Actiivity 生命周期

Actiivity 生命周期，如下图所示: onCreate　　onStart (onRestarted)　　onResume　　onPaused(to onResume(User navigates to the activity))　　onStop　　onDestroy

onCreate 方法： 一定要实现OnCreate方法，初始化 组件和布局。

onPause 方法：用户离开activity是的第一个被触发的方法。

onDestroy 方法： 系统也许直接kill了process，这个方法未必会被调用。

An activity can exist in essentially three states:

Resumed

The activity is in the foreground of the screen and has user focus. (This state is also sometimes referred to as "running".)

Paused

Another activity is in the foreground and has focus, but this one is still visible. That is, another activity is visible on top of this one and that activity is partially transparent or doesn't cover the entire screen. A paused activity is completely alive (the Activity object is retained in memory, it maintains all state and member information, and remains attached to the window manager), but can be killed by the system in extremely low memory situations.

Stopped

The activity is completely obscured by another activity (the activity is now in the "background"). A stopped activity is also still alive (the Activity object is retained in memory, it maintains all state and member information, but is not attached to the window manager). However, it is no longer visible to the user and it can be killed by the system when memory is needed elsewhere.

两个Activity跳转时的生命周期方法调用顺序， 例如 Activity A 跳转到 Activity B：

1. Activity A's onPause() method executes.   　　//先执行A的onPause方法
   
2. Activity B's onCreate(), onStart(), and onResume() methods execute in sequence. (Activity B now has user focus.)  //Activity B's onCreate、onStart、onResume方法
   
3. Then, if Activity A is no longer visible on screen, its onStop() method executes. // 执行A的onStop方法
   

综上，如果有数据交换、交互，请在A的onPause里面执行，否则B启动了也找不到新数据。