[#979] Prevent removal of actors that are killed and readded during t…

…he same frame (#1016)

Closes #979

## Changes:

- Add check to ensure Actor is still killed before removing it from drawing tree.

CHANGELOG.md

@@ -9,13 +9,16 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- Add `Scene.isActorInDrawTree` method to determine if an actor is in the scene's draw tree.
+
 ### Changed
 
 ### Deprecated
 
 ### Fixed
 
-- Fixed missing `exitviewport/enterviewport` events on Actors.on/once/off signatures. ([#978](https://github.com/excaliburjs/Excalibur/issues/978))
+- Fixed missing `exitviewport/enterviewport` events on Actors.on/once/off signatures ([#978](https://github.com/excaliburjs/Excalibur/issues/978))
+- Fix issue where Actors would not be properly added to a scene if they were removed from that scene during the same frame ([#979](https://github.com/excaliburjs/Excalibur/issues/979))
 
 <!--------------------------------- DO NOT EDIT BELOW THIS LINE --------------------------------->
 

src/engine/Scene.ts

@@ -390,10 +390,13 @@ export class Scene extends Class implements ICanInitialize, ICanActivate, ICanDe
     // Remove actors from scene graph after being killed
     var actorIndex: number;
     for (let killed of killQueue) {
-      actorIndex = collection.indexOf(killed);
-      if (actorIndex > -1) {
-        this._sortedDrawingTree.removeByComparable(killed);
-        collection.splice(actorIndex, 1);
+      //don't remove actors that were readded during the same frame they were killed
+      if (killed.isKilled()) {
+        actorIndex = collection.indexOf(killed);
+        if (actorIndex > -1) {
+          this._sortedDrawingTree.removeByComparable(killed);
+          collection.splice(actorIndex, 1);
+        }
       }
     }
     killQueue.length = 0;
@@ -737,6 +740,13 @@ export class Scene extends Class implements ICanInitialize, ICanActivate, ICanDe
     this._sortedDrawingTree.add(actor);
   }
 
+  /**
+   * Checks if an actor is in this scene's sorted draw tree
+   */
+  public isActorInDrawTree(actor: Actor): boolean {
+    return this._sortedDrawingTree.find(actor);
+  }
+
   public isCurrentScene(): boolean {
     if (this.engine) {
       return this.engine.currentScene === this;

src/spec/SceneSpec.ts

@@ -225,6 +225,19 @@ describe('A scene', () => {
     expect(scene.actors.length).toBe(1);
   });
 
+  it('will still be in the draw tree if it is killed and then added in the same frame', () => {
+    var actor = new ex.Actor();
+    scene.add(actor);
+    actor.kill();
+    scene.add(actor);
+
+    scene.update(engine, 10); //call _processKillQueue
+
+    expect(scene.actors.indexOf(actor)).toBe(0);
+    expect(scene.actors.length).toBe(1);
+    expect(scene.isActorInDrawTree(actor)).toBe(true);
+  });
+
   it('will update Actors that were added in a Timer callback', () => {
     var updated = false;
     var initialized = false;