如何在SpriteKit中进行平滑的垂直滚动？

Question

当点击屏幕时，球在空中弹跳，如果高于某一高度，我滚动屏幕。在我的情况下，滚动总是不平滑，有一些滞后，任何想法做得更好？非常感谢你，如果你有更好的想法或其他想法！

以下是代码：

let world = SKNode()
var player : SKSpriteNode!
var firsttap = true

override func didMoveToView(view: SKView) {

    physicsWorld.gravity = CGVectorMake(0.0, -9.9)
    physicsWorld.contactDelegate = self

    self.createPlayer()
    addChild(world)
    world.addChild(player)

    let floor = SKSpriteNode(color: SKColor.blackColor(), size: 320, 20))
    floor.physicsBody = SKPhysicsBody(rectangleOfSize: 320, 20))
    floor.physicsBody?.dynamic = false
    floor.position = CGPointMake(CGRectGetMidX(frame), 0)

    addChild(floor)
}

func createPlayer() {
    self.player = SKSpriteNode(imageNamed: "Ball")
    let body = SKPhysicsBody(circleOfRadius:13.0)
    body.dynamic = false
    body.contactTestBitMask = PlayerCategory
    body.allowsRotation = false
    player.physicsBody = body
    player.position = CGPointMake(CGRectGetMidX(frame), CGRectGetMidY(frame) * 0.75)
}

override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
    for touch: AnyObject in touches {
        if firsttap {
            player.physicsBody?.dynamic = true
            firsttap = false
        } 
        let touchLoc = touch.locationInNode(self)
        var right = true
        if touchLoc.x > CGRectGetMidX(frame) {
            right = false
        }

        if right {
            player.physicsBody?.velocity = CGVectorMake(0,0)
            player.physicsBody?.applyImpulse(CGVectorMake(-2, 14))
        }else{
            player.physicsBody?.velocity = CGVectorMake(0,0)
            player.physicsBody?.applyImpulse(CGVectorMake(2, 14))
        }
    }
}

override func update(currentTime: NSTimeInterval) {
    let pos : CGFloat = -player.position.y + CGRectGetMidY(frame)
    if pos < world.position.y {
        world.position.y = pos
    }
}

func didBeginContact(contact: SKPhysicsContact) {
    let nextLevel = GameScene((size: frame.size))
    let transition = SKTransition.fadeWithDuration(0.5)
    view?.presentScene(nextLevel, transition:transition)
}

第一次碰巧是平滑的，但是在呈现相同的场景时不是以下的那些，我不明白为什么：（