Activity声明周期:
整体:onCreate-onStart-onResume-onPause-onStop-onDestroy.
Activity新建后被覆盖:onCreate-onStart-onResume-onPause-onStop.
然后又重新获得焦点:onRestart-onStart-onResume.
Activity新建后被覆盖失去焦点,但可见:onCreate-onStart-onResume-onPause.
然后重新获得焦点:onResume
失去焦点,可见:调用onPause.
失去焦点,不可见;调用onPause-onStop