Android Dialogs HOWTO

I read almost everything from Android dev site here, and you should too. Here I summarize some practices that I find reasonable for my project.

Make it a separate class

I need multiple dialogs in the application, most of them contain some text description, and either one “OK” button, or two buttons (“YES” or “NO”). If you have multiple similar dialogs like I do, you should make a generic class extend DialogFragment, and make different dialogs based on it. Otherwise, you have to define a lot of dialogs and lots of duplicate code.

A class for dialog would look like this:

public class TwoButtonDialogFragment extends DialogFragment {
    ....
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        AlertDialog.Builder builder = 
			new AlertDialog.Builder(getActivity());
	    builder.setMessage(...)
			.setTitle(...)
			.setIcon(...)
			.setPositiveButton(...)
        return builder.create();
    }
}

The onCreateDialog is responsible for making the dialog.

A constructor of the class

To customize the dialog (e.g. different messages, title, etc) one needs to pass arguments into the constructor of the class. DialogFragment class supports static constructor:

public static TwoButtonDialogFragment newInstance(Account account, String message) {
	TwoButtonDialogFragment f = new TwoButtonDialogFragment();

	// Supply account_type argument
	Bundle args = new Bundle();
	args.putString("message", message);
	f.setArguments(args);
	return f;
}

And in onCreateDialog call getArguments().getString("message"); to retreive the argument you put in.

Note now you have create the dialog like this:

TwoButtonDialogFragment accountinfodialog = 
	TwoButtonDialogFragment.newInstance(account.getDescription());

And show it:

String dialog_tag = "accountinfodialog";
FragmentManager fm = getSupportFragmentManager();
accountinfodialog.show(fm, dialog_tag);

Now you can create different dialogs at different places as you like. The dialog_tag is very important as it helps you identify different dialogs if for example you have created several in one activity.

Pass onClick event back to activity

I need the activity which starts the dialog to do stuff after people click buttons on the dialog, therefore it would be convenient if we could directly implement the onClick function in the activity, to let activity do their stuff.

To have such functon, the DialogFragment should have a public interface for the onCLickListener it has. Then other acitivies just have to implement the interface and its functions.

A public interface for two button dialog could look like this:

public interface TwoButtonDialogListener {
    public void onDialogPositiveClick(DialogFragment dialog);
    public void onDialogNegativeClick(DialogFragment dialog);
}

Also add onAttach to the dialog class:

// Use this instance of the interface to deliver action events
TwoButtonDialogListener mListener;

// Override the Fragment.onAttach() method to 
// instantiate the TwoButtonDialogListener
@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    // Verify that the host activity implements the callback interface
    try {
        // Instantiate the TwoButtonDialogListener 
		// so we can send events to the host
        mListener = (TwoButtonDialogListener) activity;
    } catch (ClassCastException e) {
        // The activity doesn't implement the interface, throw exception
        throw new ClassCastException(activity.toString()
                + " must implement TwoButtonDialogListener");
    }
}

In the onClick function of onCreateDialog, then you just add mListener.onDialogPositiveClick(TwoButtonDialogFragment.this);, so that the class know the onClick function will be implemented by whoever implements the interface.

Now in other activities, they need to add implements TwoButtonDialogListener at the class definition, and implements the two functions, if it is for the TwoButtonDialogListener interface.