android onSaveInstanceState调用时机详细总结

首先给出onSaveInstanceState会调用的情况：

1、当用户按下HOME键时。

2、长按HOME键，选择运行其他的程序时（即进入后台管理界面）。

3、按下电源按键（关闭屏幕显示）时。

4、从activity A中启动一个新的activity时。

5、屏幕方向切换时，例如从竖屏切换到横屏时。

下面总结下：
1、看下官方解释

/**
     * Called to retrieve per-instance state from an activity before being killed
     * so that the state can be restored in {@link #onCreate} or
     * {@link #onRestoreInstanceState} (the {@link Bundle} populated by this method
     * will be passed to both).
     *
     * 
This method is called before an activity may be killed so that when it
     * comes back some time in the future it can restore its state.  For example,
     * if activity B is launched in front of activity A, and at some point activity
     * A is killed to reclaim resources, activity A will have a chance to save the
     * current state of its user interface via this method so that when the user
     * returns to activity A, the state of the user interface can be restored
     * via {@link #onCreate} or {@link #onRestoreInstanceState}.
     *
     * 
Do not confuse this method with activity lifecycle callbacks such as
     * {@link #onPause}, which is always called when an activity is being placed
     * in the background or on its way to destruction, or {@link #onStop} which
     * is called before destruction.  One example of when {@link #onPause} and
     * {@link #onStop} is called and not this method is when a user navigates back
     * from activity B to activity A: there is no need to call {@link #onSaveInstanceState}
     * on B because that particular instance will never be restored, so the
     * system avoids calling it.  An example when {@link #onPause} is called and
     * not {@link #onSaveInstanceState} is when activity B is launched in front of activity A:
     * the system may avoid calling {@link #onSaveInstanceState} on activity A if it isn't
     * killed during the lifetime of B since the state of the user interface of
     * A will stay intact.
     *
     * 
If called, this method will occur before {@link #onStop}.  There are
     * no guarantees about whether it will occur before or after {@link #onPause}.
     */

从中可以得知：

• 该方法是在Activity被杀之前调用以便之后恢复状态
• 举例：如果activity B启用后位于activity A的之前，当activity A因为系统回收资源的问题要被杀掉，A通过onSaveInstanceState将有机会保存其用户界面状态，使得将来用户返回到activity A时能通过onCreate(Bundle)或者onRestoreInstanceState(Bundle)恢复界面的状态。
• 要区分开onPause（Activity切到后台或者销毁时必定调用）和onStop（Activity销毁时必定调用）举例：会调用onPause()和onStop()，但不调用onSaveInstanceState的例子是当用户从Activity B返回到Activity A时：没有必要调用B的onSaveInstanceState(Bundle)，因为B实例永远不会被恢复，因此系统会避免调用它。调用onPause()但不调用onSaveInstanceState的例子是当Activity B启动并处在activity A的前端：如果在B的整个生命周期里A的用户界面状态都没有被破坏的话，系统是不会调用Activity A的onSaveInstanceState(Bundle)的。
• onSaveInstanceState的调用时机不能保证是在onPause之前还是之后，但是一定会在onStop()前被触发

2、总结来说就是onSaveInstanceState会在未经用户主动操作而导致Activity 销毁（亦可理解为Activity异常的销毁）的情况下系统会调用该方法以便保存状态在将来恢复 。此处的主动操作是指主动调用finish()方法销毁activity或者按下返回键退出