Unexpected behaviour creating DialogFragment instance

Question

I have this android.app.DialogFragment:

public class MyDialog extends DialogFragment {

   private Callback callback;

    public static MyDialog newInstance() {
      return new MyDialog();
    }

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        AlertDialog dialog = new AlertDialog.Builder(getActivity())
            //set more things
            .setPositiveButton("Ok", new DialogInterface.OnClickListener(){
                callback.run(); //callback is null        
            }).create();
        return dialog;
    }

    public void setCallback(Callback callback) { this.callback = callback; }

}

and from my activity:

MyDialog dialog = MyDialog.newInstance();
dialog.setCallback(myCallback);
dialog.show(getFragmentManager(), "dialog");

But when I click "Ok" button, it crashes beacuse callback is null. myCallback is never null

I found something. onCreateDialog is called twice and the second time callback is null:

  @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Log.d("test", "onCreateDialog (" + this + ") -> " + callback);
        //more code
    }

It prints:

onCreateDialog (MyDialog{18ea188e}) -> com.house.hehe.MyActivity$3@3afbe8af

onCreateDialog (MyDialog{22199eaa}) -> null

How is it possible? Why are there two different instances? Why is onCreateDialog called twice?

Answer 1

I don't know what is happening but I have solved by doing callback an static static:

private static Callback callback;

public static MyDialog newInstance(Callback callback) {
    MyDialog.callback = callback;      
    return new MyDialog();
}

It is an ugly solution... oh well...