从源码角度分析Android的消息机制

 public void set(T value) {
  Thread t = Thread.currentThread();
  ThreadLocalMap map = getMap(t);
  if (map != null)
   map.set(this, value);
  else
   createMap(t, value);
 }
 
  public T get() {
  Thread t = Thread.currentThread();
  ThreadLocalMap map = getMap(t);
  if (map != null) {
   ThreadLocalMap.Entry e = map.getEntry(this);
   if (e != null) {
    @SuppressWarnings("unchecked")
    T result = (T)e.value;
    return result;
   }
  }
  return setInitialValue();
 }

// Handler.java

 public Handler() {
  this(null, false);
 }
 // callback 消息回调；async 是否同步
 public Handler(Callback callback, boolean async) {
  ...
  // 1. 首先获取looper
  mLooper = Looper.myLooper();
  if (mLooper == null) {
   throw new RuntimeException(
    "Can't create handler inside thread " + Thread.currentThread()
      + " that has not called Looper.prepare()");
  }
  // 2. 获取MessggeQueue
  MQueue = mLooper.mQueue;
  mCallback = callback;
  mAsynchronous = async;
 }

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

 public static void prepare() {
  prepare(true);
 }

 private static void prepare(boolean quitAllowed) {
  // 在创建looper之前，判断looper是否与threadloacal绑定过，这也是prepare只能设置一遍的原因。
  if (sThreadLocal.get() != null) {
   throw new RuntimeException("Only one Looper may be created per thread");
  }
  sThreadLocal.set(new Looper(quitAllowed));
 }

 public static void prepareMainLooper() {
  // 这里其实还是调用的prepare方法
  prepare(false);
  synchronized (Looper.class) {
   if (sMainLooper != null) {
    throw new IllegalStateException("The main Looper has already been prepared.");
   }
   sMainLooper = myLooper();
  }
 }

public static void main(String[] args) {
  ...

  //1. 初始化Looper对象
  Looper.prepareMainLooper();
  
  // 2. 开启无限循环
  Looper.loop();
  throw new RuntimeException("Main thread loop unexpectedly exited");
 }

 private Looper(boolean quitAllowed) {
  mQueue = new MessageQueue(quitAllowed);
  mThread = Thread.currentThread();
 }

 public static void loop() {
  final Looper me = myLooper();
  // 1. 判断looper是否存在
  if (me == null) {
   throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
  }
  final MessageQueue queue = me.mQueue;
  ...
  
  //2. 开启一个死循环
  for (;;) {
   Message msg = queue.next(); // might block
   if (msg == null) {
    // No message indicates that the message queue is quitting.
    return;
   }
   ...
   try {
    msg.target.dispatchMessage(msg);
    dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
   } finally {
    if (traceTag != 0) {
     Trace.traceEnd(traceTag);
    }
   }
  ...
  }
 }

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

 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;
  ...
  return enqueueMessage(queue, msg, uptimeMillis);
 }
 
 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
  // 关键代码来了！
  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 {
    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;
 }

 Message next() {
  final long ptr = mPtr;
  if (ptr == 0) {
   return null;
  }

  int pendingIdleHandlerCount = -1; // -1 only during first iteration
  int nextPollTimeoutMillis = 0;
  for (;;) {

   nativePollOnce(ptr, nextPollTimeoutMillis);
   ...
  }
 }