我试图在SpriteKit（Xcode 7测试版）中制作一个像Doodle Jump这样的垂直无休止的跑步者，但不知道如何不断地向上移动Node。

override func didMoveToView(view: SKView) {

    makeBackground()

    // Bubble

    let bubbleTexture = SKTexture(imageNamed: "bubble.png")

    bubble = SKSpriteNode(texture: bubbleTexture)

    bubble.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame))

    bubble.physicsBody = SKPhysicsBody(circleOfRadius: bubbleTexture.size().height/2)
    bubble.physicsBody!.dynamic = false
    bubble.physicsBody!.allowsRotation = false

    let float = SKAction.moveToY(+yScale, duration: 0.2)

    bubble.SKAction.repeatActionForever(runAction(float))

    self.addChild(bubble)
}