1> 内部调用 sendMessageDelayed 函数

  public final boolean postDelayed(Runnable r, long delayMillis){
     return sendMessageDelayed(getPostMessage(r), delayMillis);
 }

2> sendMessageDelayed 函数内部调用 sendMessageDelayed

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

3> sendMessageDelayed 内部调用 sendMessageAtTime

 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);
}

4> sendMessageAtTime 内部调用 enqueueMessa

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

5> enqueueMessage 函数内部调用queue.enqueueMessage

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;
}

核心代码在 enqueueMessage，通过分析可以得到如下的结论

• 消息队列是在一个同步的代码块中 synchronized (this) {}
• 如果发送的消息为空、延迟时间为0等，标识了这条信息就是一条普通消息，不进行特殊处理，体现在：if (p == null || when == 0 || when < p.when) {}
• 如果发送的消息含有延迟信息，则使用 nativeWake (native 函数) 函数进行唤醒

实践结论总结：调用 Handler.postDelayed 函数不会造成主线程阻塞，请看下面例子，应用会先展示onCreate—1、onCreate—3、onCreate—2，且延迟消息的也是一个有序的消息队列（先入先出）

handler.postDelayed(new Runnable() {
  @Override
  public void run() {
      Toast.makeText(MainActivity.this,"onCreate---3", Toast.LENGTH_SHORT).show();
   }
   },2000);

   handler.postDelayed(new Runnable() {
   @Override
   public void run() {
       Toast.makeText(MainActivity.this, "onCreate---2", Toast.LENGTH_SHORT).show();
    }
  }, 2000);

  Toast.makeText(MainActivity.this, "onCreate---1", Toast.LENGTH_SHORT).show();

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