Step 4.11: Add 'Restorable' class

resources/scripts/engine/restorable.js

@@ -0,0 +1,21 @@
+Engine.Restorable = class Restorable {
+  // Acts the same as canvas's save() and restore() API.
+  // 'restorable' props are defined in the constructor
+  constructor(...restorableProps) {
+    this._restorableProps = restorableProps;
+    this._restorableStates = [];
+  }
+
+  // Save current state in the stack
+  save() {
+    this._restorableStates.push(this._restorableProps.reduce((state, prop) => {
+      state[prop] = this[prop];
+      return state;
+    }, {}));
+  }
+
+  // Pop most recent state and apply it
+  restore() {
+    _.extend(this, this._restorableStates.pop());
+  }
+};

views/game.html

@@ -9,6 +9,7 @@
 
     <!-- Scripts -->
     <script type="text/javascript" src="/scripts/namespaces.js"></script>
+    <script type="text/javascript" src="/scripts/engine/restorable.js"></script>
     <script type="text/javascript" src="/scripts/engine/sprite.js"></script>
     <script type="text/javascript" src="/scripts/engine/animations/keyframe.js"></script>
     <script type="text/javascript" src="/scripts/engine/key_states.js"></script>