When posting or sending to a Handler, you can either allow the item to be processed as soon as the message queue is ready to do so, or specify a delay before it gets processed or absolute time for it to be processed. The latter two allow you to implement timeouts, ticks, and other timing-based behavior.
When a process is created for your application, its main thread is dedicated to running a message queue that takes care of managing the top-level application objects (activities, broadcast receivers, etc) and any windows they create. You can create your own threads, and communicate back with the main application thread through a Handler. This is done by calling the same post or sendMessage methods as before, but from your new thread. The given Runnable or Message will then be scheduled in the Handler's message queue and processed when appropriate.
Handler是用于发送和处理消息和一个线程的MessageQueue相关联的Runable对象。每个Handler实例关联到一个单一线程和线程的messagequeue。当您创建一个Handler,从你创建它的时候开始，它就绑定到创建它的线程以及对应的消息队列，handler将发送消息到消息队列，并处理从消息队列中取出的消息。
Handler的主要用途有两个：(1)、在将来的某个时刻执行消息或一个runnable,(2)、为运行在不同线程中的多个任务排队。
主要依靠post(Runnable)、postAtTime(Runnable, long)、postDelayed(Runnable, long)、sendEmptyMessage(int)、sendMessage(Message)、sendMessageAtTi(Message)、sendMessageDelayed(Message, long)这些方法来来完成消息调度。post方法是当到Runable对象到达就被插入到消息队列；sendMessage方法允许你把一个包含有信息的Message插入队列，而且它会Handler的handlerMessage(Message)方法中执行(该方法要求在Handler的子类中实现)。
当向Handler post或者send消息的时候，你可以在消息队列准备好的时候立刻执行，或者指定一个延迟之前得到处理或绝对时间对它进行处理，后两个是实现了timeout、ticks或者其他timing-based的行为。
当你的应用创建一个进程时，其主线程(UI线程)会运行一个消息队列,负责管理优先级最高的应用程序对象(活动、广播接收器等)和任何他们创建的windows。你也可以创建自己的线程，通过handler与主线程进行通信，通过在你创建的线程调用的post或sendMessage方法。传入的Runnable或者消息会被插入到消息队列并且在适当的时候得到处理。
先看下类利用使用的全局变量：
[java]
final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
IMessenger mMessenger;
都会在构造方法里面赋值：
[java]
/**
* Default constructor associates this handler with the queue for the
* current thread.
*
* If there isn't one, this handler won't be able to receive messages.
*/
public Handler() {
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 = null;
}
/**
* Constructor associates this handler with the queue for the
* current thread and takes a callback interface in which you can handle
* messages.
*/
public Handler(Callback callback) {
if (FIND_POTENTIAL_LEAKS) {
final Class< extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Lo