(每次學習一點xamarin就做個學習筆記和視頻來加深記憶鞏固知識)
如有不正確的地方,請幫我指正。
安卓異步消息處理簡介
有時候需要執行一些耗時的操作,例如從遠程服務器讀取數據,讀取的時間的長度由很多因素決定,我們不希望主線程被阻塞程序無法進行其它工作,而且Android中只能在主線程進行UI操作,不能在子線程操作,如果要根據子線程執行的結果來更新UI時,這時就需要用到安卓異步消息處理機制。
異步消息原理
安卓異步消息處理主要有四個類:Message、Handler、Looper和MessageQueue,
我們直接打交道的是前兩個類,後面兩個類在內部自動處理的。
類 | 說明 |
---|---|
Message | 用於攜帶數據 |
MessageQueue | 是一個存放消息的隊列 |
Looper | 用於不斷的從隊列中取消息,交給消息關聯的Handler對象去處理 |
Handler | 用於處理消息 |
如圖所示,首先在主線程中創建一個Handler實例對象並重寫了HandlerMessage方法,然後在子線程中執行run方法並創建Message對象攜帶好數據,通過消息對象關聯的hander對象的SendMessage方法將Message對象發送到消息隊列,Looper會自動去從隊列中取消息,取到後自動交給Handler的HandlerMessage方法處理。
下面我提取了Android中這四個類的源碼的主要屬性和方法,配上個人加的註釋(如有註釋不對的地方,請大家指正)
Message類
//用於攜帶數據
public final class Message implements Parcelable {
public int what;//一個整型標識
public int arg1;//可保存一個整數
public int arg2;//可保存一個整數
public Object obj;//可保存任意數據
long when;//消息應該被處理的時間
Handler target;//保存與它關聯的Handlder對象的引用
Runnable callback;//保存處理消息的回調方法的引用
Message next;//指向下一個消息,因爲消息隊列是一個單鏈表的數據結構
private static final Object sPoolSync = new Object();//用於同步鎖的對象
private static Message sPool;//緩存對象
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
public Message() { }
public static Message obtain() {//比上面構造方法效率高,因爲用了緩存對象
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
public long getWhen() {return when;}
public void setTarget(Handler target) {this.target = target;}
public Handler getTarget() {return target;}
public Runnable getCallback() {return callback;}
public void sendToTarget() {target.sendMessage(this);}
}
MessageQueue類
//消息隊列(以Message的when值來排序的鏈表結構)
public final class MessageQueue {
private final boolean mQuitAllowed;
Message mMessages;//鏈表中最前頭的Message對象(when值最小)
private boolean mQuitting;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;
//一些本地方法
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
}
//從隊列中取消息
Message next() {
final long ptr = mPtr;
if (ptr == 0) {return null;}
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {//以同步的方式取,確保同一時刻只有一個線程訪問
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {//如果消息關聯的Handlder對象沒有
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());//那麼只取異步消息
}
if (msg != null) {
if (now < msg.when) {//如果沒到消息應該被處理的時間
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);// 計算一個超時時間來等待
} else {//到達消息應該被處理的時間就返回消息
mBlocked = false;
if (prevMsg != null) {prevMsg.next = msg.next;}//取出一個消息後整理好鏈表
else {mMessages = msg.next;}
msg.next = null;
msg.markInUse();
return msg;
}
} else {
nextPollTimeoutMillis = -1;
}
}
nextPollTimeoutMillis = 0;
}
}
//添加一個新的消息到隊列中
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {throw new IllegalArgumentException("Message must have a target.");}
if (msg.isInUse()) {throw new IllegalStateException(msg + " This message is already in use.");}
synchronized (this) {//以同步方式添加消息
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {//如果鏈表爲空或者新來消息比隊列中所有消息的優先級更高
msg.next = p;//把原先鏈表中最前頭的消息放到新的消息後面
mMessages = msg;//設置新消息爲鏈表中最前頭的消息
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;//從鏈表第二個開始找合適的位置(第一個已經在上面的when < p.when判斷過了)
if (p == null || when < p.when) {
break;//找到合適鏈表位置p後退出循環
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p;
prev.next = msg;//這兩行代碼是把新消息插到prev與p之間
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
}
Handler類
//用於處理消息
public class Handler {
final Looper mLooper; //保存着Looper對象的引用
final MessageQueue mQueue; //保存着Looper對象中MessageQueue對象的引用
final Handler.Callback mCallback; //保存着構造方法傳入的回調方法的引用
public interface Callback { //一個接口,規定了消息處理的方法的規範
public boolean handleMessage(Message msg);
}
public Handler() {
this(null, false);
} //構造方法
public Handler(Handler.Callback callback, boolean async) {//構造方法
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;
}
public void handleMessage(Message msg)
{
//如果dispatchMessage方法中的msg.callback==null 或 mCallback==null
//那麼子類應該重寫此方法來處理消息
}
public void dispatchMessage(Message msg) {//分發消息給回調方法處理
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
public final boolean sendMessage(Message msg) { //發送一個消息
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)//以指定的延遲時間發送消息
{
if (delayMillis < 0) {delayMillis = 0;}
//當前時間加上延遲時間就是消息應該被處理的時間
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {//在指定的時間發送消息
MessageQueue queue = mQueue;
if (queue == null) { return false;}
return enqueueMessage(queue, msg, uptimeMillis);
}
//將消息發送到消息隊列中
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
//通過MessageQueue對象的enqueueMessage方法把消息添加到消息隊列
return queue.enqueueMessage(msg, uptimeMillis);
}
}
Looper類
//用於不斷的從隊列中取消息,交給消息關聯的Handler對象去處理
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//以鍵值對形式存儲(key:線程 value:此線程的Looper對象)
private static Looper sMainLooper;
final MessageQueue mQueue;//自己持有的消息隊列
final Thread mThread;//自己所在的線程
private Looper(boolean quitAllowed) {//私有構造方法
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("每個線程只能有一個Looper");
}
sThreadLocal.set(new Looper(quitAllowed));
}
public static void prepareMainLooper() {//準備好主Looper對象
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
//開啓循環
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
for (;;) {
Message msg = queue.next(); //從消息隊列中取出一個消息
if (msg == null) {
return;
}
try {
msg.target.dispatchMessage(msg);//將消息交給關聯的Handlder對象去處理
} finally {
}
msg.recycleUnchecked();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}//取出Looper
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
public boolean isCurrentThread() {return Thread.currentThread() == mThread;}
public void quit() {mQueue.quit(false);}
public @NonNull Thread getThread() {return mThread;}
public @NonNull MessageQueue getQueue() {return mQueue;}
}
我們直接打交道的是Message類和Handlder類,那麼MessageQueue和Looper類是怎麼來的,這就要看ActivityThread類程序的入口方法main,貼上部分代碼如下:
public final class ActivityThread {
//..........省略其它代碼
public static void main(String[] args) {
//..........省略其它代碼
Looper.prepareMainLooper();//創建好Looper對象,它構造方法裏同時也創建了MessageQueue對象
ActivityThread thread = new ActivityThread();//創建好主線程
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
Looper.loop(); //開啓無限循環,不斷從MessageQueue中取消息交給消息關聯的Handler處理
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
大概瞭解了這四大類和它們的機制後,接下來就開始做個簡單的異步消息處理程序。
//處理消息的類
public class MyHandler : Handler
{
private Action<Message> _action;//指向一個回調方法的委託
public MyHandler(Action<Message> action)
{
_action = action;//構造方法中傳入處理消息的回調方法
}
//重寫父類的HandleMessage方法
public override void HandleMessage(Message msg)
{
//if (_action != null)
//{
// _action.Invoke(msg);
//}
_action?.Invoke(msg); //C#6.0新增判斷空值的方式
}
}
//模擬從服務器讀取數據(實現Java.Lang.IRunnable接口可讓線程使用)
public class ReadDataTask : Java.Lang.Object, Java.Lang.IRunnable
{
private Handler _handler;
public ReadDataTask(Handler handler)
{
_handler = handler;
}
public void Run()//子線程要運行的任務
{
try
{
Java.Lang.Thread.Sleep(4000);// 模擬任務,比如從遠程服務器讀取數據
Message msg = _handler.ObtainMessage();//創建一個消息對象
msg.What = 123; //設置消息對象的標識
msg.Obj = "已讀取到數據";//消息對象攜帶一個字符串數據
_handler.SendMessage(msg);// 將消息對象發送到主線程的消息隊列(MessageQueue)中
}
catch (Java.Lang.InterruptedException e)
{
Log.Debug("ReadInfoTask", e.Message);
}
}
}
上面兩個類準備好後,接下來就在Activity中使用了
public class MainActivity : AppCompatActivity,IOnClickListener
{
private MyHandler _myHandler;
private EditText _editText;
public void OnClick(View v)
{
switch (v.Id)
{
case Resource.Id.button1:
Java.Lang.Thread thread = new Java.Lang.Thread(new ReadDataTask(_myHandler));
thread.Start();//啓動一個子線程去執行耗時任務
break;
}
}
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
SetContentView(Resource.Layout.Main);
Button btn1 = this.FindViewById<Button>(Resource.Id.button1);
btn1.SetOnClickListener(this);
_editText = this.FindViewById<EditText>(Resource.Id.editText1);
_myHandler = new MyHandler(RefreshUI); //在主線程實例化一個Handler對象
}
private void RefreshUI(Message msg)//在主線程更新UI
{
switch (msg.What)
{
case 123:
string s = Convert.ToString(msg.Obj);
_editText.Text = s;
break;
case 456:
//……………………
break;
}
}
}
代碼和視頻在我上傳的CSDN資源中http://download.csdn.net/download/junshangshui/10022514