onSaveInstanceState and onRestoreInstanceState

Question

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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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       gt  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               gt 1234567890          private void FinishActivity bool bResult        Intent oIntent   new Intent        Bundle oBundle   new Bundle        oBundle PutInt  MYVALUE   iValue      gt 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       gt  1234567890

User · Answer

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

User · Answer

If you are handling activity s orientation changes with android configChanges  orientation screenSize  and onConfigurationChanged Configuration newConfig   onRestoreInstanceState   will not be called

User · Answer

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

User · Answer

From the documentation Restore activity UI state using saved instance state it is stated as   Instead of restoring the state during onCreate   you may choose to implement onRestoreInstanceState    which the system calls after the onStart   method  The system calls onRestoreInstanceState   only if there is a saved state to restore  so you do not need to check whether the Bundle is null     IMO  this is more clear way than checking this at onCreate  and better fits with single responsiblity principle

User · Answer

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

[android] onSaveInstanceState () and onRestoreInstanceState ()

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