Ios 在使用Swift的SpriteKit中，如何设置子节点数量的特定限制？

我有一个简单的场景，当用户触摸某个区域时，它会向场景中添加一个弹跳球。我们的目标是将球弹起，以便在特定区域“得分”

我刚刚开始编写应用程序，但我还没有找到一种方法来限制用户可以生成的球的数量。目前，它们可以生成任意数量的球，并使场景过度饱和，从而导致FPS下降和非常简单的游戏

override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
    /* Called when a touch begins */

    for touch: AnyObject in touches {
        let location = touch.locationInNode(self)

        let ball = SKSpriteNode(imageNamed:"ball")

        ball.xScale = 0.2
        ball.yScale = 0.2
        ball.position = location

        ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.size.height / 2.0)
        ball.physicsBody!.dynamic = true

        ball.physicsBody!.friction = 0
        ball.physicsBody!.restitution = 0.8

        ball.name = "ball"

        self.addChild(ball)

    }
}

override func update(currentTime: CFTimeInterval) {
    /* Called before each frame is rendered */

}

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我知道我需要询问场景中有多少个球节点，然后在球达到极限时移除球，但我尝试的一切都会导致错误。

有很多方法可以做到这一点，但这可能是最简单的方法。 注意：除非你在某个时候从场景中移除球，否则你将达到10个球的极限并被卡住

override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
    /* Called when a touch begins */

    for touch: AnyObject in touches {
        let location = touch.locationInNode(self)

        // ball limit
        let limit = 10

        // setup counter
        var i = 0

        // loop through the ball nodes added to the scene
        self.enumerateChildNodesWithName("ball", usingBlock: {
            _, _ in  // we dont need to use these variables right now   
            i += 1  // increment the counter
        })

        // if we haven't hit our limit, add a ball (YOUR CODE HERE)
        if i <= limit {

            let ball = SKSpriteNode(imageNamed:"ball")

            ball.xScale = 0.2
            ball.yScale = 0.2
            ball.position = location

            ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.size.height / 2.0)
            ball.physicsBody!.dynamic = true

            ball.physicsBody!.friction = 0
            ball.physicsBody!.restitution = 0.8

            ball.name = "ball"

            self.addChild(ball)

        }

    }
}

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我尝试的每件事都会带来错误您能否显示您的尝试并描述您收到的错误消息？