refactor: collider tracks collisions

packages/physics/traits/collider.trait.js

@@ -26,6 +26,7 @@ export class Collider extends decorate(Trait) {
   initialize() {
     this._collisionGroup = this.params.get('collisionGroup');
     this._collidesWithGroups = this.params.get('collidesWithGroups').toJS();
+    this._isCollidingWith = [];
     this._isSensor = this.params.get('isSensor');
   }
 
@@ -37,10 +38,29 @@ export class Collider extends decorate(Trait) {
     return this._collidesWithGroups;
   }
 
+  get isCollidingWith() {
+    return this._isCollidingWith;
+  }
+
   get isSensor() {
     return this._isSensor;
   }
 
+  listeners() {
+    return {
+
+      collisionEnd: (other) => {
+        const index = this._isCollidingWith.indexOf(other);
+        this._isCollidingWith.splice(index, 1);
+      },
+
+      collisionStart: (other) => {
+        this._isCollidingWith.push(other);
+      },
+
+    };
+  }
+
   methods() {
     return {