Calls to onsaveinstancestate and onrestoreinstancestate methods

Only when Activity When it ends in an abnormal state , The system will call onSaveInstance Method , Used to save the current Activity The state of , So that the subsequent reconstruction is abnormally ended Activity Called when the onRestoreInstatnceState Methods to restore the scene .

onSaveInstance The method is only in Activity By End of exception Called when , And it is in onStop Before .onSaveInstance Method will save the data in Bundle In the object , When it ends abnormally Activity After being rebuilt , The system will call onRestoreInstanceState Method , And put this Bundle Objects are also passed to onRestoreInstanceState and onCreate Method . therefore onRestoreInstanceState Method can be used to recovery Activity The state of .onRestoreInstanceState Method is called at onStart after .

that , say concretely onSaveInstance When will the method be called ？

I also mentioned above onSaveInstance The method is in onstop Before execution , That is, it will be called when the user switches to the desktop or other applications .onSaveInstance The method is only in Activity Called when the exception ends , In fact, when your application may end abnormally , This method will be called .

The following are all cases where the application may end abnormally , At this time, the system will call onSaveInstance：

1、 User presses home Key to switch to the desktop

2、 Switch to another application

3、 Press the power button / Breath screen

4、 Horizontal and vertical screen switching

Life cycle of horizontal and vertical screen switching ：onPause()->onSaveInstanceState()->onStop()->onDestroy()->onCreate()->onStart()->onRestoreInstanceState->onResume()

It can be seen that the horizontal and vertical screen switching experience Activity Destruction and reconstruction of . If we don't want horizontal and vertical screens to cause Activity Destruction and reconstruction of , We can do that ：

stay AndroidManifest Of documents Activity Specify the following properties in ：

android:configChanges = "orientation| screenSize"

In this case, when the application switches between horizontal and vertical screens , You don't go through Activity Destroyed and rebuilt , Instead, call back the following methods ：

@Override
public void onConfigurationChanged(Configuration newConfig){
    super.onConfigurationChanged(newConfig);
}

onRestoreInstatnceState Method is used for Activity It is called to restore the scene when it is created again after the exception . But here's the thing ,onRestoreInstanceState The method is not necessarily and onSaveInstance Methods appear in pairs ,onSaveInstance The method is to judge in the system Activity It may be called when it ends abnormally , and onRestoreInstance The method is to have Activity It can only be called when it is really destroyed by the exception and rebuilt . For example, the user first Activity a Switch to Activity b, Then immediately return to Activity a, Because from a Switch to b When , The system judges that it may be destroyed abnormally , So I call onSaveInstance Method , But because Activity Not really destroyed , So from Activity b Back to Activity a When ,Activity a Not invoke onRestoreInstance.onRestoreInstance The call to is based on Activity In fact, it was destroyed abnormally .

in addition , Because of the call onSaveInstanceState Method Bundle Objects will be passed to onCreate and onRestoreInstanceState Method , So it can also be in onCreate Method to recover data .onCreate and onRestoreInstanceState Method recovery Activity The difference is that ：onRestoreInstanceState Callback indicates Bundle Object is not empty , Don't add non empty judgment , and onCreate Medium requires non empty judgment , Therefore, it is generally recommended to use onRestoreInstanceState Method to recover data .