* This method is only for use in very special circumstances -- it
* can easily starve the message queue, cause ordering problems, or have
* other unexpected side-effects.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
postAtTime在指定时间uptimeMillis把runnable插入到队列中去，另一个postAtTime方法又加了一个Object类型的token，在下面的sendMessageAtTime中具体分析。postDelayed在延迟delayMillis时间后插入队列。postAtFrontOfQueue把Runnable插入到队首，下一次轮询就会被执行。
下面是从队列中删除对应的runable：
[java]
/**
* Remove any pending posts of Runnable r that are in the message queue.
*/
public final void removeCallbacks(Runnable r)
{
mQueue.removeMessages(this, r, null);
}
/**
* Remove any pending posts of Runnable r with Object
* token that are in the message queue. If token is null,
* all callbacks will be removed.
*/
public final void removeCallbacks(Runnable r, Object token)
{
mQueue.removeMessages(this, r, token);
}
下面就是重头戏SendMessage：
[java]
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
把一个消息插入到当前所有正在等待执行的消息的后面。它会在当前线程所关联的handler的handleMessage方法中被处理。我们看到这个方法主要是调用了441行的sendMessageDelayed方法（延迟0秒发送消息）：
[java]
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
这个方法主要是在delayMillis时间后发送消息。调用的是467行的sendMessageAtTime方法：
[java]
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) uptimeMillis.
* The time-base is {@link android.os.SystemClock#uptimeMillis}.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {