问题
I\'m trying to save and restore the state of an Activity using the methods onSaveInstanceState() and onRestoreInstanceState().
The problem is that it never enters the onRestoreInstanceState() method. Can anyone explain to me why this is?
回答1:
Usually you restore your state in onCreate(). It is possible to restore it in onRestoreInstanceState() as well, but not very common. (onRestoreInstanceState() is called after onStart(), whereas onCreate() is called before onStart().
Use the put methods to store values in onSaveInstanceState():
protected void onSaveInstanceState(Bundle icicle) {
super.onSaveInstanceState(icicle);
icicle.putLong("param", value);
}
And restore the values in onCreate():
public void onCreate(Bundle icicle) {
if (icicle != null){
value = icicle.getLong("param");
}
}
You do not have to store view states, as they are stored automatically by calling super.onSaveInstanceState(icicle);.
回答2:
onRestoreInstanceState() is called only when recreating activity after it was killed by the OS. Such situation happen when:
- orientation of the device changes (your activity is destroyed and recreated)
- there is another activity in front of yours and at some point the OS kills your activity in order to free memory (for example). Next time when you start your activity
onRestoreInstanceState()will be called.
In contrast: if you are in your activity and you hit Back button on the device, your activity is finish()ed (i.e. think of it as exiting desktop application) and next time you start your app it is started "fresh", i.e. without saved state because you intentionally exited it when you hit Back.
Other source of confusion is that when an app loses focus to another app onSaveInstanceState() is called but when you navigate back to your app onRestoreInstanceState() may not be called. This is the case described in the original question, i.e. if your activity was NOT killed during the period when other activity was in front onRestoreInstanceState() will NOT be called because your activity is pretty much "alive".
All in all, as stated in the documentation for onRestoreInstanceState():
Most implementations will simply use onCreate(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 by onSaveInstanceState(Bundle).
As I read it: There is no reason to override onRestoreInstanceState() unless you are subclassing Activity and it is expected that someone will subclass your subclass.
回答3:
The state you save at onSaveInstanceState() is later available at onCreate() method invocation. So use onCreate (and its Bundle parameter) to restore state of your activity.
回答4:
As a workaround, you could store a bundle with the data you want to maintain in the Intent you use to start activity A.
Intent intent = new Intent(this, ActivityA.class);
intent.putExtra("bundle", theBundledData);
startActivity(intent);
Activity A would have to pass this back to Activity B. You would retrieve the intent in Activity B's onCreate method.
Intent intent = getIntent();
Bundle intentBundle;
if (intent != null)
intentBundle = intent.getBundleExtra("bundle");
// Do something with the data.
Another idea is to create a repository class to store activity state and have each of your activities reference that class (possible using a singleton structure.) Though, doing so is probably more trouble than it's worth.
回答5:
The main thing is that if you don't store in onSaveInstanceState() then onRestoreInstanceState() will not be called. This is the main difference between restoreInstanceState() and onCreate(). Make sure you really store something. Most likely this is your problem.
回答6:
I found that onSaveInstanceState is always called when another Activity comes to the foreground. And so is onStop.
However, onRestoreInstanceState was called only when onCreate and onStart were also called. And, onCreate and onStart were NOT always called.
So it seems like Android doesn't always delete the state information even if the Activity moves to the background. However, it calls the lifecycle methods to save state just to be safe. Thus, if the state is not deleted, then Android doesn't call the lifecycle methods to restore state as they are not needed.
Figure 2 describes this.
回答7:
I think this thread was quite old. I just mention another case, that onSaveInstanceState() will also be called, is when you call Activity.moveTaskToBack(boolean nonRootActivity).
回答8:
If you are handling activity's orientation changes with android:configChanges="orientation|screenSize" and onConfigurationChanged(Configuration newConfig), onRestoreInstanceState() will not be called.
回答9:
It is not necessary that onRestoreInstanceState will always be called after onSaveInstanceState.
Note that : onRestoreInstanceState will always be called, when activity is rotated (when orientation is not handled) or open your activity and then open other apps so that your activity instance is cleared from memory by OS.
回答10:
In my case, onRestoreInstanceState was called when the activity was reconstructed after changing the device orientation. onCreate(Bundle) was called first, but the bundle didn't have the key/values I set with onSaveInstanceState(Bundle).
Right after, onRestoreInstanceState(Bundle) was called with a bundle that had the correct key/values.
回答11:
I just ran into this and was noticing that the documentation had my answer:
"This function will never be called with a null state."
https://developer.android.com/reference/android/view/View.html#onRestoreInstanceState(android.os.Parcelable)
In my case, I was wondering why the onRestoreInstanceState wasn't being called on initial instantiation. This also means that if you don't store anything, it'll not be called when you go to reconstruct your view.
回答12:
I can do like that (sorry it's c# not java but it's not a problem...) :
private int iValue = 1234567890;
function void MyTest()
{
Intent oIntent = new Intent (this, typeof(Camera2Activity));
Bundle oBundle = new Bundle();
oBundle.PutInt("MYVALUE", iValue); //=> 1234567890
oIntent.PutExtras (oBundle);
iRequestCode = 1111;
StartActivityForResult (oIntent, 1111);
}
AND IN YOUR ACTIVITY FOR RESULT
private int iValue = 0;
protected override void OnCreate(Bundle bundle)
{
Bundle oBundle = Intent.Extras;
if (oBundle != null)
{
iValue = oBundle.GetInt("MYVALUE", 0);
//=>1234567890
}
}
private void FinishActivity(bool bResult)
{
Intent oIntent = new Intent();
Bundle oBundle = new Bundle();
oBundle.PutInt("MYVALUE", iValue);//=>1234567890
oIntent.PutExtras(oBundle);
if (bResult)
{
SetResult (Result.Ok, oIntent);
}
else
SetResult(Result.Canceled, oIntent);
GC.Collect();
Finish();
}
FINALLY
protected override void OnActivityResult(int iRequestCode, Android.App.Result oResultCode, Intent oIntent)
{
base.OnActivityResult (iRequestCode, oResultCode, oIntent);
iValue = oIntent.Extras.GetInt("MYVALUE", -1); //=> 1234567890
}
来源：https://stackoverflow.com/questions/4096169/onsaveinstancestate-and-onrestoreinstancestate