Activity 生命周期

/**
 * 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 
 */