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前言
Android中的消息機制是指線程之間的通信機制。我們都知道,如果我們在UI主線程中做耗時的操作而無法及時處理時,程序會彈出ANR全名Application Not Responding, 也就是”應用無響應”的對話框。
首先來一張圖,從整體上來看一下android消息機制。
Handler:用於發送消息和處理消息
MessageQueue: 一個先進先出的消息隊列
Looper:循環者,它不斷的循環的遍歷查詢消息隊列
Looper中會創建一個消息隊列,並進入消息循環,不斷的從消息隊列中取出消息,然後分發消息給對應的消息處理函數,如果消息隊列爲空,它會進入阻塞等待,直到有新的消息到來,然後被喚醒。
源碼分析
Looper
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
這就是Looper的創建函數,它創建了一個Looper實例並放到ThreadLocal中。
ThreadLocal是一個線程共享和線程安全的,ThreadLocal變量在不同的線程中有不同的副本。
這裏,首先檢查線程是否有Looper,如果已經有,就報”Only one Looper may be created per thread”異常。也就是說一個線程只能有一個Looper,不能重複創建。
進入Looper的構造函數
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的構造函數中主要是創建了一個消息隊列,和賦值當前線程變量。
開啓消息循環
public static void loop() {
//取出循環者
final Looper me = myLooper();
//開始循環之前,必須先創建循環者
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//從循環者中取出消息隊列
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
//進入循環,不斷的從消息隊列中取出消息,如果沒有消息會進入阻塞狀態
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
//將消息分發給對應的handler處理
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
//釋放資源
msg.recycleUnchecked();
}
}
Looper#loop方法的工作,在代碼中已經進行註釋說明。
Looper#loop中會將消息分發給對應的handler處理。
msg.target.dispatchMessage(msg);
現在我們進入handler。
Handler
Handler handler = new Handler(Looper.myLooper());
首先看Handler的構造函數,可以知道Handler是怎麼和Looper取得關聯的。
public Handler(Looper looper, Callback callback, boolean async)
{
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
主要爲Handler的四個變量賦值,其中確定了Handler是和哪一個Looper關聯,和Handler發送消息到對應的哪個消息隊列。可以知道,一個Handler只有一個Looper和對應的MessageQueue。而不同的Handler可以共享同一個Looper和MessageQueue,這就看你在初始化Handler時與哪個Looper關聯了。
Handler無參數的構造函數是和哪個Looper關聯呢?
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
//取出當前線程的循環者
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Handler無參數的構造函數仍然主要是爲那四個變量賦值。它會首先取出當前線程的消息循環者,如果線程沒有循環者,會報一個異常。
發送消息到循環隊列
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
sendMessage中會調用sendMessageDelayed,sendMessageDelayed再調用sendMessageAtTime,最後會調用enqueueMessage將消息入隊。post開頭的方法是調用相應send開頭的方法的。
進入Handler#enqueueMessage
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在分析Looper#loop時,其中有將消息的分發給相應的Handler處理的邏輯,而正是在第2行代碼時,它們取得聯繫的。然後將消息放入Handler關聯的Looper中的消息隊列。
在MessageQueue#enqueueMessage中,消息入隊時,如果消息隊列是阻塞休眠狀態,會喚醒消息隊列。
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
}
在Looper#loop中,會將消息分發給對應的Handler處理函數dispatchMessage處理
msg.target.dispatchMessage(msg);
進入Handler#dispatchMessage
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
java.lang.Callback
public interface Runnable {
public abstract void run();
}
Handler#Callback
public interface Callback {
public boolean handleMessage(Message msg);
}
優先調用Message的callback接口,如果Handler有Callback,調用Callback,否則會調用handleMessage方法。
Handler#handleMessage
public void handleMessage(Message msg) {
}
這是一個空方法,具體的消息邏輯由我們自己定義。
到此,這個流程已經解釋完畢
後話
在非UI線程中只要找好時機也是可以更新UI的。這個會在源碼再分析。