Fixes issues in demo game and adds heads up display

packages/flutter/example/game/lib/game_demo_world.dart

@@ -27,6 +27,7 @@ class GameDemoWorld extends NodeWithSize {
   sky.Image _imgNebula;
 
   SpriteSheet _spriteSheet;
+  SpriteSheet _spriteSheetUI;
   Navigator _navigator;
 
   // Inputs
@@ -46,8 +47,10 @@ class GameDemoWorld extends NodeWithSize {
   int _numFrames = 0;
   bool _isGameOver = false;
 
-  GameDemoWorld(App app, this._navigator, ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {
+  // Heads up display
+  Hud _hud;
 
+  GameDemoWorld(App app, this._navigator, ImageMap images, this._spriteSheet, this._spriteSheetUI) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {
     // Fetch images
     _imgNebula = images["assets/nebula.png"];
 
@@ -81,6 +84,10 @@ class GameDemoWorld extends NodeWithSize {
 
     userInteractionEnabled = true;
     handleMultiplePointers = true;
+
+    _hud = new Hud(_spriteSheetUI);
+    _hud.zPosition = 1000.0;
+    addChild(_hud);
   }
 
   // Methods for adding game objects
@@ -585,6 +592,49 @@ class StarField extends Node {
   }
 }
 
+class Hud extends NodeWithSize {
+  SpriteSheet spriteSheetUI;
+  Sprite sprtBgScore;
+  Sprite sprtBgShield;
+
+  int _score = 0;
+
+  int get score => _score;
+
+  set score(int score) {
+    _score = score;
+    _updateHud();
+  }
+
+  Hud(this.spriteSheetUI) {
+    pivot = Point.origin;
+
+    sprtBgScore = new Sprite(spriteSheetUI["scoreboard.png"]);
+    sprtBgScore.pivot = Point.origin;
+    sprtBgScore.scale = 0.6;
+    addChild(sprtBgScore);
+
+    sprtBgShield = new Sprite(spriteSheetUI["bar_shield.png"]);
+    sprtBgShield.pivot = new Point(1.0, 0.0);
+    sprtBgShield.scale = 0.6;
+    addChild(sprtBgShield);
+  }
+
+  void spriteBoxPerformedLayout() {
+    // Set the size and position of HUD display
+    position = spriteBox.visibleArea.topLeft;
+    size = spriteBox.visibleArea.size;
+
+    // Position hud objects
+    sprtBgScore.position = new Point(20.0, 20.0);
+    sprtBgShield.position = new Point(size.width - 20.0, 20.0);
+  }
+
+  void _updateHud() {
+
+  }
+}
+
 class Nebula extends Node {
 
   Nebula.withImage(sky.Image img) {

packages/flutter/example/game/main.dart

@@ -51,6 +51,7 @@ main() async {
 class GameDemoApp extends App {
 
   NavigationState _navigationState;
+  GameDemoWorld _game;
 
   void initState() {
     _navigationState = new NavigationState([
@@ -84,16 +85,18 @@ class GameDemoApp extends App {
   }
 
   Widget _buildGameScene(navigator, route) {
-    return new SpriteWidget(
-      new GameDemoWorld(_app, navigator, _loader, _spriteSheet)
-    );
+    if (_game == null) _game = new GameDemoWorld(_app, navigator, _loader, _spriteSheet);
+    return new SpriteWidget(_game);
   }
 
   Widget _buildMainScene(navigator, route) {
     return new Center(
       child: new RaisedButton(
         child: new Text("Play"),
-        onPressed: () => navigator.pushNamed('/game')
+        onPressed: () {
+          _game = new GameDemoWorld(_app, navigator, _loader, _spriteSheet, _spriteSheetUI);
+          navigator.pushNamed('/game');
+        }
       )
     );
   }