Android之Activity的onSaveInstanceState和onRestoreInstanceState

0.先看看官方文档是怎么描述的

protected voidonSaveInstanceState(BundleoutState)

Called to retrieve per-instance state from an activity before being killed so that the state can be restored in onCreate(Bundle) or onRestoreInstanceState(Bundle) (the 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 onCreate(Bundle) or onRestoreInstanceState(Bundle).

在activity被杀掉之前调用保存每个实例的状态,以保证该状态可以在onCreate(Bundle)或者onRestoreInstanceState(Bundle) (传入的Bundle参数是由onSaveInstanceState封装好的)中恢复。这个方法在一个activity被杀死前调用，当该activity在将来某个时刻回来时可以恢复其先前状态。例如，如果activity B启用后位于activity A的前端，在某个时刻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。

Do not confuse this method with activity lifecycle callbacks such as onPause(), which is always called when an activity is being placed in the background or on its way to destruction, or onStop() which is called before destruction. One example of when onPause() and 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 onSaveInstanceState(Bundle) on B because that particular instance will never be restored, so the system avoids calling it. An example when onPause() is called and not onSaveInstanceState(Bundle) is when activity B is launched in front of activity A: the system may avoid calling onSaveInstanceState(Bundle) 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.
不要将这个方法和activity生命周期回调如onPause()或onStop()搞混淆了，onPause()在activtiy被放置到背景或者自行销毁时总会被调用，onStop()在activity被销毁时被调用。一个会调用onPause()和onStop()，但不触发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)，此时的B实例永远不会被恢复，因此系统会避免调用它。一个调用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。

The default implementation takes care of most of the UI per-instance state for you by calling onSaveInstanceState() on each view in the hierarchy that has an id, and by saving the id of the currently focused view (all of which is restored by the default implementation of onRestoreInstanceState(Bundle)). If you override this method to save additional information not captured by each individual view, you will likely want to call through to the default implementation, otherwise be prepared to save all of the state of each view yourself. If called, this method will occur before onStop(). There are no guarantees about whether it will occur before or after onPause().

默认的实现负责了大部分UI实例状态(的保存)，采用的方式是调用UI层上每个拥有id的view的onSaveInstanceState() ，并且保存当前获得焦点的view的id(所有保存的状态信息都会在默认的onRestoreInstanceState(Bundle)实现中恢复)。如果你覆写这个方法来保存额外的没有被各个view保存的信息，你可能想要在默认实现过程中调用或者自己保存每个视图的所有状态。如果被调用，这个方法会在onStop()前被触发，但系统并不保证是否在onPause()之前或者之后触发。

protected voidonRestoreInstanceState(BundlesavedInstanceState)

This method is called afteronStart()when the activity is being re-initialized from a previously saved state, given here insavedInstanceState. Most implementations will simply useonCreate(Bundle)to restore their state, but it is sometimes convenient to do it here after all of the initialization has been done or to allow subclasses to decide whether to use your default implementation. The default implementation of this method performs a restore of any view state that had previously been frozen byonSaveInstanceState(Bundle).

This method is called betweenonStart()andonPostCreate(Bundle).

当Activity从之前的保存状态被重新初始化会调用onRestoreInstanceState这个方法，大多数的实现会仅仅简单的使用onCreate来恢复他们的状态，但是有时候在所有的初始化已经完成或者允许子类来决定是否使用默认的实现的情况下会比较方便，默认实现这个方法来执行任何一个被onSaveInstanceState冻结的状态。

这个方法在onStart与onPostCreate之间调用。

1.下面是引用别人的观点：

当某个activity（假定为activity A）显示在当前task的最上层时，其onSaveInstanceState方法会在什么时候被执行，有这么几种情况：
1、当用户按下HOME键时。这是显而易见的，系统不知道你按下HOME后要运行多少其他的程序，自然也不知道activity A是否会被销毁，故系统会调用onSaveInstanceState，让用户有机会保存某些非永久性的数据。以下几种情况的分析都遵循该原则
2、长按HOME键，选择运行其他的程序时。
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。在屏幕切换之前，系统会销毁activity A，在屏幕切换之后系统又会自动地创建activity A，所以onSaveInstanceState一定会被执行

总而言之，onSaveInstanceState的调用遵循一个重要原则，即当系统“未经你许可”时销毁了你的activity，则 onSaveInstanceState会被系统调用，这是系统的责任，因为它必须要提供一个机会让你保存你的数据（当然你不保存那就随便你了）。
至于onRestoreInstanceState方法，需要注意的是，onSaveInstanceState方法和 onRestoreInstanceState方法“不一定”是成对的被调用的，onRestoreInstanceState被调用的前提 是，activity A“确实”被系统销毁了，而如果仅仅是停留在有这种可能性的情况下，则该方法不会被调用，例如，当正在显示activity A的时候，用户按下HOME键回到主界面，然后用户紧接着又返回到activity A，这种情况下activity A一般不会因为内存的原因被系统销毁，故activity A的onRestoreInstanceState方法不会被执行
另外，onRestoreInstanceState的bundle参数也会传递到onCreate方法中，你也可以选择在onCreate方法中做数据还原

2.总结一下

Activity的 onSaveInstanceState() 和 onRestoreInstanceState()并不是生命周期方法，它们不同于 onCreate()、onPause()等生命周期方法，它们并不一定会被触发。当应用遇到意外情况（如：内存不足、用户直接按Home键）由系统销毁一个Activity时，onSaveInstanceState()才会被调用。但是当用户主动去销毁一个Activity时，例如在应用中按返回键，onSaveInstanceState()就不会被调用。因为在这种情况下，用户的行为决定了不需要保存Activity的状态。通常onSaveInstanceState()只适合用于保存一些临时性的状态，而onPause()适合用于数据的持久化保存。

另外，当屏幕的方向发生了改变， Activity会被摧毁并且被重新创建，如果你想在Activity被摧毁前缓存一些数据，并且在Activity被重新创建后恢复缓存的数据。可以重写Activity的 onSaveInstanceState() 和 onRestoreInstanceState()方法，如下：

public class PreferencesActivity extends Activity { private String name; protected void onRestoreInstanceState(Bundle savedInstanceState) { name = savedInstanceState.getString("name"); //被重新创建后恢复缓存的数据 super.onRestoreInstanceState(savedInstanceState); } protected void onSaveInstanceState(Bundle outState) { outState.putString("name", "liming");//被摧毁前缓存一些数据 super.onSaveInstanceState(outState); } }