Java: Accessing resources and the Law Of Demeter

Question

Overview

In my (Android) Java game, I have a class called resources. As the name suggests, this class holds the resources for the game. All of my OpenGL objects (Sprites) are created here

It's looks something like the following (obviously, this is a simplified version compared to that which appears in the real project):

public class Resources {

    Hero hero;
    Enemy enemy;
    MenuButtons mainMenuButtons;
    Background background;
    Scene mainMenu;

    public void createObjects(){

        hero = new Hero();
        enemy = new Enemy();
        mainMenuButtons = new MenuButtons();
        background = new Background();
        mainMenu = new Scene(this);

    }
}

So, within my mainMenu scene, I need access my objects, so we may see something like this:

public class mainMenu implements Scene {

    Resources resources;

    public mainMenu(Resources resources){

        this.resources = resources;

    }

    @Override
    public void render(){

        resources.background.draw();
        resources.hero.draw();
        resources.enemy.draw();
        mainMenuButtons.draw();           

    }

    @Override
    public void updateLogic(){

        resources.hero.move();
        resources.enemy.move();
        resources.mainMenubuttons.animate();
    }

}

Now, the above method is just one way to get access to the objects in resources and their methods. But does this actually break the Law of Demeter? If not, why not? If so, what is the best way to get access to these objects in a way that does not violate the LOD?

Accessors?

One option (which I've ruled out TBH - see below) is placing accessor methods into my resources class. So that I could do something like:

resources.drawBackround();

I have a lot of objects and I need an accessor for each method/variable of each object. Not really practical, it seems like I'm writing a ton of extra code and most importantly, it makes the resources class ridiculously long as it becomes filled with these accessors. Therefore, I'm not going down this road.

Passing in objects into the scene's constructor

Of course, I can also do something like this:

    hero = new Hero();
    enemy = new Enemy();
    mainMenuButtons = new MenuButtons();
    background = new Background();
    mainMenu = new Scene(hero, enemy, mainMenuButtons, background);

So I can simply do this:

    background.draw(); //etc....

This is workable for simple scene's (such as menu systems that don't require a lot of objects) but for the main game, it could quickly become a mess as I'd have to pass references to some 30+ objects into the constructor which doesn't really sound quite right......

So I would really appreciate if someone could point out the best way to proceed and why.

Answer 1

So I would really appreciate if someone could point out the best way to proceed and why.

The best way, in my opinion, is to keep the Resources class, make all objects private to not break the law and write accessors (but not for every object like you already ruled out).

I have a lot of objects and I need an accessor for each method/variable of each object. Not really practical, it seems like I'm writing a ton of extra code and most importantly, it makes the resources class ridiculously long as it becomes filled with these accessors. Therefore, I'm not going down this road.

I assume many objects are of the same class. So you do not have to make an accessor for every object what would really blow up the class. I a game you normally have a hero, one or more enemies and many sprites.

public class Resources {

    private Hero hero;
    private Enemy enemy;
    private MenuButtons mainMenuButtons;
    private Background background;
    private Scene mainMenu;

    public void createObjects(){

        hero = new Hero();
        enemy = new Enemy();
        mainMenuButtons = new MenuButtons();
        background = new Background();
        mainMenu = new Scene(this);

    }

    public Hero getBackground() {

        return background;
    }

    public Hero getHero() {

        return hero;
    }

    public List<Enemy> getEnemies() {

        ArrayList<Enemy> list = new ArrayList<Enemy>();
        list.add(enemy);
        list.add(next_enemy);
        return list;
    }

    public List<Sprite> getSprites() {

        ArrayList<Sprite> list = new ArrayList<Sprite>();
        list.addAll(enemy.getActiveSprites());
        return list;
    }

}

Instead of getHero() and getEnemy() you could also make a getActor() method if Hero and Enemy are derived from the same class. The getSprites() method is just an example how it could look like.

If that solution is not going to work for you, I have another suggestion.

Make the Resources class do some work.

public class ResourceManager {

    private Hero hero;
    private Enemy enemy;
    private MenuButtons mainMenuButtons;
    private Background background;
    private Scene mainMenu;

    public void createObjects(){

        hero = new Hero();
        enemy = new Enemy();
        mainMenuButtons = new MenuButtons();
        background = new Background();
        mainMenu = new Scene(this);

    }

    public void render(Scene scene) {

        this.background.draw();
        if (scene != mainMenu) {

            this.hero.draw();
            this.enemy.draw();
        }
        this.mainMenuButtons.draw();           

    }

    public void updateLogic(Scene scene){

        this.hero.move();
        this.enemy.move();
        this.mainMenubuttons.animate();
    }

}

The mainMenu then calls logic methods directly in the RescourceManager class.

public class mainMenu implements Scene {

    ResourceManager resourceManager;

    public mainMenu(ResourceManager resourceManager){

        this.resourceManager = resourceManager;
    }

    @Override
    public void render(){

        resourceManager.render(this);
    }

    @Override
    public void updateLogic(){

        resourceManager.updateLogic(this);
    }

}

I hope my suggestions helped you a bit figure out how to continue with your project.

Answer 2

You could use dependency injection and eliminate your Resources class. Then you can call functions on your fields and wouldn't be in violation of the Law of Demeter.

Here is an example using constructor injection:

public class MainMenu implements Scene {

   Background background;
   Hero hero;  
   Enemy enemy; 
   MenuButtons buttons

    public mainMenu(Background background, Hero hero,  Enemy enemy, MenuButtons buttons){

       this.background = background;
       this.hero = hero;
       this.enemy = enemy;
       this.buttons = buttons;   
    }

    @Override
    public void render(){

        this.background.draw();
        this.hero.draw();
        this.enemy.draw();
        this.mainMenuButtons.draw();           

    }

    @Override
    public void updateLogic(){

        this.hero.move();
        this.enemy.move();
        this.mainMenubuttons.animate();
    }

}

With dependency injection, you pass instances into constructors and functions instead of "newing" them inside your class. You need to manage your instances somewhere though, and there are plenty of libraries that will do that for you. Dagger is a popular one for Android: http://square.github.io/dagger/

Answer 3

The idea of passing a list isn't a bad first step, but it's not sufficient. Game developers have a (somewhat controversial) concept of a structure called a "scene graph" that helps them keep track of their resources (among other things). https://en.wikipedia.org/?title=Scene_graph

It's a pretty complicated concept, but you're going to need to learn about it sooner or later. There's a lot of good advice on gamedev.stackexchange.com, so I'd suggest you take a peek over there.

Here's a nice YouTube video tutorial on the subject. https://www.youtube.com/watch?v=ktz9AlMSEoA

Answer 4

You could create an Drawer class that handles the drawing of all the objects. Your scene objects simply need to feed the Drawer the objects that I assume are Drawable.

public class Drawer {
   public void drawObjects(Drawable... objects) {
      for(Drawable drawable : objects) {
         drawable.draw();
      }
   }
}

This is then used by Scene to draw those objects.

public class mainMenu implements Scene {
   Resources resources;
   Drawer drawer;

   ...

   public void render() {
      drawer.drawObjects(resources.background, 
                         resources.hero, 
                         resources.enemy, 
                         resources.mainMenuButtons);
   }

   ...
}

A similar strategy, using an Updater, can applied for the other methods. If your updateLogic() method makes as simple of calls as it looks, you can definitely do the same thing, by making all those objects inherit from an Updateable interface.

public interface Updateable {
   void update();
}

Hero's and Enemy's update() methods could simply call their move() methods, while MenuButtons's update() could delegate to animate(), etc.

Obviously, if you like, you can use some sort of collection instead of varargs for the parameter of Drawer's drawObjects(). I just like the nice fluency made possible by the varargs, since you don't have to create the collection.

For other tips for keeping code in line with the Law of Demeter, check out this article: Law of Demeter and How to Work With It

Answer 5

I like the concept of a ResourceManager.
But a ResourceManager should be responsilbe for loading Resources, caching and freeing them.
Rendering is definitly a Method of a Render Object.

So the Scence - render Method could delegate the rendering to it after
instantiating a Renderer and feed it with Drawables as the Renderer does not
render Resources but renderable objects.

Say:

class MainMenu implements Scene {
    Renderer sceneRenderer = new Renderer();
    AnimatedRenderer animatedRenderer = new AnimatedRenderer();
    ResourceManager resourceManager = ResourceManager.getInstance();
    List<Resource> resources;
    List<Drawable> renderedObjects;
    GameObjectController gameObjectController;


    void initializeScene() {
          resources = resourceManager.getResources();
          renderedObjects = resourcesAsRenderables();
          sceneRenderer.setDrawables(renderedObjects);
    }

    List<Drawable> resourcesAsRenderables() {
      // if resources are not directly renderable, do decoration etc
      // and return a List of Drawable
    }

    @Override
    public void render(){
         sceneRenderer.render();
    }
    @Override
    public void updateLogic(){
       moveGameObjects();
       doAnimations();

    }
    protected void moveGameObjects() {
        gameObjectController.moveAllObjects(this, resources);
    }
    protected void doAnimations() {
        animatedRenderer.render(resources);
    }


    class ResourceManager {
       private static ResourceManager instance = null;
       List<Resource> resources;
       public ResourceManager getInstance() {
          if(instance == null) {
             instance = new ResourceManager();
             instance.loadResources();
          }
          return instance;
       }
       private void loadResources() { 
           resources = new LinkedList<Resource>();
           resources.add(new Hero());
           ....
       }
       public List<Resource> getResources() {
          return resources;
       }
    }

This clearly separates the logic and responsibilities for the tasks carried out during the scene lifecycle. A resource manager which is responsible for retrieving resources and as they may take long loading times does things like caching or freeing in low memory situations hiding the details from the client. A renderer which is responsible for displaying the objects and a controller which is responsible for moving the objects. The controller itself may implement handlers for keyboard events but that is not something which must be transparent to the scene. The renderer may swap backgrounds in or out or scale or set lighting effects but the scene only calls its render method. The animated renderer is responsible for starting , rendering and stopping animations.

Answer 6

As can be seen your Resources dont need to be recreated, instead they do use some resources that cant be reloaded (probably images).

You should share the images object within a Resource class, and create your objects within a Scene class, on the constructor of the entities you can get the shared resource that is pre-loaded.

Answer 7

Change this:

 public void render(){

        resources.background.draw();
        resources.hero.draw();
        resources.enemy.draw();
        mainMenuButtons.draw();           

    }
 @Override
    public void updateLogic(){

        resources.hero.move();
        resources.enemy.move();
        resources.mainMenubuttons.animate();
    }

With this:

public void render(){
            resources.render();
    } 

@Override
    public void updateLogic(){
        resources.update();
} 

ResourceManager don't have to know what's inside Resources. It may be one enemy or ten, it doesn't care to ResourceManager.

And so in 'Resource' you can do:

  public void update(){
        hero.update();// Cause hero may, or may not move, he makes the choice
        enemy.update();//...
        mainMenubuttons.update();//.
    }
  public void render(){
  ...
  }

More than this! you could change the "Resource" implementation with an interface and you will be programming for interfaces and not for implementations, which is cool! This way you can have a 'Resources' for in-game and another one for menus that will be used in same way: Only changing, at runtime, the concrete Resources you will be in a menu or in game!

Anyway, not always is needed to fill Demeter.