separate logic and GUI in java

Question

I'm implementing a game in Java, using the classes shown below to control the game logic. I try to be very clear when explaining my question.

• GamePanel

  • I use this class to start the game loop with a thread (only game loop)

    public void run() {
    init(); //initialize gamePanel components
    // game loop
    while (running) {
          start = System.nanoTime();
          gameManager.update();
          KeyInput.update();
          MouseInput.update();
          gameManager.draw(graphic);
          Graphics g2 = getGraphics();
          g2.drawImage(image, 0, 0, null);
          g2.dispose();
    
          elapsed = System.nanoTime() - start;
          wait = targetTime - elapsed / 1000000;
          if (wait < 0)
        wait = 5;
         try {
             Thread.sleep(wait);
         } catch (Exception e) {
         e.printStackTrace();
       }
    
    }
    

• GameManager

  • I made this class to control each aspect of the game as (update,graphics)

    private void loadState(int state) {
    
    if (state == States.MENU.getValue())
    gameState = new Menu(this);
    else if (state == States.GAME.getValue())
    gameState = new Game(this);
    else if (state == States.PATHSELECT.getValue())
    gameState = new SelectPath(this);
    }
    public void update() {
    if (gameState != null)
    gameState.update();
    }
    
    public void draw(java.awt.Graphics2D graphic) {
     if (gameState != null)
    gameState.draw(graphic);
     else {
    JOptionPane.showMessageDialog(null, JOptionPane.ERROR_MESSAGE);
         }
    }
    

• Gamestate

  • GameState is an abstract class that receives, as a constructor parameter, an instance of GameManager and it has abstract methods implemented in each GameState (Menu,Game,MultiPlayer,etc..)

  • this class has these abstract methods:

    • controller() -> that verify the state logic

    • update() -> in case of the currentState is a MultyPlayer , this method call the update method of the class Player , etc...

    • draw(Graphics g) -> draw to screen the objects of the currentState

I thought of making a utility class singleton that implements all the draw() of each state, but it is very nasty to insert all in a single class.

Example:

instead of calling gameState.draw(graphic); I call Render.getRenderInstance().draw(graphic,gameState); this solutions works, but I don't like it.

So how can I divide the draw method from the rest of the logic in a pretty way?

Some advice? Thank you.

Answer 1

The usual approach is to divide the game into two parts:

• the logic, which looks to be done OK in your case

• the scene, where things are drawn

Here in pseudocode is how I do it:

the main loop:
  gamestate.update()
  scene.render();

In the scene you should have all your graphics operations. You should be able to add new graphics to the scene using a call similar to this:

scene.addObject(SomeGraphic graphic);

So the scene would render that graphic every main loop iteration.

To let this happen you should keep a list or another collection inside the Scene class and render every object every tick. So the actuall scene.render() would look like so:

public void render() {
  for(SomeGraphic graphic : objects) {
    graphic.draw()
  }
}

You will be able then to control what is on the scene from the game logic - you will be able to add and delete objects etc.