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nathan edited this page Aug 22, 2015 · 1 revision

The different steps

The most common way to create an interpolation is the following:

  1. Implement the TweenAccessor interface for your objects, and register the implementations, so the engine will know them,
  2. Create a TweenManager, to easility handle their life-cycles and updates of your tweens and timelines,
  3. Create some tweens with Tween.to(), Tween.from() or Tween.set() and add them to the manager,
  4. (repeat) Call the update(int delta) method of your manager periodically.

Example (with code)

Let's say that we have some particles drawn, and we want to move them around with acceleration and/or deceleration. We'll use the Tween Engine of course. But before any tween can be applied on our particle, we must tell the engine how to tween those particles: we need to make the particles tweenable !

The Particle class

This Particle class is very simple (for the example) and only defines a position with an "x" and an "y" field, like this:

public class Particle {
    private float x, y;
    
    public float getX() {
        return x;
    }
    
    public float getY() {
        return y;
    }

    public void setX() {
        this.x = x;
    }

    public void setY() {
        this.y = y;
    }
}

Making the particles tweenable

The recommended way is to create a new class which implements the TweenAccessor interface. Of course, you can also directly implement this interface in the Particle class, but the philosophy of the Tween Engine is to let your code unchanged.

// We implement TweenAccessor<Particle>. Note the use of a generic.

public class ParticleAccessor implements TweenAccessor<Particle> {

    // The following lines define the different possible tween types.
    // It's up to you to define what you need :-)

    public static final int POSITION_X = 1;
    public static final int POSITION_Y = 2;
    public static final int POSITION_XY = 3;

    // TweenAccessor implementation

    @Override
    public int getValues(Particle target, int tweenType, float[] returnValues) {
        switch (tweenType) {
            case POSITION_X: returnValues[0] = target.getX(); return 1;
            case POSITION_Y: returnValues[0] = target.getY(); return 1;
            case POSITION_XY:
                returnValues[0] = target.getX();
                returnValues[1] = target.getY();
                return 2;
            default: assert false; return -1;
        }
    }
    
    @Override
    public void setValues(Particle target, int tweenType, float[] newValues) {
        switch (tweenType) {
            case POSITION_X: target.setX(newValues[0]); break;
            case POSITION_Y: target.setY(newValues[0]); break;
            case POSITION_XY:
                target.setX(newValues[0]);
                target.setY(newValues[1]);
                break;
            default: assert false; break;
        }
    }
}

Before anything else, don't forget to register this new TweenAccessor implementation to the engine, else it won't know it:

Tween.registerAccessor(Particle.class, new ParticleAccessor());

Let's move the particles!

It's over, we can now easily apply interpolations directly on our particles.

In the initialization code:

// We need a manager to handle every tween.

manager = new TweenManager();

// We can now create as many interpolations as we need !

Tween.to(particle1, ParticleAccessor.POSITION_XY, 1.0f)
    .target(100, 200)
    .start(manager);
    
Tween.to(particle2, ParticleAccessor.POSITION_XY, 0.5f)
    .target(0, 0)
    .ease(Bounce.OUT)
    .delay(1.0f)
    .repeatYoyo(2, 0.5f)
    .start(manager);

In a loop (usually the render loop in games):

manager.update(ellapsedSeconds);
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