Sunday, December 2, 2007

OSGi Services for Dynamic Applications (III)

Or: use OSGi to write a dueling card game, part 3

Adding Monster cards to the game and a simple UI

In the previous posting I've created the hardest bit of the game. The actual game controller. In this posting I'm going to create a small bundle that contains a few Monster cards and a simple UI bundle so that we can actually start playing.

Creating the Monster Deck bundle
Every card in my Dueling game is an OSGi Service.
In the Duel Bundle I used OSGi Declarative Services to activate the bundle and consume the services, and you can use DS also to register services, but in this case I'm going to use the plain OSGi API's for this. Registering a service is so simple in OSGi that in this case I'm just going to go API.

My monster cards instances are realized by a class called MonsterCard, which implements the Card, Fighter and Command interfaces. It's nice and simple, also because it extends AbstractCommandCard, which means that Command.execute() for the Standby and Recover phase are implemented as placing the card on the table. Other basic Card requirements such as accessors to the name, ID and description and proper implementations of equals(), hashcode() and toString() are handled by AbstractCard, the superclass of AbstractCommandCard. My MonsterCard class basically adds the implementation of the Fighter interface into the mix. It also specifies when the card is enabled in conditions() - in the Standby phase and the Recover phase, and also in the Battle phase, but only when it's on the table.
public class MonsterCard extends AbstractCommandCard implements Fighter, Card, Command {
  private int attack;
  private int defense;

  public MonsterCard(String name, String desc,
      int att, int def) {
    super(CardType.MONSTER, name, desc);

    attack = att;
    defense = def;
  public void damage(Game game, int amount) {
    // nothing to do on damage
    // the battle code will discard when hit

  public int getAttack() {
    return attack;

  public int getDefense() {
    return defense;

  public Condition [] conditions() {
    return new Condition [] {
      new Condition(Phase.STANDBY),
      new Condition(Area.TABLE, Phase.BATTLE),
      new Condition(Phase.RECOVER)

  public boolean responding(Game game) {
    return true; // can respond to any attacker

To create any straight monster without any special abilities, I can simply use this class. I'm going to create 3 different monster cards, Super Hero, Floppie and Diamond Buster:
  new MonsterCard("Super Hero",
    "This is the invincible super hero.", 60, 35));
  new MonsterCard("Floppie",
    "Floppie joppie.", 15, 25));
  new MonsterCard("Diamond Buster",
    "Diamond buster is as strong as a rock.", 40, 55));
To turn these POJO's into OSGi services, I need to register them as services with the BundleContext.registerService(), defined as:
  registerService(java.lang.String clazz,
    java.lang.Object service,
    java.util.Dictionary properties)

Simply pass in the name of the class as the first argument, the actual object and optionally a map of properties. The properties can be used when selecting services as a service consumer, but I'm not using them here.

So to get 5 of each card registered when the MonsterDeck bundle is started, a small OSGi Activator class does it all:
package monsterdeck;

import game.model.Card;
import java.util.Hashtable;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;

public class Activator implements BundleActivator {
  public void start(BundleContext ctx) throws Exception {
    for (int i = 0; i < 5; i++) {
      registerCard(ctx, new MonsterCard("Super Hero",
          "This is the invincible super hero.", 60, 35));
      registerCard(ctx, new MonsterCard("Floppie",
          "Floppie joppie.", 15, 25));
      registerCard(ctx, new MonsterCard("Diamond Buster",
          "As strong as a rock.", 40, 55));

  private void registerCard(BundleContext ctx, Card c) {
    ctx.registerService(Card.class.getName(), c,
        new Hashtable());

  public void stop(BundleContext context)
      throws Exception { }

The only thing left to do it create the bundle-manifest in the META-INF/MANIFEST.MF file:
  Bundle-Name: MonsterDeck Bundle
  Bundle-SymbolicName: MonsterDeck
  Bundle-Version: 1.0.0
  Bundle-Activator: monsterdeck.Activator
  Import-Package: game.model,
  Export-Package: monsterdeck

I have to import the org.osgi.framework package since in my Activator I'm using the BundleActivator and BundleContext classes from this package. I'm exporting the monsterdeck package from this bundle so that I can use the MonsterCard class later in another bundle.

Adding a simple UI
I've tried to create the simplest UI possible. Its written using SWT and simply implements UserInterface. I just wanted to keep this part as simple as possible, but (as always) GUI classes tend to become relatively big. My GUI looks like this:

At the top you see the cards your opponent has on the table.
Below that is a status area where messages are displayed.
Then the cards you have on the table are shown and at the bottom are the cards that you have in your hand.
On the right hand side there is a status panel where each player's life points are shown, the current game phase is displayed and buttons are present to draw a card, end the current phase and take the damage of a fight.
Monster cards are shown red and spell cards are green. To select a card, you simply click on it. Generally only cards and actions that make sense in the current context are enabled. Attacking monsters have their names shown in italics and card descriptions can be read by hovering with the mouse over a card.

The UI bundles comes with a little factory that is launched as soon as the bundles is started. Type your name in the UI Factory and it will create a User Interface for you which is then registered as a UserInterface Service with OSGi.

You can get this UI implementation by checking it out the SimpleUI bundle from the usual SVN location:

Running the game
This is really easy if you're using Eclipse. What you should have is 4 OSGi Bundle Projects:

Select the Run -> Open Run Dialog... menu and double-click on the OSGi Framework item in the tree. This will create a new OSGi launch configuration for you. By default it will have all the OSGi bundles in the system selected, which is waaaay too much, so I normally deselect the root 'Target Platform' node and then click 'Add Required Bundles':

Hit 'Run' and you're off. The UI Controller will show up and after creating two UI's the game will start.
The game controller prints the names of the users as soon as they register their UI services and when two users are in, it will shuffle the cards and start the game. My console shows:
  Adding user interface for: Pinky
  Adding user interface for: Brain
  Shuffling deck...

I can Draw a card:

Play a monster (e.g. Super Hero) on the table:

and engage in battle: my opponent attacks me with Diamond Buster, I can choose to defend with Super Hero, but he will die, alternatively I can take the damage myself and lose life points.

Before I forget: closing all the windows will not actually exit the system. The OSGi container will keep running. The command to stop the OSGi container is implementation-specific, if you're using Equinox just type in exit in the console.

With only 3 different cards in the game it is a little bit boring to say the least. In the next posting I'll add some cool spell cards and a bunch more monsters too!


trisa said...

great blogposts! I really enjoyed reading it. I heard the Eclipse platform (and therefore Eclipse RCP, too) does not completely comply to this approach. I was told that there are still several service registry that are not dynamic yet. I did not test it yet though. Have you any experiences that deny or comply to that opinion?

davidb said...

Hi Trisa, Equinox itself definitely supports all the dynamics of OSGi Services, but I agree that there are probably still some components in the greater Eclipse platform that don't completely utilize the OSGi Service Dynamics. A clue towards this is that Eclipse still asks for a restart when you add or remove features. Have you tried posting this question on the eclipse.platform newsgroup? Cheers, David

trisa said...

Hi davidb, I already did some year ago and got no answer. I did several RCP apps but never tried this dynamic approach myself. Anyway, great posts, thanks!