Robust way to pass value back from Dialog to Activity on Android?

Question

This question has come up several times and I've read all the answers, but I haven't seen a truly robust way to handle this. In my solution, I am using listeners from the calling Activity to the AlertDialog like so:

public class MyDialogFragment extends DialogFragment {

    public interface MyDialogFragmentListener {
        public void onReturnValue(String foo);
    }

    public void init(boolean someValue)
    {
        sSomeValue = someValue;
        listeners = new ArrayList<MyDialogFragmentListener>();
    }
    static boolean sSomeValue;
    private static ArrayList<MyDialogFragmentListener> listeners;

    public void addMyDialogFragmentListener(MyDialogFragmentListener l)
    {
        listeners.add(l);
    }

    public void removeMyDialogFragmentListener(MyDialogFragmentListener l)
    {
        listeners.remove(l);
    }

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        builder.setTitle(R.string.title)
           .setPositiveButton(R.string.ok, new DialogInterface.OnClickListener() {
               @Override
               public void onClick(DialogInterface dialog, int id) {
                   for (MyDialogFragmentListener listener : listeners) {
                       listener.onReturnValue("some value");
                   }
               }
           })
           .setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() {
               public void onClick(DialogInterface dialog, int id) {
                   // User cancelled the dialog
                   // Nothing to do but exit
               }
           });
        if (sSomeValue) {
            builder.setMessage(R.string.some_value_message);
        } else {
            builder.setMessage(R.string.not_some_value_message);
        }
        // Create the AlertDialog object and return it
        return builder.create();
    }
}

Then in the calling Activity, I instantiate the object normally, pass in any arguments through init and set my listener.

Here's the problem: when you rotate the device and change orientation while the dialog is open, both the Activity and MyDialogFragment objects get re-created. To ensure that the input values don't get screwed up, I am setting my initialized values as static. This feels hacky to me, but since there will only be one such dialog at a time, I am ok with it. Where the problem comes in is with the return value. The original listener will get called. That's fine because the object still exists, but if there is a requirement to update the UI on the Activity (which there is), it won't get updated because the new Activity instance is now controlling the UI.

One solution I am considering is casting getActivity() in the dialog class to my Activity and forcing the dialog itself to add a listener, rather than having the calling Activity do it. But this just feels like a snowballing of hacks.

What is the best practice for handling this gracefully?

Answer 1

You are on the right track, I follow the method recommended by the Android Developers - Using DialogFragments article.

You create your DialogFragment and define an interface that the Activity will implement, like you have done above with this:

public interface MyDialogFragmentListener {
    public void onReturnValue(String foo);
}

Then in the DialogFragment when you want to return the result to the Activity you cast the activity to the interface:

@Override
public void onClick(DialogInterface dialog, int id) {
    MyDialogFragmentListener activity = (MyDialogFragmentListener) getActivity();
    activity.onReturnValue("some value");
}

Then in the Activity you implement that interface and grab the value:

public class MyActivity implements MyDialogFragmentListener {
    ...
    @Override
    public void onReturnValue(String foo) {
        Log.i("onReturnValue", "Got value " + foo + " back from Dialog!");
    }
}

Answer 2

You can fire off an Intent from your Dialog's onClickListner which the Activity will be listening for.

Take a look at this tutorial on Broadcasting and Receiving Intents