onSaveInstanceState 一些知识

先来看下官方文档对于onSaveInstanceState的解释

the method onSaveInstanceState(Bundle) is called before placing the activity in such a background state, allowing you to save away any dynamic instance state in your activity into the given Bundle, to be later received in onCreate(Bundle) if the activity needs to be re-created.

大概意思就是如果 内存不够了，那么系统就会强制杀死这个Activity，但是会调用onSaveInstanceState方法来保存Activity的状态。

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。 

 默认的实现负责了大部分UI实例状态(的保存)，比如文本信息，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)。 

有这么几种情况，会调用onSaveInstanceState(Bundle)： 

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。

2、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。

6、键盘弹出（这个不确定）。

可如果用户主动去销毁一个Activity（back），那么，说明了用户不需要保存这个Activity的状态，那么那么onSaveInstanceState(Bundle)不会被调用，所以onSaveInstanceState(Bundle)只适合保存一些临时的状态，比如说文本输入。所以最好还是在OnPause中去保存永久性的一些数据。

但

Note that it is important to save persistent data in onPause() instead of onSaveInstanceState(Bundle) because the latter is not part of the lifecycle callbacks, so will not be called in every situation as described in its documentation.

官方建议是需要持久化数据的操作应该放在onPause里面，因为onSaveInstanceState(Bundle)并不是生命周期的一部分，有时候可能没有被调用。

写这篇文章的原因是因为遇到了这样的一个问题：

java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState

上网查了以后，好像是在切换fragment的时候，系统因为未知原因把这个呈现fragment的Activity给销毁了。

如果在保存完状态后再给Activity添加Fragment就会出错，一种解决方法是把commit换成commitAllowingStateLoss。

在stackoverflow上，有个回答：

Please check my answer here. Basically I just had to :

@Overrideprotected void onSaveInstanceState(Bundle outState) {    //No call for super(). Bug on API Level > 11.}

don’t make the call to super() on the saveInstanceState method. This was messing things up…

EDIT: after some more research, this is a know bug in the support package.

If you need to save the instance, and add something to your outState Bundle you can use the following :

@Overrideprotected void onSaveInstanceState(Bundle outState) {    outState.putString("WORKAROUND_FOR_BUG_19917_KEY", "WORKAROUND_FOR_BUG_19917_VALUE");    super.onSaveInstanceState(outState);}

EDIT2: in the end the proper solution was (as seen in the comments) to use :

transaction.commitAllowingStateLoss();

when adding or performing the FragmentTransaction that was causing the Exception.