Sprite Kit中的节点在屏幕外生成

Question

I am building a ios game with swift and I have run into a bit of a problem. I am trying to spawn balls from the top of the screen and have them come down towards the ground. They are supposed to have random x values and go down at random rates but instead of spawning on the screen the nodes spawn on an x value which is not encompassed by the screen. Please help me as I think I have done everything right. Here is the code for my addball function...

func addBall(){

        //create ball sprite
        var ball = SKSpriteNode(imageNamed: "ball.png")

        //create physics for ball
        ball.physicsBody = SKPhysicsBody(rectangleOfSize: ball.size) // 1
        ball.physicsBody?.dynamic = true // 2
        ball.physicsBody?.categoryBitMask = PhysicsCategory.Ball // 3
        ball.physicsBody?.contactTestBitMask = PhysicsCategory.Person & PhysicsCategory.Ground
        ball.physicsBody?.collisionBitMask = PhysicsCategory.None // 5

        //generate random postion along x axis for ball to spawn
        let actualX = random(min:ball.frame.size.width/2+1, max: self.frame.size.width - ball.frame.size.width/2-1)
        println(actualX)
        //set balls positon
        ball.position = CGPoint(x: actualX, y: size.height - ball.size.width/2)

        //add ball to scene
        addChild(ball)

        //determine speed of ball
        let actualDuration = random(min: CGFloat(3.0), max: CGFloat(5.0))

        //create movement actions
        let actionMove = SKAction.moveTo(CGPoint(x:actualX, y:  -ball.size.width/2), duration: NSTimeInterval(actualDuration))

        let actionMoveDone = SKAction.removeFromParent()


        ball.runAction(SKAction.sequence([actionMove, actionMoveDone]), withKey: "action")


    }

here is the code for my random functions

func random() -> CGFloat {
        return CGFloat(Float(arc4random()) / 0xFFFFFFFF)
    }

    func random(#min: CGFloat, max: CGFloat) -> CGFloat {
        return random() * (max - min) + min
    }

Answer 1

The problem here is that your SKScene likely takes up much more space than the screen of your device. Thus, when you calculate a random value using the whole scene, some of the time the ball will spawn in the area of the scene not visible to you.

The two main properties that control the scene's size are its size and scaleMode properties. The scaleMode property relates to how the scene is mapped. Unless you initialized and presented this scene yourself, you can check the scaleMode in your view controller. It will likely be set to aspectFill , which according to Apple means:

The scaling factor of each dimension is calculated and the larger of the two is chosen. Each axis of the scene is scaled by the same scaling factor. This guarantees that the entire area of the view is filled but may cause parts of the scene to be cropped.

If you don't like this, there are other scaleModes. However, in most cases this mode would actually be preferable since SpriteKit's internal scaling is able to make universal apps. If this is fine for you, then the easiest thing to do is set hardcoded values for something like the spawn locations for your ball node.