Android Handler消息派发机制源码分析

 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);}

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

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;}

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;}

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

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(); }}

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

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