Le blog de Fabien DUMINY

Dialog box under Android

19 April, 2012 (09:00) | Android | By: fabien

With Java / Swing, each showXxxDialog method in the JOptionPane class creates a modal dialog box waiting for a user action. It’s modal because the user can’t do any action outside this one. Let’s take the following example :

int response = JOptionPane.showConfirmDialog(null,
                                "Do you want to delete this file ?",
                                "Confirmation",
                                JOptionPane.YES_OPTION);
if (response == JOptionPane.YES_NO_OPTION) {
    // ... action if response = Yes
}

We can see the call is synchronous, which means the method returns a value that allow to process the response immediately.

Under Android, dialog boxes are also modals. But the processing of the response must be done asynchronously.
So, one must use a different programming strategy, based on callback. In the example of a Yes/No dialog box, a callback for the Yes answer is needed and eventually another one for the No answer. The code for Android looks like this :

Activity activity = .....; // find the current Activity
AlertDialog.Builder builder = new AlertDialog.Builder(activity);
builder.setMessage("Do you want to delete this file ?");

// definition of the callback for the Yes answer
builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
    @Override
    public void onClick(DialogInterface dialog, int which) {
        // ... action if answer = Yes
    }
});

builder.show(); // doesn't return user's choice
// after this line, the user may not have already answered
// to the question (because the call is asynchronous)

In order to make the code a bit clearer, I create an abstract class called Command :

abstract public class Command implements DialogInterface.OnClickListener {
    @Override
    public final void onClick(DialogInterface dialog, int which) {
        execute();
        dialog.dismiss();
    }

    abstract public void execute();
}

I also write the following method :

public void displayYesNoDialog(Activity context, int messageId,
                                               Command yesCommand) {
    AlertDialog.Builder builder = new AlertDialog.Builder(context);
    builder.setMessage(context.getString(messageId));
    builder.setPositiveButton("Yes", yesCommand);
    builder.show();
}

I use it like that :

Activity activity = .....; // find the current Activity
int messageId = .....; // find the message identifier
Command yesCommand = new Command() {
    public void execute() {
    // ... action if answer = Yes
}

displayYesNoDialog(activity, messageId, yesCommand);

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