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);
   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
   // @R_616_6296@e 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) {
}