/**** 
* Classes
****/
// Starfield background class
var Starfield = Container.expand(function () {
	var self = Container.call(this);
	var starfieldGraphics = self.createAsset('starfield', 'Scrolling starfield', 0, 0);
	self.speed = 1;
	self.move = function () {
		starfieldGraphics.y += self.speed;
		// Reset position when it scrolls off screen
		if (starfieldGraphics.y >= game.height) {
			starfieldGraphics.y = -game.height;
		}
	};
});
// Hero ship class
var HeroShip = Container.expand(function () {
	var self = Container.call(this);
	var shipGraphics = self.createAsset('heroShip', 'Cute smiley ship', 0.5, 0.5);
	self.speed = 5;
	self.isShooting = false;
	self.shootingInterval = 30;
	self._shootingCounter = 0;
	self.shoot = function () {
		var bullet = new HeroBullet();
		bullet.x = self.x;
		bullet.y = self.y - self.height / 2;
		game.addChild(bullet);
		return bullet;
	};
});
// Hero bullet class
var HeroBullet = Container.expand(function () {
	var self = Container.call(this);
	var bulletGraphics = self.createAsset('heroBullet', 'Heart bullet', 0.5, 1);
	self.speed = -10;
	self.move = function () {
		self.y += self.speed;
	};
});
// Enemy ship class
var EnemyShip = Container.expand(function () {
	var self = Container.call(this);
	var shipGraphics = self.createAsset('enemyShip', 'Evil smiley', 0.5, 0.5);
	self.speed = 2;
	self.move = function () {
		self.y += self.speed;
	};
});

/**** 
* Initialize Game
****/
var game = new LK.Game({
	backgroundColor: 0xFEF3BD // Init game with black background
});

/**** 
* Game Code
****/
// Initialize starfield
var starfield = game.addChild(new Starfield());
starfield.x = 0;
starfield.y = -game.height; // Start off screen

// Initialize important asset arrays
var heroBullets = [];
var enemyShips = [];

// Create hero ship
var heroShip = game.addChild(new HeroShip());
heroShip.x = game.width / 2;
heroShip.y = game.height - 450;

// Create enemies
function spawnEnemy() {
	var enemy = new EnemyShip();
	enemy.x = Math.random() * (game.width - enemy.width) + enemy.width / 2;
	enemy.y = -enemy.height / 2;
	enemyShips.push(enemy);
	game.addChild(enemy);
}

// Spawn initial enemies
for (var i = 0; i < 5; i++) {
	spawnEnemy();
}

// Game tick event
LK.on('tick', function () {
	// Move the starfield
	starfield.move();

	// Move hero bullets
	for (var i = heroBullets.length - 1; i >= 0; i--) {
		var bullet = heroBullets[i];
		bullet.move();
		if (bullet.y < -bullet.height) {
			bullet.destroy();
			heroBullets.splice(i, 1);
		} else {
			// Check collision with enemies
			for (var j = enemyShips.length - 1; j >= 0; j--) {
				if (bullet.intersects(enemyShips[j])) {
					bullet.destroy();
					enemyShips[j].destroy();
					heroBullets.splice(i, 1);
					enemyShips.splice(j, 1);
					break;
				}
			}
		}
	}

	// Move enemy ships
	for (var i = enemyShips.length - 1; i >= 0; i--) {
		var enemy = enemyShips[i];
		enemy.move();
		if (enemy.y > game.height + enemy.height / 2) {
			enemy.destroy();
			enemyShips.splice(i, 1);
			spawnEnemy();
		}
	}

	// Shoot bullets
	if (heroShip._shootingCounter % heroShip.shootingInterval == 0) {
		if (heroShip.isShooting) {
			heroBullets.push(heroShip.shoot());
			heroShip._shootingCounter = 0;
		}
	}
	heroShip._shootingCounter += 1;
});

// Touch controls to make heroShip follow the pointer
game.on('move', function (obj) {
	var touchPos = obj.event.getLocalPosition(game);
	heroShip.x = touchPos.x;
});
game.on('down', function (obj) {
	heroShip._shootingCounter = 0;
	heroShip.isShooting = true;
});
game.on('up', function (obj) {
	heroShip.isShooting = false;
});

===================================================================
--- original.js
+++ change.js
@@ -1,7 +1,20 @@
 /**** 
 * Classes
 ****/
+// Starfield background class
+var Starfield = Container.expand(function () {
+	var self = Container.call(this);
+	var starfieldGraphics = self.createAsset('starfield', 'Scrolling starfield', 0, 0);
+	self.speed = 1;
+	self.move = function () {
+		starfieldGraphics.y += self.speed;
+		// Reset position when it scrolls off screen
+		if (starfieldGraphics.y >= game.height) {
+			starfieldGraphics.y = -game.height;
+		}
+	};
+});
 // Hero ship class
 var HeroShip = Container.expand(function () {
 	var self = Container.call(this);
 	var shipGraphics = self.createAsset('heroShip', 'Cute smiley ship', 0.5, 0.5);
@@ -45,8 +58,13 @@
 
 /**** 
 * Game Code
 ****/
+// Initialize starfield
+var starfield = game.addChild(new Starfield());
+starfield.x = 0;
+starfield.y = -game.height; // Start off screen
+
 // Initialize important asset arrays
 var heroBullets = [];
 var enemyShips = [];
 
@@ -70,8 +88,11 @@
 }
 
 // Game tick event
 LK.on('tick', function () {
+	// Move the starfield
+	starfield.move();
+
 	// Move hero bullets
 	for (var i = heroBullets.length - 1; i >= 0; i--) {
 		var bullet = heroBullets[i];
 		bullet.move();