Android Handler消息派發機制源碼分析

注：這裡只是說一下sendmessage的一個過程，post就類似的
如果我們需要發送消息，會調用sendMessage方法

 public final boolean sendMessage(Message msg)
{
 return sendMessageDelayed(msg, 0);
} 

這個方法會調用如下的這個方法 

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
 if (delayMillis < 0) {
  delayMillis = 0;
 }
 return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

接下來設定延遲時間，然後繼續調用sendMessageAtTime方法 

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);
}

這裡獲得了消息隊列，檢查隊列是否存在，然後返回enqueMessage的方法的執行結果，這個結果是說明消息能否進入隊列的一個布爾值 

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
 msg.target = this;
 if (mAsynchronous) {
  msg.setAsynchronous(true);
 }
 return queue.enqueueMessage(msg, uptimeMillis);
}

這裡是對消息進行入隊處理，下面就是在MessageQueue中對消息進行入隊 

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) {
  if (mQuitting) {
   IllegalStateException e = new IllegalStateException(
     msg.target + " sending message to a Handler on a dead thread");
   Log.w(TAG, e.getMessage(), e);
   msg.recycle();
   return false;
  }

  msg.markInUse();
  msg.when = when;

  Message p = mMessages;
  boolean needWake;
  if (p == null || when == 0 || when < p.when) {
   // New head, wake up the event queue if blocked.
   msg.next = p;
   mMessages = msg;
   needWake = mBlocked;
  } else {
   // Inserted within the middle of the queue. Usually we don't have to wake
   // up the event queue unless there is a barrier at the head of the queue
   // and the message is the earliest asynchronous message in the queue.
   needWake = mBlocked && p.target == null && msg.isAsynchronous();
   Message prev;
   for (;;) {
    prev = p;
    p = p.next;
    if (p == null || when < p.when) {
     break;
    }
    if (needWake && p.isAsynchronous()) {
     needWake = false;
    }
   }
   msg.next = p; // invariant: p == prev.next
   prev.next = msg;
  }

  // We can assume mPtr != 0 because mQuitting is false.
  if (needWake) {
   nativeWake(mPtr);
  }
 }
 return true;
}

就是對傳遞過來的消息進行一些封裝然後放到隊列中，至此我們的sendMessage處理完畢，返回的結果是進隊是否成功的布爾值，那麼究竟消息之後是如何被處理的呢？
我們可以看到在Handler構造的時候記錄了一個Looper對象，也記錄了一個回掉函數 

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());
  }
 }

 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;
}

這裡的myLooper方法返回的是當前線程關聯的一個Looper對象

 public static @Nullable Looper myLooper() {
 return sThreadLocal.get();
} 

當Looper實例化了以後會執行自己的prepare方法然後執行loop方法，loop方法就是不斷的讀取消息隊列中的消息然後執行相應的操作的方法，因為是在其他線程中執行的循環所以不會影響其他線程 

public static void loop() {
 final Looper me = myLooper();
 if (me == null) {
  throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
 }
 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();

 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
  Printer logging = me.mLogging;
  if (logging != null) {
   logging.println(">>>>> Dispatching to " + msg.target + " " +
     msg.callback + ": " + msg.what);
  }

  msg.target.dispatchMessage(msg);

  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);
  }

  msg.recycleUnchecked();
 }
}

在循環中如果讀取到了消息，就會執行dispatchMessage方法，然後分派完消息之後再執行一次recycleUnchecked方法來重用這個Message，我們看到dispatchMessage方法 

public void dispatchMessage(Message msg) {
 if (msg.callback != null) {
  handleCallback(msg);
 } else {
  if (mCallback != null) {
   if (mCallback.handleMessage(msg)) {
    return;
   }
  }
  handleMessage(msg);
 }
}

這裡看到直接執行了一個handlerMessage方法，這個方法是一個回調方法，我們是必須實現的，否則Handler什麼都不會做，為什麼呢？還記得剛剛說構造Handler的時候我們記錄了一個CallBack的回掉嗎？Handler中的這個handlerMessage方法是一個空方法，如果我們重寫了這個方法，在回調的時候就會執行我們先寫下的代碼，也就是接收到消息之後要做什麼。 

public interface Callback {
 public boolean handleMessage(Message msg);
}

public void handleMessage(Message msg) {
}

這裡簡單說下整個過程： 
當我們實例化一個Handler的子類並重寫handleMessage方法之後，這個時候系統已經幫我們做了幾個事情 
1.實例化了一個消息隊列MessageQueue 
2.實例化了一個關聯的Looper對象，並讓Looper不斷的讀取消息隊列
3.把我們重寫的handleMessage方法記錄為我們需要回調的方法 
當我們執行Handler的sendMessage方法的時候，系統會把我們傳過去的Message對象添加到消息隊列，這個時候如果Looper讀取到了消息，就會把消息派發出去，然後回調handleMessage方法，執行我們設定的代碼。

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