protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            {
                this.Exit();
            }

            // update our virtual thumbsticks
            VirtualThumbsticks.Update();

            // countdown until we spawn more enemies
            spawnTimer -= gameTime.ElapsedGameTime;
            if (spawnTimer <= TimeSpan.Zero)
            {
                // we spawn 1-3 enemies per spawn
                int numToSpawn = rand.Next(1, 3);

                for (int i = 0; i < numToSpawn; i++)
                {
                    // create the enemy
                    EnemyShip enemy = new EnemyShip(Content.Load <Texture2D>("alien"));

                    // target the player's ship
                    enemy.Player = player;

                    // we randomly pick either the left or right side of the screen
                    // to place the enemy
                    if (rand.Next() % 2 == 0)
                    {
                        enemy.Position.X = -worldWidth / 2f - (graphicsWidthHalf + 10);
                    }
                    else
                    {
                        enemy.Position.X = worldWidth / 2f + (graphicsWidthHalf + 10);
                    }

                    // we randomly pick either the top or bottom side of the screen
                    // to place the enemy
                    if (rand.Next() % 2 == 0)
                    {
                        enemy.Position.Y = -worldHeight / 2f - (graphicsHeightHalf + 10);
                    }
                    else
                    {
                        enemy.Position.Y = worldHeight / 2f + (graphicsHeightHalf + 10);
                    }

                    // add the enemy to our list
                    enemies.Add(enemy);
                }

                // reset our timer
                spawnTimer = spawnRate;
            }

            // update the player
            player.Update(gameTime);

            // update all the enemies
            foreach (var enemy in enemies)
            {
                enemy.Update(gameTime);
            }

            // create a couple lists to hold the bullets and enemies
            // we need to remove due to collisions
            List <Bullet>    bulletsToRemove = new List <Bullet>();
            List <EnemyShip> enemiesToRemove = new List <EnemyShip>();

            // figure out what bullets and enemies are colliding
            foreach (var b in player.Bullets)
            {
                foreach (var e in enemies)
                {
                    // check if the enemy contains the bullet's position
                    if (e.ContainsPoint(b.Position))
                    {
                        // add the bullet and enemy to the lists to be removed
                        bulletsToRemove.Add(b);
                        enemiesToRemove.Add(e);

                        // break the inner loop so bullets only collide
                        // with one enemy
                        break;
                    }
                }
            }

            // remove all marked bullets and enemies
            foreach (var b in bulletsToRemove)
            {
                player.Bullets.Remove(b);
            }
            foreach (var e in enemiesToRemove)
            {
                enemies.Remove(e);
            }

            base.Update(gameTime);
        }
        protected override void Update(GameTime gameTime)
		{
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

			// update our virtual thumbsticks
			VirtualThumbsticks.Update();

			// countdown until we spawn more enemies
			spawnTimer -= gameTime.ElapsedGameTime;
			if (spawnTimer <= TimeSpan.Zero)
			{
				// we spawn 1-3 enemies per spawn
				int numToSpawn = rand.Next(1, 3);

				for (int i = 0; i < numToSpawn; i++)
				{
					// create the enemy
					EnemyShip enemy = new EnemyShip(Content.Load<Texture2D>("alien"));

					// target the player's ship
					enemy.Player = player;

					// we randomly pick either the left or right side of the screen
					// to place the enemy
					if (rand.Next() % 2 == 0)
					{
						enemy.Position.X = -worldWidth / 2f - (graphicsWidthHalf + 10);
					}
					else
					{
						enemy.Position.X = worldWidth / 2f + (graphicsWidthHalf + 10);
					}

					// we randomly pick either the top or bottom side of the screen
					// to place the enemy
					if (rand.Next() % 2 == 0)
					{
						enemy.Position.Y = -worldHeight / 2f - (graphicsHeightHalf + 10);
					}
					else
					{
						enemy.Position.Y = worldHeight / 2f + (graphicsHeightHalf + 10);
					}

					// add the enemy to our list
					enemies.Add(enemy);
				}

				// reset our timer
				spawnTimer = spawnRate;
			}

			// update the player
			player.Update(gameTime);

			// update all the enemies
			foreach (var enemy in enemies)
				enemy.Update(gameTime);

			// create a couple lists to hold the bullets and enemies
			// we need to remove due to collisions
			List<Bullet> bulletsToRemove = new List<Bullet>();
			List<EnemyShip> enemiesToRemove = new List<EnemyShip>();

			// figure out what bullets and enemies are colliding
			foreach (var b in player.Bullets)
			{
				foreach (var e in enemies)
				{
					// check if the enemy contains the bullet's position
					if (e.ContainsPoint(b.Position))
					{
						// add the bullet and enemy to the lists to be removed
						bulletsToRemove.Add(b);
						enemiesToRemove.Add(e);

						// break the inner loop so bullets only collide
						// with one enemy
						break;
					}
				}
			}

			// remove all marked bullets and enemies
			foreach (var b in bulletsToRemove)
				player.Bullets.Remove(b);
			foreach (var e in enemiesToRemove)
				enemies.Remove(e);

			base.Update(gameTime);
		}