Invalidate和postInvalidate

1.4.Invalidate和postInvalidate
API的描述：Invalidate the whole view. If the view is visible, onDraw(Canvas) will be called at somepoint in the future. This must be called from a UI thread. To call from anon-UI thread, call postInvalidate()。
即，在UI线程的话可以直接调用invalidate去刷新布局，其他的非UI线程的话要去调用postInvalidete()。