Ytukyg

I have two activities, A and B. A starts B by calling startActivityForResult(). B has theme @android:style/Theme.Dialog.

Thus, B is displayed "on top of" A, but A is still visible (as B is a dialog).

When both activities are launched, I force recreation by switching to another task and back (I have enabled the "Don't keep activities" option in the android developer settings. I see OnCreate is called on A and B when I return to my task.)

When I click the button in Activity B, it calls setResult() and finish(), but onActivityResult() is not called on A.

The problem does not appear

• if I do not force recreation of the activities

or

• if I remove the dialog theme from Activity B.

I tested this on a Google Pixel with Android 9.

Is this expected behavior or a bug in Android?

This is the code I used to test this (Xamarin Android):

[Activity(Label = "@string/app_name")]

public class ActivityA : Activity

{



    protected override void OnCreate(Bundle savedInstanceState)

    {

        Kp2aLog.Log("OnCreate A");

        base.OnCreate(savedInstanceState);

        SetContentView(Resource.Layout.yubichall_test);



        {

            FindViewById<Button>(Resource.Id.btn_yubichall).Text = "Start B";

            FindViewById<Button>(Resource.Id.btn_yubichall).Click += (sender, args) =>

            {



                var chalIntent = new Intent(this, typeof(ActivityB));



                StartActivityForResult(chalIntent, 123);



            };

        }





    }



    protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)

    {

        base.OnActivityResult(requestCode, resultCode, data);

        Android.Util.Log.Debug("KP2A", "OnActivityResult A: " + requestCode);



    }

}

[Activity(Label = "@string/app_name", Theme = "@android:style/Theme.Dialog")]

public class ActivityB : Activity

{



    protected override void OnCreate(Bundle savedInstanceState)

    {

        Kp2aLog.Log("OnCreate B");

        base.OnCreate(savedInstanceState);

        SetContentView(Resource.Layout.yubichall_test);



        FindViewById<Button>(Resource.Id.btn_yubichall).Text = "Return result to A";



        {

            FindViewById<Button>(Resource.Id.btn_yubichall).Click += (sender, args) =>

            {

                SetResult(Result.Ok);

                Finish();



            };

        }





    }

}

where the layout yubicall_test is

    <?xml version="1.0" encoding="utf-8" ?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

              android:layout_width="fill_parent"

              android:orientation="vertical"

              android:layout_height="wrap_content">

  <Button android:id="@+id/btn_yubichall"

          android:layout_width="wrap_content"

          android:layout_height="wrap_content"

          android:text="yubi challenge"

  />



  <TextView

    android:id="@+id/text_result"

    android:layout_width="wrap_content"

    android:layout_height="wrap_content"

  />



</LinearLayout>

I will try to explain what is happening here. Don't keep activity is used to mimic low memory behaviour on Device, so that when activity goes into background state (onPause, onStop) and gets killed due to memory quench, it will save the required state. Most of the view states are saved by the Android OS itself and is responsible for restoring them when the activity comes back to foreground.

Now as per your scenario, the theme does not seem to be creating any problem in plain Vanilla android when launching activity B. It will pause and stop activity A as it is not a DialogActivity. It just has a theme of Dialog. You should verify this with Xamarin, if it treats Dialog Theme differently then this could be an issue.

Now, when you put Activity B into background (onPause, onStop), it will also get killed as you have Dont keep activity on. But restoring all activities (i.e. B and everything before that with correct View states), is job of Android OS. Even if you see Activity A getting created while restoring state, you will see all the states saved in the bundle getting restored(parameter of onCreate of ActivityA).

Another possibility of this happening could be device specific. As discussed earlier, it doesn't happen on Plain Android but manufacturers (Xiaomi, Samsung) do make modifications to Android OS and this could be very well be your issue. Its a good idea to first develop your app for plain Android and then customize your code for each manufacturer.