Handler有什么作用
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed at some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
- 安排message与runnable在将来某个时刻执行(传递Message)
- 在不同的线程去操作入队列与执行(切换线程,经常使用的就是子线程通知UI线程更新UI,这个的前提是Handler持有的Looper是mainLooper)
