcast()。
processNextBroadcast()的代碼邏輯大體是這樣的：先嚐試處理BroadcastQueue中的“平行廣播”部分。這需要遍歷並行列表（mParallelBroadcasts）的每一個BroadcastRecord以及其中的receivers列表。對於平行廣播而言，receivers列表中的每個子節點是個BroadcastFilter。我們直接將廣播遞送出去即可：
- <code class="hljs gradle" style=""><span class="hljs-keyword" style="">while</span> (mParallelBroadcasts.<span class="hljs-keyword" style="">size</span>() > <span class="hljs-number" style="">0</span>)
- {
- r = mParallelBroadcasts.remove(<span class="hljs-number" style="">0</span>);
- r.dispatchTime = SystemClock.uptimeMillis();
- r.dispatchClockTime = System.currentTimeMillis();
- <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">int</span> N = r.receivers.<span class="hljs-keyword" style="">size</span>();
- . . . . . .
- <span class="hljs-keyword" style="">for</span> (<span class="hljs-keyword" style="">int</span> i=<span class="hljs-number" style="">0</span>; i<N; i++)
- {
- Object target = r.receivers.get(i);
- . . . . . .
- deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, <span class="hljs-keyword" style="">false</span>);
- }
- . . . . . .
- }</code>
3.2.1 用deliverToRegisteredReceiverLocked()遞送到平行動態receiver
deliverToRegisteredReceiverLocked()的代碼截選如下：
【frameworks/base/services/java/com/android/server/am/BroadcastQueue.java】
- <code class="hljs swift" style=""><span class="hljs-keyword" style="">private</span> <span class="hljs-keyword" style="">final</span> void deliverToRegisteredReceiverLocked(<span class="hljs-type" style="">BroadcastRecord</span> r,
- <span class="hljs-type" style="">BroadcastFilter</span> <span class="hljs-built_in" style="">filter</span>,
- boolean ordered)
- {
- . . . . . .
- . . . . . .
- <span class="hljs-keyword" style="">if</span> (!skip)
- {
- <span class="hljs-keyword" style="">if</span> (ordered)
- {
- r.receiver = <span class="hljs-built_in" style="">filter</span>.receiverList.receiver.asBinder();
- r.curFilter = <span class="hljs-built_in" style="">filter</span>;
- <span class="hljs-built_in" style="">filter</span>.receiverList.curBroadcast = r;
- r.state = <span class="hljs-type" style="">BroadcastRecord</span>.<span class="hljs-type" style="">CALL_IN_RECEIVE</span>;
- <span class="hljs-keyword" style="">if</span> (<span class="hljs-built_in" style="">filter</span>.receiverList.app != null)
- {
- r.curApp = <span class="hljs-built_in" style="">filter</span>.receiverList.app;
- <span class="hljs-built_in" style="">filter</span>.receiverList.app.curReceiver = r;
- mService.updateOomAdjLocked();
- }
- }
- . . . . . .
- performReceiveLocked(<span class="hljs-built_in" style="">filter</span>.receiverList.app,
- <span class="hljs-built_in" style="">filter</span>.receiverList.receiver,
- new <span class="hljs-type" style="">Intent</span>(r.intent), r.resultCode,
- r.resultData, r.resultExtras,
- r.ordered, r.initialSticky);
- <span class="hljs-keyword" style="">if</span> (ordered)
- {
- r.state = <span class="hljs-type" style="">BroadcastRecord</span>.<span class="hljs-type" style="">CALL_DONE_RECEIVE</span>;
- }
- . . . . . .
- }</code>
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">private</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">static</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">performReceiveLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(ProcessRecord app, IIntentReceiver receiver,
- Intent intent, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode, String data, Bundle extras,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky)</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">throws</span></span></span><span class="hljs-function" style=""> RemoteException
- </span></span>{
- <span class="hljs-comment" style="">// Send the intent to the receiver asynchronously using one-way binder calls.</span>
- <span class="hljs-keyword" style="">if</span> (app != <span class="hljs-keyword" style="">null</span> && app.thread != <span class="hljs-keyword" style="">null</span>)
- {
- <span class="hljs-comment" style="">// If we have an app thread, do the call through that so it is</span>
- <span class="hljs-comment" style="">// correctly ordered with other one-way calls.</span>
- app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode,
- data, extras, ordered, sticky);
- }
- <span class="hljs-keyword" style="">else</span>
- {
- receiver.performReceive(intent, resultCode, data, extras, ordered, sticky);
- }
- }</code>
終於通過app.thread向用戶進程傳遞語義了。注意scheduleRegisteredReceiver()的receiver參數，它對應的就是前文所說的ReceiverDispatcher的Binder實體——InnerReceiver了。
總之，當語義傳遞到用戶進程的ApplicationThread以後，走到：
- <code class="hljs java" style=""><span class="hljs-comment" style="">// This function exists to make sure all receiver dispatching is</span>
- <span class="hljs-comment" style="">// correctly ordered, since these are one-way calls and the binder driver</span>
- <span class="hljs-comment" style="">// applies transaction ordering per object for such calls.</span>
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">scheduleRegisteredReceiver</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(IIntentReceiver receiver, Intent intent,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode, String dataStr, Bundle extras, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky)</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">throws</span></span></span><span class="hljs-function" style=""> RemoteException
- </span></span>{
- receiver.performReceive(intent, resultCode, dataStr, extras, ordered, sticky);
- }</code>
終於走到ReceiverDispatcher的InnerReceiver了：
- <code class="hljs java" style=""><span class="hljs-keyword" style="">static</span> <span class="hljs-keyword" style="">final</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">ReceiverDispatcher</span></span></span><span class="hljs-class" style="">
- </span></span>{
- <span class="hljs-keyword" style="">final</span> <span class="hljs-keyword" style="">static</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">InnerReceiver</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">extends</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">IIntentReceiver</span></span></span><span class="hljs-class" style="">.</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">Stub</span></span></span><span class="hljs-class" style="">
- </span></span>{
- . . . . . .
- . . . . . .
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">performReceive</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Intent intent, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode,
- String data, Bundle extras,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- LoadedApk.ReceiverDispatcher rd = mDispatcher.get();
- . . . . . .
- <span class="hljs-keyword" style="">if</span> (rd != <span class="hljs-keyword" style="">null</span>) {
- rd.performReceive(intent, resultCode, data, extras,
- ordered, sticky);
- }
- . . . . . .
- }
- }
- . . . . . .
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">performReceive</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Intent intent, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode,
- String data, Bundle extras,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- . . . . . .
- Args args = <span class="hljs-keyword" style="">new</span> Args(intent, resultCode, data, extras, ordered, sticky);
- <span class="hljs-keyword" style="">if</span> (!mActivityThread.post(args)) <span class="hljs-comment" style="">// 請注意這一句！</span>
- {
- <span class="hljs-keyword" style="">if</span> (mRegistered && ordered)
- {
- IActivityManager mgr = ActivityManagerNative.getDefault();
- <span class="hljs-keyword" style="">if</span> (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,
- <span class="hljs-string" style="">"Finishing sync broadcast to "</span> + mReceiver);
- args.sendFinished(mgr);
- }
- }
- }
- }</code>
請注意mActivityThread.post(args)一句，這樣，事件泵最終會回調Args參數的run()成員函數：
- <code class="hljs java" style=""><span class="hljs-keyword" style="">final</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">class</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">Args</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">extends</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">BroadcastReceiver</span></span></span><span class="hljs-class" style="">.</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">PendingResult</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">implements</span></span></span><span class="hljs-class" style=""> </span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">Runnable</span></span></span><span class="hljs-class" style="">
- </span></span>{
- . . . . . .
- . . . . . .
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">run</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">()</span></span></span><span class="hljs-function" style="">
- </span></span>{
- <span class="hljs-keyword" style="">final</span> BroadcastReceiver receiver = mReceiver;
- . . . . . .
- <span class="hljs-keyword" style="">try</span> {
- ClassLoader cl = mReceiver.getClass().getClassLoader();
- intent.setExtrasClassLoader(cl);
- setExtrasClassLoader(cl);
- receiver.setPendingResult(<span class="hljs-keyword" style="">this</span>);
- receiver.onReceive(mContext, intent); <span class="hljs-comment" style="">// 回調具體receiver的onReceive()</span>
- } <span class="hljs-keyword" style="">catch</span> (Exception e) {
- . . . . . .
- }
- <span class="hljs-keyword" style="">if</span> (receiver.getPendingResult() != <span class="hljs-keyword" style="">null</span>) {
- finish();
- }
- . . . . . .
- }
- }</code>
其中的那句receiver.onReceive(this)，正是回調我們具體receiver的onReceive()成員函數的地方。噢，終於看到應用程序員熟悉的onReceive()了。這部分的示意圖如下：
3.2.2 靜態receiver的遞送
說完動態遞送，我們再來看靜態遞送。對於靜態receiver，情況會複雜很多，因爲靜態receiver所從屬的進程有可能還沒有運行起來呢。此時BroadcastRecord節點中記錄的子列表的節點是ResolveInfo對象。
- <code class="hljs tcl" style="">ResolveInfo <span class="hljs-keyword" style="">info</span> = (ResolveInfo)nextReceiver;
- . . . . . .
- r.state = BroadcastRecord.APP_RECEIVE;
- String targetProcess = <span class="hljs-keyword" style="">info</span>.activityInfo.processName;</code>
那麼我們要先找到receiver所從屬的進程的進程名。
processNextBroadcast()中啓動進程的代碼如下：
- <code class="hljs kotlin" style="">ProcessRecord app = mService.getProcessRecordLocked(targetProcess,
- info.activityInfo.applicationInfo.uid);
- . . . . . .
- <span class="hljs-keyword" style="">if</span> (app != <span class="hljs-literal" style="">null</span> && app.thread != <span class="hljs-literal" style="">null</span>)
- {
- . . . . . .
- app.addPackage(info.activityInfo.packageName);
- processCurBroadcastLocked(r, app);
- <span class="hljs-keyword" style="">return</span>;
- . . . . . .
- }
- r.curApp = mService.startProcessLocked(targetProcess,
- info.activityInfo.applicationInfo, <span class="hljs-literal" style="">true</span>,
- r.intent.getFlags() | Intent.FLAG_FROM_BACKGROUND,
- <span class="hljs-string" style="">"broadcast"</span>, r.curComponent,
- (r.intent.getFlags()&Intent.FLAG_RECEIVER_BOOT_UPGRADE) != <span class="hljs-number" style="">0</span>,
- <span class="hljs-literal" style="">false</span>)
- . . . . . .
- mPendingBroadcast = r;
- mPendingBroadcastRecvIndex = recIdx;</code>
如果目標進程已經存在了，那麼app.thread肯定不爲null，直接調用processCurBroadcastLocked()即可，否則就需要啓動新進程了。啓動的過程是異步的，可能很耗時，所以要把BroadcastRecord節點記入mPendingBroadcast。
3.2.2.1 processCurBroadcastLocked()
我們先說processCurBroadcastLocked()。
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">private</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">processCurBroadcastLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(BroadcastRecord r,
- ProcessRecord app)</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">throws</span></span></span><span class="hljs-function" style=""> RemoteException
- </span></span>{
- . . . . . .
- r.receiver = app.thread.asBinder();
- r.curApp = app;
- app.curReceiver = r;
- . . . . . .
- . . . . . .
- app.thread.scheduleReceiver(<span class="hljs-keyword" style="">new</span> Intent(r.intent), r.curReceiver,
- mService.compatibilityInfoForPackageLocked(r.curReceiver.applicationInfo),
- r.resultCode, r.resultData, r.resultExtras, r.ordered);
- . . . . . .
- started = <span class="hljs-keyword" style="">true</span>;
- . . . . . .
- }</code>
其中最重要的是調用app.thread.scheduleReceiver()的那句。在IApplicationThread接口中，是這樣定義scheduleReceiver()函數原型的：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">scheduleReceiver</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Intent intent, ActivityInfo info,
- CompatibilityInfo compatInfo,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode, String data,
- Bundle extras, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sync)</span></span></span><span class="hljs-function" style="">
- </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">throws</span></span></span><span class="hljs-function" style=""> RemoteException</span></span>;</code>
其中ActivityInfo info參數，記錄着目標receiver的信息。可以看到，遞送靜態receiver時，是不會用到RecevierDispatcher的。
用戶進程裏handleMessage()
- <code class="hljs css" style=""><span class="hljs-selector-tag" style="">case</span> <span class="hljs-selector-tag" style="">RECEIVER</span>:
- <span class="hljs-selector-tag" style="">Trace</span><span class="hljs-selector-class" style="">.traceBegin</span>(<span class="hljs-selector-tag" style="">Trace</span><span class="hljs-selector-class" style="">.TRACE_TAG_ACTIVITY_MANAGER</span>, "<span class="hljs-selector-tag" style="">broadcastReceiveComp</span>");
- <span class="hljs-selector-tag" style="">handleReceiver</span>((<span class="hljs-selector-tag" style="">ReceiverData</span>)<span class="hljs-selector-tag" style="">msg</span><span class="hljs-selector-class" style="">.obj</span>);
- <span class="hljs-selector-tag" style="">maybeSnapshot</span>();
- <span class="hljs-selector-tag" style="">Trace</span><span class="hljs-selector-class" style="">.traceEnd</span>(<span class="hljs-selector-tag" style="">Trace</span><span class="hljs-selector-class" style="">.TRACE_TAG_ACTIVITY_MANAGER</span>);
- <span class="hljs-selector-tag" style="">break</span>;</code>
ActivityThread中，會運用反射機制，創建出BroadcastReceiver對象，而後回調該對象的onReceive()成員函數。
- <code class="hljs haskell" style=""><span class="hljs-title" style="">private</span> void handleReceiver(<span class="hljs-type" style="">ReceiverData</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">) </span></span>
- {
- . . . . . .
- <span class="hljs-type" style="">IActivityManager</span> mgr = <span class="hljs-type" style="">ActivityManagerNative</span>.getDefault();
- <span class="hljs-type" style="">BroadcastReceiver</span> receiver;
- try {
- java.lang.<span class="hljs-type" style="">ClassLoader</span> cl = packageInfo.getClassLoader();
- <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.intent.setExtrasClassLoader(</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">cl</span></span></span><span class="hljs-class" style="">);</span></span>
- <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.setExtrasClassLoader(</span><span class="hljs-title" style=""><span class="hljs-class" style=""><span class="hljs-title" style="">cl</span></span></span><span class="hljs-class" style="">);</span></span>
- receiver = (<span class="hljs-type" style="">BroadcastReceiver</span>)cl.loadClass(component).newInstance();
- } catch (<span class="hljs-type" style="">Exception</span> e) {
- . . . . . .
- }
- try {
- . . . . . .
- receiver.setPendingResult(<span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">);</span></span>
- receiver.onReceive(context.getReceiverRestrictedContext(), <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.intent);</span></span>
- } catch (<span class="hljs-type" style="">Exception</span> e) {
- . . . . . .
- } finally {
- sCurrentBroadcastIntent.set(null);
- }
- <span class="hljs-keyword" style="">if</span> (receiver.getPendingResult() != null) {
- <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.finish();</span></span>
- }
- }</code>
3.2.2.2 必要時啓動新進程
現在我們回過頭來看，在目標進程尚未啓動的情況下，是如何完成遞送的。剛剛我們已經看到調用startProcessLocked()的句子了，只要不出問題，目標進程成功啓動後就會調用AMS的attachApplication()。
有關attachApplication()的詳情，請參考其他關於AMS的文檔，此處我們只需知道它裏面又會調用attachApplicationLocked()函數。
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">private</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">boolean</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">attachApplicationLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(IApplicationThread thread, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> pid)</span></span></span></span></code>
attachApplicationLocked()內有這麼幾句：
- <code class="hljs php" style=""><span class="hljs-comment" style="">// Check if a next-broadcast receiver is in this process...</span>
- <span class="hljs-keyword" style="">if</span> (!badApp && isPendingBroadcastProcessLocked(pid)) {
- <span class="hljs-keyword" style="">try</span> {
- didSomething = sendPendingBroadcastsLocked(app);
- } <span class="hljs-keyword" style="">catch</span> (<span class="hljs-keyword" style="">Exception</span> e) {
- <span class="hljs-comment" style="">// If the app died trying to launch the receiver we declare it 'bad'</span>
- badApp = <span class="hljs-keyword" style="">true</span>;
- }
- }</code>
它們的意思是，如果新啓動的進程就是剛剛mPendingBroadcast所記錄的進程的話，此時AMS就會執行sendPendingBroadcastsLocked(app)一句。
- <code class="hljs cpp" style=""><span class="hljs-comment" style="">// The app just attached; send any pending broadcasts that it should receive</span>
- <span class="hljs-function" style=""><span class="hljs-function" style="">boolean </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">sendPendingBroadcastsLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(ProcessRecord app)</span></span></span><span class="hljs-function" style=""> </span></span>{
- boolean didSomething = <span class="hljs-literal" style="">false</span>;
- <span class="hljs-keyword" style="">for</span> (BroadcastQueue <span class="hljs-built_in" style="">queue</span> : mBroadcastQueues) {
- didSomething |= <span class="hljs-built_in" style="">queue</span>.sendPendingBroadcastsLocked(app);
- }
- <span class="hljs-keyword" style="">return</span> didSomething;
- }</code>
BroadcastQueue的sendPendingBroadcastsLocked()函數如下：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">boolean</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">sendPendingBroadcastsLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(ProcessRecord app)</span></span></span><span class="hljs-function" style=""> </span></span>{
- <span class="hljs-keyword" style="">boolean</span> didSomething = <span class="hljs-keyword" style="">false</span>;
- <span class="hljs-keyword" style="">final</span> BroadcastRecord br = mPendingBroadcast;
- <span class="hljs-keyword" style="">if</span> (br != <span class="hljs-keyword" style="">null</span> && br.curApp.pid == app.pid) {
- <span class="hljs-keyword" style="">try</span> {
- mPendingBroadcast = <span class="hljs-keyword" style="">null</span>;
- processCurBroadcastLocked(br, app);
- didSomething = <span class="hljs-keyword" style="">true</span>;
- } <span class="hljs-keyword" style="">catch</span> (Exception e) {
- . . . . . .
- }
- }
- <span class="hljs-keyword" style="">return</span> didSomething;
- }</code>
可以看到，既然目標進程已經成功啓動了，那麼mPendingBroadcast就可以賦值爲null了。接着，sendPendingBroadcastsLocked()會調用前文剛剛闡述的processCurBroadcastLocked()，其內再通過app.thread.scheduleReceiver()，將語義發送到用戶進程，完成真正的廣播遞送。這部分在上一小節已有闡述，這裏就不多說了。
3.2.3 說說有序廣播是如何循環起來的？
我們知道，平行廣播的循環很簡單，只是在一個while循環裏對每個動態receiver執行deliverToRegisteredReceiverLocked()即可。而對有序廣播來說，原則上每次processNextBroadcast()只會處理一個BroadcastRecord的一個receiver而已。當然，此時摘下的receiver既有可能是動態註冊的，也有可能是靜態的。
對於動態註冊的receiver，目標進程處理完廣播之後，會間接調用am.finishReceiver()向AMS發出反饋，關於這一步，其實在前面羅列ReceiverDispatcher的performReceive()時已經出現過了，我們再列一下：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">performReceive</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Intent intent, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode,
- String data, Bundle extras,
- </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> ordered, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> sticky)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- . . . . . .
- Args args = <span class="hljs-keyword" style="">new</span> Args(intent, resultCode, data, extras, ordered, sticky);
- <span class="hljs-keyword" style="">if</span> (!mActivityThread.post(args))
- {
- <span class="hljs-keyword" style="">if</span> (mRegistered && ordered)
- {
- IActivityManager mgr = ActivityManagerNative.getDefault();
- . . . . . .
- args.sendFinished(mgr); <span class="hljs-comment" style="">// 請注意這一句！</span>
- }
- }
- }</code>
Args繼承於BroadcastReceiver.PendingResult，它調用的sendFinished()就是PendingResult的sendFinished()：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">sendFinished</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(IActivityManager am)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- <span class="hljs-keyword" style="">synchronized</span> (<span class="hljs-keyword" style="">this</span>) {
- <span class="hljs-keyword" style="">if</span> (mFinished) {
- <span class="hljs-keyword" style="">throw</span> <span class="hljs-keyword" style="">new</span> IllegalStateException(<span class="hljs-string" style="">"Broadcast already finished"</span>);
- }
- mFinished = <span class="hljs-keyword" style="">true</span>;
- <span class="hljs-keyword" style="">try</span> {
- <span class="hljs-keyword" style="">if</span> (mResultExtras != <span class="hljs-keyword" style="">null</span>) {
- mResultExtras.setAllowFds(<span class="hljs-keyword" style="">false</span>);
- }
- <span class="hljs-keyword" style="">if</span> (mOrderedHint) {
- am.finishReceiver(mToken, mResultCode, mResultData, mResultExtras,
- mAbortBroadcast);
- } <span class="hljs-keyword" style="">else</span> {
- <span class="hljs-comment" style="">// This broadcast was sent to a component; it is not ordered,</span>
- <span class="hljs-comment" style="">// but we still need to tell the activity manager we are done.</span>
- am.finishReceiver(mToken, <span class="hljs-number" style="">0</span>, <span class="hljs-keyword" style="">null</span>, <span class="hljs-keyword" style="">null</span>, <span class="hljs-keyword" style="">false</span>);
- }
- } <span class="hljs-keyword" style="">catch</span> (RemoteException ex) {
- }
- }
- }</code>
代碼中的am.finishReceiver()會通知AMS，表示用戶側receiver已經處理好了，或者至少告一段落了，請AMS進行下一步動作。
而對於動態註冊的receiver，情況是類似的，最終也是調用am.finishReceiver()向AMS發出回饋的，只不過發起的動作是在ActivityThread的handleReceiver()動作中。前文已經列過這個函數了，大家注意下面的句子即可：
- <code class="hljs haskell" style=""><span class="hljs-title" style="">private</span> void handleReceiver(<span class="hljs-type" style="">ReceiverData</span> <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">) </span></span>
- {
- . . . . . .
- receiver.setPendingResult(<span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">);</span></span>
- receiver.onReceive(context.getReceiverRestrictedContext(),
- <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.intent);</span></span>
- . . . . . .
- <span class="hljs-keyword" style="">if</span> (receiver.getPendingResult() != null) {
- <span class="hljs-class" style=""><span class="hljs-keyword" style=""><span class="hljs-class" style=""><span class="hljs-keyword" style="">data</span></span></span><span class="hljs-class" style="">.finish();</span></span>
- }
- }</code>
ReceiverData也是繼承於BroadcastReceiver.PendingResult的，它調用的finish()是PendingResult的finish()：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">finish</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">()</span></span></span><span class="hljs-function" style="">
- </span></span>{
- <span class="hljs-keyword" style="">if</span> (mType == TYPE_COMPONENT) {
- . . . . . .
- } <span class="hljs-keyword" style="">else</span> <span class="hljs-keyword" style="">if</span> (mOrderedHint && mType != TYPE_UNREGISTERED) {
- <span class="hljs-keyword" style="">if</span> (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,
- <span class="hljs-string" style="">"Finishing broadcast to "</span> + mToken);
- <span class="hljs-keyword" style="">final</span> IActivityManager mgr = ActivityManagerNative.getDefault();
- sendFinished(mgr);
- }
- }</code>
此處的sendFinished()內部最終也會調用到am.finishReceiver()，向AMS通告receiver已經處理好了。
AMS側在收到finishReceiver語義後，執行：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">finishReceiver</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(IBinder who, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">int</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultCode, String resultData,
- Bundle resultExtras, </span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> resultAbort)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- . . . . . .
- <span class="hljs-keyword" style="">try</span> {
- <span class="hljs-keyword" style="">boolean</span> doNext = <span class="hljs-keyword" style="">false</span>;
- BroadcastRecord r = <span class="hljs-keyword" style="">null</span>;
- <span class="hljs-keyword" style="">synchronized</span>(<span class="hljs-keyword" style="">this</span>) {
- r = broadcastRecordForReceiverLocked(who);
- <span class="hljs-keyword" style="">if</span> (r != <span class="hljs-keyword" style="">null</span>) {
- doNext = r.queue.finishReceiverLocked(r, resultCode,
- resultData, resultExtras, resultAbort, <span class="hljs-keyword" style="">true</span>);
- }
- }
- <span class="hljs-keyword" style="">if</span> (doNext) {
- r.queue.processNextBroadcast(<span class="hljs-keyword" style="">false</span>);
- }
- trimApplications();
- } <span class="hljs-keyword" style="">finally</span> {
- Binder.restoreCallingIdentity(origId);
- }
- }</code>
可以看到，如有必要，會繼續調用processNextBroadcast()，從而完成有序廣播的循環處理。
3.2.4 說說有序廣播的timeout處理
因爲AMS很難知道一次廣播究竟能不能完全成功遞送出去，所以它必須實現一種“時限機制”。前文在闡述broadcastIntentLocked()時，提到過new一個BroadcastRecord節點，並插入一個BroadcastQueue裏的“平行列表”或者“有序列表”。不過當時我們沒有太細說那個BroadcastQueue，現在我們多加一點兒說明。
實際上系統中有兩個BroadcastQueue，一個叫做“前臺廣播隊列”，另一個叫“後臺廣播隊列”，在AMS裏是這樣定義的：
- <code class="hljs nginx" style=""><span class="hljs-attribute" style="">BroadcastQueue</span> mFgBroadcastQueue;
- <span class="hljs-attribute" style="">BroadcastQueue</span> mBgBroadcastQueue;</code>
爲什麼要搞出兩個隊列呢？我認爲這是因爲系統對“廣播時限”的要求不同導致的。對於前臺廣播隊列而言，它裏面的每個廣播必須在10秒之內把廣播遞送給receiver，而後臺廣播隊列的時限比較寬，只需60秒之內遞送到就可以了。具體時限值請看BroadcastQueue的mTimeoutPeriod域。注意，這個10秒或60秒限制是針對一個receiver而言的。比方說“前臺廣播隊列”的某個BroadcastRecord節點對應了3個receiver，那麼在處理這個廣播節點時，只要能在30秒（3 x 10）之內搞定就可以了。事實上，AMS系統考慮了更多東西，所以它給一個BroadcastRecord的總時限是其所有receiver時限之和的2倍，在此例中就是60秒（2 x 3 x 10）。
對於平行receiver而言，時限的作用小一點兒，因爲動態receiver是直接遞送到目標進程的，它不考慮目標端是什麼時候處理完這個廣播的。
然而對於有序receiver來說，時限就比較重要了。因爲receiver之間必須是串行處理的，也就是說上一個receiver在沒處理完時，系統是不會讓下一個receiver進行處理的。從processNextBroadcast()的代碼來看，在處理有序receiver時，BroadcastRecord裏的nextReceiver域會記錄“下一個應該處理的receiver”的標號。只有在BroadcastRecord的所有receiver都處理完後，或者BroadcastRecord的處理時間超過了總時限的情況下，系統纔會把這個BroadcastRecord節點從隊列裏刪除。因此我們在processNextBroadcast()裏看到的獲取當前BroadcastRecord的句子是寫死爲r = mOrderedBroadcasts.get(0)的。
在拿到當前BroadcastRecord之後，利用nextReceiver值拿到當前該處理的receiver信息：
- <code class="hljs cs" style=""><span class="hljs-keyword" style="">int</span> recIdx = r.nextReceiver++;
- . . . . . .
- Object nextReceiver = r.receivers.<span class="hljs-keyword" style="">get</span>(recIdx);</code>
當然，一開始，nextReceiver的值只會是0，表示第一個receiver有待處理，此時會給BroadcastRecord的dispatchTime域賦值。
- <code class="hljs cpp" style=""><span class="hljs-keyword" style="">int</span> recIdx = r.nextReceiver++;
- r.receiverTime = SystemClock.uptimeMillis();<span class="hljs-keyword" style="">if</span> (recIdx == <span class="hljs-number" style="">0</span>) {
- r.dispatchTime = r.receiverTime;
- r.dispatchClockTime = System.currentTimeMillis();
- . . . . . .
- }</code>
也就是說，dispatchTime的意義是標記實際處理BroadcastRecord的起始時間，那麼這個BroadcastRecord所能允許的最大時限值就是：
dispatchTime + 2 * mTimeoutPeriod * 其receiver總數
一旦超過這個時限，而BroadcastRecord又沒有處理完，那麼就強制結束這個BroadcastRecord節點：
- <code class="hljs actionscript" style=""><span class="hljs-keyword" style="">if</span> ((numReceivers > <span class="hljs-number" style="">0</span>) &&
- (now > r.dispatchTime + (<span class="hljs-number" style="">2</span>*mTimeoutPeriod*numReceivers)))
- {
- . . . . . .
- broadcastTimeoutLocked(<span class="hljs-literal" style="">false</span>); <span class="hljs-comment" style="">// forcibly finish this broadcast</span>
- forceReceive = <span class="hljs-literal" style="">true</span>;
- r.state = BroadcastRecord.IDLE;
- }</code>
此處調用的broadcastTimeoutLocked()的參數是boolean fromMsg，表示這個函數是否是在處理“時限消息”的地方調用的，因爲當前是在processNextBroadcast()函數裏調用broadcastTimeoutLocked()的，所以這個參數爲false。從這個參數也可以看出，另一處判斷“處理已經超時”的地方是在消息處理機制裏，在那個地方，fromMsg參數應該設爲true。
大體上說，每當processNextBroadcast()準備遞送receiver時，會調用setBroadcastTimeoutLocked()設置一個延遲消息：
- <code class="hljs bash" style="">long timeoutTime = r.receiverTime + mTimeoutPeriod;
- . . . . . .
- <span class="hljs-built_in" style="">set</span>BroadcastTimeoutLocked(timeoutTime);</code>
setBroadcastTimeoutLocked()的代碼如下：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">setBroadcastTimeoutLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(</span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">long</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> timeoutTime)</span></span></span><span class="hljs-function" style="">
- </span></span>{ <span class="hljs-keyword" style="">if</span> (! mPendingBroadcastTimeoutMessage)
- {
- Message msg = mHandler.obtainMessage(BROADCAST_TIMEOUT_MSG, <span class="hljs-keyword" style="">this</span>);
- mHandler.sendMessageAtTime(msg, timeoutTime);
- mPendingBroadcastTimeoutMessage = <span class="hljs-keyword" style="">true</span>;
- }
- }</code>
只要我們的receiver能及時處理廣播，系統就會cancel上面的延遲消息。這也就是說，但凡事件泵的handleMessage()開始處理這個消息，就說明receiver處理超時了。此時，系統會放棄處理這個receiver，並接着嘗試處理下一個receiver。
- <code class="hljs java" style=""><span class="hljs-keyword" style="">final</span> Handler mHandler = <span class="hljs-keyword" style="">new</span> Handler()
- {
- <span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">public</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">handleMessage</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(Message msg)</span></span></span><span class="hljs-function" style=""> </span></span>{
- <span class="hljs-keyword" style="">switch</span> (msg.what)
- {
- . . . . . .
- <span class="hljs-keyword" style="">case</span> BROADCAST_TIMEOUT_MSG:
- {
- <span class="hljs-keyword" style="">synchronized</span> (mService)
- {
- broadcastTimeoutLocked(<span class="hljs-keyword" style="">true</span>);
- }
- } <span class="hljs-keyword" style="">break</span>;
- }
- }
- };</code>
broadcastTimeoutLocked()的代碼截選如下：
- <code class="hljs java" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">final</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-keyword" style="">void</span></span></span><span class="hljs-function" style=""> </span><span class="hljs-title" style=""><span class="hljs-function" style=""><span class="hljs-title" style="">broadcastTimeoutLocked</span></span></span><span class="hljs-params" style=""><span class="hljs-function" style=""><span class="hljs-params" style="">(</span></span><span class="hljs-keyword" style=""><span class="hljs-function" style=""><span class="hljs-params" style=""><span class="hljs-keyword" style="">boolean</span></span></span></span><span class="hljs-function" style=""><span class="hljs-params" style=""> fromMsg)</span></span></span><span class="hljs-function" style="">
- </span></span>{
- <span class="hljs-keyword" style="">if</span> (fromMsg) {
- mPendingBroadcastTimeoutMessage = <span class="hljs-keyword" style="">false</span>;
- }
- <span class="hljs-keyword" style="">if</span> (mOrderedBroadcasts.size() == <span class="hljs-number" style="">0</span>) {
- <span class="hljs-keyword" style="">return</span>;
- }
- <span class="hljs-keyword" style="">long</span> now = SystemClock.uptimeMillis();
- BroadcastRecord r = mOrderedBroadcasts.get(<span class="hljs-number" style="">0</span>);
- . . . . . .
- . . . . . .
- finishReceiverLocked(r, r.resultCode, r.resultData,
- r.resultExtras, r.resultAbort, <span class="hljs-keyword" style="">true</span>);
- scheduleBroadcastsLocked();
- . . . . . .
- }</code>
可以看到，當一個receiver超時後，系統會放棄繼續處理它，並再次調用scheduleBroadcastsLocked()，嘗試處理下一個receiver。
4 尾聲
有關Android的廣播機制，我們就先說這麼多吧。品一杯紅茶，說一段代碼，管他雲山霧罩，任那瑣碎冗長，我自冷眼看安卓，瞧他修短隨化。