Swift-SKAction序列遇到问题

Question

I'm trying to move my background image down and then after a 3 second delay, move it back up the screen. And I want this to occur forever.

It slides down the screen perfectly fine, but then nothing else happens. Can anyone see what I'm doing wrong here?

 override func didMoveToView(view: SKView) {

    background.anchorPoint = CGPoint(x: 0, y: 0)
    background.size = CGSize(width: frame.size.width, height: frame.size.height)
    background.name = "bg"
    addChild(background)

    let moveBG1 = SKAction.moveToY(-(size.height), duration: 3.0)

    let moveBGback = SKAction.moveToY(size.height, duration: 3.0)


    runAction(SKAction.repeatActionForever(
        SKAction.sequence([
            SKAction.runAction(moveBG1, onChildWithName: "bg"),
            SKAction.waitForDuration(3.0),
            SKAction.runAction(moveBGback, onChildWithName: "bg")
            ])
        ))

Answer 1

According with your anchorPoint, the first action is correct. As nickfalk write to you, the second action must to be:

let moveBGback = SKAction.moveToY(0, duration: 3.0) 

if you want to see your BG come back, so you have wrong the Y position, otherwise your BG is putted at the TOP of the screen (so not visible)