Handler 是主线程与子线程沟通的桥梁

* <p>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 <em>post</em> or <em>sendMessage</em> 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.

即使你在后台线程中调用 handler.post()，任务也还是会被送到 Handler 所绑定的线程（通常是主线程）中执行。

Handler 是主线程与子线程沟通的桥梁

这样避免了多线程通信的麻烦