add photonAddedEmitter and photonRemovedEmitter to PhotonSystem, #47

js/common/model/PhotonSystem.ts

@@ -16,6 +16,7 @@ import TReadOnlyProperty from '../../../../axon/js/TReadOnlyProperty.js';
 import HydrogenAtom from './HydrogenAtom.js';
 import Vector2 from '../../../../dot/js/Vector2.js';
 import { Color } from '../../../../scenery/js/imports.js';
+import Emitter from '../../../../axon/js/Emitter.js';
 
 export default class PhotonSystem extends PhetioObject {
 
@@ -28,6 +29,10 @@ export default class PhotonSystem extends PhetioObject {
   // the collection of photons that appear in the zoomed-in box
   private readonly photons: ObservableArray<Photon>;
 
+  // notify when a photon is added or removed
+  public readonly photonAddedEmitter: Emitter<[ Photon ]>;
+  public readonly photonRemovedEmitter: Emitter<[ Photon ]>;
+
   public constructor( zoomedInBox: ZoomedInBox, hydrogenAtomProperty: TReadOnlyProperty<HydrogenAtom>, tandem: Tandem ) {
 
     super( {
@@ -41,6 +46,13 @@ export default class PhotonSystem extends PhetioObject {
 
     //TODO https://github.com/phetsims/models-of-the-hydrogen-atom/issues/47 replace ObservableArray
     this.photons = createObservableArray<Photon>();
+
+    this.photonAddedEmitter = new Emitter<[ Photon ]>( {
+      parameters: [ { name: 'photon', valueType: Photon } ]
+    } );
+    this.photonRemovedEmitter = new Emitter<[ Photon ]>( {
+      parameters: [ { name: 'photon', valueType: Photon } ]
+    } );
   }
 
   public reset(): void {
@@ -51,7 +63,7 @@ export default class PhotonSystem extends PhetioObject {
    * Emits a photon from the light source.
    */
   public emitPhotonFromLight( wavelength: number, position: Vector2, direction: number ): void {
-    this.photons.add( new Photon( {
+    this.addPhoton( new Photon( {
       wavelength: wavelength,
       position: position,
       direction: direction
@@ -62,7 +74,7 @@ export default class PhotonSystem extends PhetioObject {
    * Emits a photon from the hydrogen atom.
    */
   public emitPhotonFromAtom( wavelength: number, position: Vector2, direction: number, debugHaloColor: Color ): void {
-    this.photons.add( new Photon( {
+    this.addPhoton( new Photon( {
       wavelength: wavelength,
       position: position,
       direction: direction,
@@ -71,26 +83,26 @@ export default class PhotonSystem extends PhetioObject {
     } ) );
   }
 
+  private addPhoton( photon: Photon ): void {
+    this.photons.add( photon );
+    this.photonAddedEmitter.emit( photon );
+  }
+
   public removePhoton( photon: Photon ): void {
     assert && assert( this.photons.includes( photon ), 'Attempted to remove a photon that does not exist.' );
     this.photons.remove( photon );
     photon.dispose();
+    this.photonRemovedEmitter.emit( photon );
   }
 
   public removeAllPhotons(): void {
     while ( this.photons.length > 0 ) {
-      this.photons.pop()!.dispose();
+      const photon = this.photons.pop()!;
+      photon.dispose();
+      this.photonRemovedEmitter.emit( photon );
     }
   }
 
-  public addPhotonAddedListener( listener: ( photon: Photon ) => void ): void {
-    this.photons.addItemAddedListener( listener );
-  }
-
-  public addPhotonRemovedListener( listener: ( photon: Photon ) => void ): void {
-    this.photons.addItemRemovedListener( listener );
-  }
-
   /**
    * Advances the state of the photons.
    * @param dt - the time step, in seconds

js/common/view/ZoomedInBoxNode.ts

@@ -82,14 +82,14 @@ export default class ZoomedInBoxNode extends Node {
     const photonNodes: PhotonNode[] = [];
 
     // Add the PhotonNode for a Photon.
-    photonSystem.addPhotonAddedListener( photon => {
+    photonSystem.photonAddedEmitter.addListener( photon => {
       const photonNode = new PhotonNode( photon, modelViewTransform );
       photonNodes.push( photonNode );
       photonsLayer.addChild( photonNode );
     } );
 
     // Remove the PhotonNode for a Photon.
-    photonSystem.addPhotonRemovedListener( photon => {
+    photonSystem.photonRemovedEmitter.addListener( photon => {
       const photonNode = _.find( photonNodes, photonNode => ( photonNode.photon === photon ) )!;
       assert && assert( photonNode );
       photonNodes.splice( photonNodes.indexOf( photonNode ), 1 );