Android Handler消息派发机制源码分析

注：这里只是说一下sendmessage的一个过程，post就类似的
如果我们需要发送消息，会调用sendmessage方法

 public final boolean sendmessage(message msg)
{
 return sendmessagedelayed(msg, 0);
} 

这个方法会调用如下的这个方法 

public final boolean sendmessagedelayed(message msg, long delaymillis)
{
 if (delaymillis < 0) {
  delaymillis = 0;
 }
 return sendmessageattime(msg, systemclock.uptimemillis() + delaymillis);
}

接下来设定延迟时间，然后继续调用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);
}

这里获得了消息队列，检查队列是否存在，然后返回enquemessage的方法的执行结果，这个结果是说明消息能否进入队列的一个布尔值 

private boolean enqueuemessage(messagequeue queue, message msg, long uptimemillis) {
 msg.target = this;
 if (masynchronous) {
  msg.setasynchronous(true);
 }
 return queue.enqueuemessage(msg, uptimemillis);
}

这里是对消息进行入队处理，下面就是在messagequeue中对消息进行入队 

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;
}

就是对传递过来的消息进行一些封装然后放到队列中，至此我们的sendmessage处理完毕，返回的结果是进队是否成功的布尔值，那么究竟消息之后是如何被处理的呢？
我们可以看到在handler构造的时候记录了一个looper对象，也记录了一个回掉函数 

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;
}

这里的mylooper方法返回的是当前线程关联的一个looper对象

 public static @nullable looper mylooper() {
 return sthreadlocal.get();
} 

当looper实例化了以后会执行自己的prepare方法然后执行loop方法，loop方法就是不断的读取消息队列中的消息然后执行相应的操作的方法，因为是在其他线程中执行的循环所以不会影响其他线程 

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();
 }
}

在循环中如果读取到了消息，就会执行dispatchmessage方法，然后分派完消息之后再执行一次recycleunchecked方法来重用这个message，我们看到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);
 }
}

这里看到直接执行了一个handlermessage方法，这个方法是一个回调方法，我们是必须实现的，否则handler什么都不会做，为什么呢？还记得刚刚说构造handler的时候我们记录了一个callback的回掉吗？handler中的这个handlermessage方法是一个空方法，如果我们重写了这个方法，在回调的时候就会执行我们先写下的代码，也就是接收到消息之后要做什么。 

public interface callback {
 public boolean handlemessage(message msg);
}

public void handlemessage(message msg) {
}

这里简单说下整个过程： 
当我们实例化一个handler的子类并重写handlemessage方法之后，这个时候系统已经帮我们做了几个事情 
1.实例化了一个消息队列messagequeue 
2.实例化了一个关联的looper对象，并让looper不断的读取消息队列
3.把我们重写的handlemessage方法记录为我们需要回调的方法 
当我们执行handler的sendmessage方法的时候，系统会把我们传过去的message对象添加到消息队列，这个时候如果looper读取到了消息，就会把消息派发出去，然后回调handlemessage方法，执行我们设定的代码。

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