Handler源码解析(五)

我们从源码中结合注释，返回传入message的name值，默认的实现是如火message.callback不为空，就返回callback的类名，或者返回一个16进制的message的what值。
再往下，191行的obtainMessage()方法：

[java]
/**
* Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
* creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
* If you don't want that facility, just call Message.obtain() instead.
*/
public final Message obtainMessage()
{
return Message.obtain(this);
}

从一个全局消息池里面获取一个新的Message。在Message池中检索是否存在与handler实例对应的message比创建一个新的Message更高效。如果你不想创建新Message，就是用Message.obtain方法代替。
下面是几个obtainMessage的重载方法：

[java]
/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what and obj members
* of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, Object obj)
{
return Message.obtain(this, what, obj);
}

/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what, arg1 and arg2 members of the returned
* Message.
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, int arg1, int arg2)
{
return Message.obtain(this, what, arg1, arg2);
}

/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what, obj, arg1,and arg2 values on the
* returned Message.
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
{
return Message.obtain(this, what, arg1, arg2, obj);
}

和上面相同，只是参数不同，为返回的Message的一些属性赋值。
在往下就是post（）方法了：

[java]
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable 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 post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}

把传入的Runnable对象r加入到Message队列中，这个runnable对象将在handler关联的线程中执行。如果runnable对象被正确执行返回true，如果looper遍历消息队列时退出，则返回false。在这个方法中，主要是调用了sendMessageDelayed方法。在下面会有相应的分析。
接下来，看一下其他有关post的方法（从266行到353