UIScreen.mainScreen().bounds.size.height返回正确的数字，但SKSpriteNod

我正在学习SpriteKit，并尝试使用SKAction.moveToY将节点从屏幕上向上移动。我想先让它接近顶部（我想一旦我把它拿到节点的顶部，它就会仍然在屏幕上，我必须调整，但这应该不是问题）。

问题是，我已经尝试使用UIScreen.mainScreen().bounds.size.height和（在didMoveToView方法中）SKView的bounds.size.height。如果我同时使用println，它们会返回顶部的正确值（5s返回568.0）。当我将该值添加到SKAction.moveToY(screenHeightVariable, duration: 2)时，它可以正常工作并移动节点，但它会在大约75%的时间内停止到5s的顶部，在大约90%的时间内停到6+的顶部。有没有一种不同的方法可以让我得到屏幕顶部的Y值？以下是我的一些代码（尽量忽略一些更不合逻辑的东西，比如红色ufo，因为这更多的是学习和错误尝试）：

override func didMoveToView(view: SKView) {
    physicsWorld.contactDelegate = self
    println(view.bounds.size.height)
    xVal = self.frame.width/2
    ufoNode = ufo(imageNamed: "ufo")
    ufoNode!.position = CGPoint(x: xVal!, y: 30)
    ufoNode!.physicsBody?.categoryBitMask = charCat

    ufoRedNode = obstacle(imageNamed: "ufo-red")
    ufoRedNode!.position = CGPoint(x: (self.frame.width/2 - 50), y: 30)
    ufoNode!.physicsBody?.categoryBitMask = sceneCat
    self.addChild(ufoRedNode!)
    self.addChild(ufoNode!)
}
override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
    /* Called when a touch begins */
    for touch: AnyObject in touches {
        let location = touch.locationInNode(self)

        if location.x < self.frame.width/2 {
            // Left
            direction = Selector("moveLeft:")
        } else {
            // Right
            direction = Selector("moveRight:")
        }
        movementTimer = NSTimer.scheduledTimerWithTimeInterval(0.005, target: self, selector: direction!, userInfo: nil, repeats: true)
        self.fireBullet(self)
        fireBulletTimer = NSTimer.scheduledTimerWithTimeInterval(0.1, target: self, selector: "fireBullet:", userInfo: nil, repeats: true)
    }
}
func fireBullet(sender: AnyObject!) {

    var singleBullet: bullet?
    bulletY = ufoNode!.position.y + 10
    if bulletSide == "Left" {
        bulletX = (ufoNode!.position.x - ((ufoNode!.size.width/4) ))
        bulletSide = "Right"
       // println("left")
    } else {
        bulletX = (ufoNode!.position.x + ((ufoNode!.size.width/4) ))
        bulletSide = "Left"
       // println("right")
    }
    singleBullet = bullet(imageNamed: "bullet")
    singleBullet!.position = CGPoint(x: bulletX!, y: bulletY!)
    var bulletAction: SKAction = SKAction.moveToY(screenHeight, duration: 2)

    singleBullet!.runAction(bulletAction)
    self.addChild(singleBullet!)

}