Question

I would like to know if the following code would be a good pattern to use Dialogs in Android, following the guidelines of dialogs (developer.android) and promoting encapsulation.

The example shows a dialog for choosing an option. The key point is the ChooseLevel class
that only needs to be modified at the point marked HERE to add constants for the options being presented to the user.

package org.dialogs;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Bundle;
import android.widget.Toast;
import org.dialogs.ChooseLevel.Level; // see below

// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
public class MainActivity 
    extends Activity 
    implements ChooseLevel.Listener
{

    // ...........................................................................
    @Override
    public void onCreate(Bundle icicle) {

        super.onCreate(icicle);

        showDialog(ChooseLevel.ID);

    } // ()

    // ...........................................................................
    @Override
    protected Dialog onCreateDialog(int dialogId) {

        Dialog dialog;

        if (dialogId == ChooseLevel.ID) {
            dialog = new ChooseLevel (this, this).getTheDialog();
        }

        return dialog;

    }

    // ...........................................................................
    public void levelChosen(Level whatLevel) {
        Toast.makeText(this, "level = " + whatLevel.toString(), Toast.LENGTH_LONG).show();
    }

} //

// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
class ChooseLevel 
{
    // ...........................................................................
    public static final int ID = ChooseLevel.class.hashCode();

    // ...........................................................................
    public interface Listener {
        void levelChosen(Level whatLevel);
    }

    // ...........................................................................
    public enum Level {

        Easy, Medium, Expert; // HERE: write constants names for the options

        public static final String[] names;

        static {
            Level[] vals = Level.values();

            names = new String[vals.length];
            for (int i=0; i<vals.length; i++) {
                names[i] = vals[i].toString();
            }
        } // static initializer

    }

    // ...........................................................................
    private Listener theListener;

    // ...........................................................................
    private AlertDialog theDialog = null;
    public AlertDialog getTheDialog () { return this.theDialog; }

    // ...........................................................................
    public ChooseLevel (Context ctx, Listener li) {

        theListener = li;

        AlertDialog.Builder builder = new AlertDialog.Builder(ctx);

        builder.setTitle("choose a level");

        builder.setSingleChoiceItems(Level.names, -1, new DialogInterface.OnClickListener() {

            public void onClick(DialogInterface dialog, int numChosen) {
                dialog.dismiss();
                Level theLevel = Level.valueOf(Level.names[numChosen]);
                if (theListener != null) {
                    theListener.levelChosen(theLevel);
                }
            }
        });

        theDialog = builder.create();

    } // ()
} // class

Answer 1

I’ve found a much easier way of displaying dialogs for choosing options:

the user code would be:

import android.app.Activity;
import android.app.Dialog;
import android.os.Bundle;
import android.widget.Toast;

// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
public class MainActivity 
    extends Activity 
    implements Chooser.Listener
{
    // .....................................................................
    private Chooser levelChooser;

    // .....................................................................
    enum Options { one, two, tree, four};

    // .....................................................................
    // .....................................................................
    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);

        levelChooser = new Chooser (Options.class, "choose an option", this, this);

        showDialog(levelChooser.ID);
    } // ()

    // .....................................................................
    // .....................................................................
    @Override
    protected Dialog onCreateDialog(int dialogId) {
        Dialog aDialog = null;

        if (dialogId == levelChooser.ID) {
            aDialog = levelChooser.getTheDialog();
        }
        return aDialog;
    }

    // .....................................................................
    // .....................................................................
    public <T extends Enum<T>> void onChoiceMade(T theChoice) {
        Toast.makeText(this, "choice = " + theChoice, Toast.LENGTH_LONG).show();
    }
} // class

You only have to change the constants in enum Options above.
All the tricks are done in the Chooser class, that needs no modification:

import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import java.lang.reflect.Method;

public class Chooser {

    // .....................................................................
    // .....................................................................
    public static <T extends Enum<T>> String[] getNames(Class<T> c) {
        // returns the values of *any* enum in an array of String
        String[] values;
        try {
            Method m1 = c.getMethod("values");
            Object[] valuesObj;
            valuesObj = (Object[]) m1.invoke(c);
            values = new String[valuesObj.length];
            for (int i = 0; i < valuesObj.length; i++) {
                values[i] = valuesObj[i].toString();
            }
        } catch (Exception ex) {
            values = null;
        }
        return values;
    } // ()

    // .....................................................................
    // .....................................................................
    public interface Listener {
        public <T extends Enum<T>> void onChoiceMade(T theChoice);
    }

    // .....................................................................
    // .....................................................................
    public final int ID;
    private Listener theListener;
    private AlertDialog.Builder theBuilder;
    private AlertDialog theDialog = null;
    private final String[] names;

    // .....................................................................
    // .....................................................................
    public final AlertDialog getTheDialog() {
        if (this.theDialog == null) {
            this.theDialog = this.theBuilder.create();
        }
        return this.theDialog;
    }

    // .....................................................................
    // .....................................................................
    public <T extends Enum<T>> Chooser(final Class<T> theEnum, String theTitle, Context ctx, Listener listener) {

        this.ID = theEnum.hashCode();
        this.names = getNames(theEnum);
        this.theListener = listener;
        this.theBuilder = new AlertDialog.Builder(ctx);
        this.theBuilder.setTitle(theTitle);

        this.theBuilder.setSingleChoiceItems(names, -1, new DialogInterface.OnClickListener() {

            public void onClick(DialogInterface dialog, int electedNumber) {
                dialog.dismiss();
                Enum.valueOf(theEnum, names[electedNumber]);

                T theOption = (T) Enum.valueOf(theEnum, names[electedNumber]);
                if (theListener != null) {
                    theListener.onChoiceMade(theOption);
                }
            }
        });

    } // ()
} // class