/**
* 程序启动: onCreate -> onStart -> onResume
* HOME键: onSaveInstanceState -> onPause -> onStop
* BACK键: finish -> onPause -> onStop -> onDestory
* 再次执行: onRestart -> onStart -> onResume
* 横屏<-->竖屏(切换)
* onSaveInstanceState -> onPause -> onStop --> onDestory --> onCreate -> onStart -> onResume
*
* 调用finish onPause -> onStop -> onDestroy (程序结束)
* 再次执行 onCreate -> onStart -> onResume (重新启动)
* killProcess的时候系统不执行onDestroy;直接杀死
* --------------------------------------------
* 两个activity之间的跳转
* 启动A_onCreate -> A_onStart -> A_onResume
* **intent跳转(A->B) A_onSaveInstanceState -> A_onPause ->
* B_onCreate -> B_onStart -> B_onResume -> A_onStop
* **back跳转(B->A) B_onPause -> A_onRestart -> A_onStart -> A_onResume
* B_onStop -> B_destroy
*/