Scene graph

Here we have recipes for doing things with the scene graph.

Using Pointer to combine Mouse and Touch

The Pointer mechanism allows your game to react to either mouse or touch events using a lowest-common denominator API that is usually sufficient for basic interaction. To cause Pointer events to be dispatched to layers just add the following to your game:

import playn.scene.Pointer;

public class MyGame extends SceneGame {
  public final Pointer pointer;
  public MyGame (Platform plat) {
    super(plat, 33);
    // combines mouse and touch events into pointer events
    pointer = new Pointer(plat, rootLayer, true);

That’s it. See Reacting to a click on a Layer for an example of how to listen for clicks on a layer and respond to them.

Reacting to a click on a Layer

This example will use the Pointer system, but the same approach works if you want to use Mouse or Touch events directly. Note that you need to follow the instructions in Using Pointer to combine Mouse and Touch in order to use the Pointer system.

Just create a layer and connect a listener to its events signal:

import playn.scene.Pointer;
// ...
ImageLayer layer = new ImageLayer(plat.assets().getImage("button.png")); Pointer.Listener() {
  public void onStart (Pointer.Interaction iact) {
    plat.log().info("Button click started!");
  public void onEnd (Pointer.Interaction iact) {
    plat.log().info("Button click released!");
  // onDrag() notifies you of movement while the interaction is happening
  // onCancel() notifies you if the interaction was canceled; this happens on mobile
  // sometimes and you should clean up and preted like the click never happened

Note that ImageLayer “knows” its bounds. This is necessary for hit testing to work. When a mouse, pointer or touch interaction starts, the dispatcher for the appropriate input mechanism looks at the scene graph at the bounds of each layer to determine which layer is “hit” by the coordinates of the interaction.

If you create a Layer and override paintImpl, that layer will not know its bounds by default, and thus will not react to events because it will assume its size is 0x0. If you want your layer to respond to events you need to override width() and height() and return the size of your layer. This is also true of GroupLayer. It will not respond to events itself, though children of a group layer (like an ImageLayer) would respond to events because the dispatcher will keep looking down the scene graph for a layer hit by an interaction.

If you want the GroupLayer itself to respond to events, you can either override width() and height() again, or you can set a custom HitTester via Layer.setHitTester. Note that when you’re doing custom hit testing, you may want to use Layer.hitTestDefault to first check whether any children of the GroupLayer were hit before doing your custom hit testing. For example:

GroupLayer group = new GroupLayer();
group.setHitTester(new Layer.HitTester() {
  public Layer hitTest (Layer self, Point p) {
    Layer child = self.hitTestDefault(p);
    if (child != null) return child; // child was hit
    // otherwise check whether p is in your group layer's bounds
// now add children to the group, some of which react to events...