Swift Game Scene alter vertically moving background with time? - ios

Question

I have a moving background which is 1500 x 600 pixels and constantly moves vertically down the screen using this code:
let bgTexture = SKTexture(imageNamed: "bg.png")
let moveBGanimation = SKAction.move(by: CGVector(dx: 0, dy: -bgTexture.size().height), duration: 4)
let shiftBGAnimation = SKAction.move(by: CGVector(dx: 0, dy: bgTexture.size().height), duration: 0)
let moveBGForever = SKAction.repeatForever(SKAction.sequence([moveBGanimation, shiftBGAnimation]))
var i: CGFloat = 0
while i < 3 {
bg = SKSpriteNode(texture: bgTexture)
bg.position = CGPoint(x: self.frame.midX, y: bgTexture.size().height * i)
bg.size.width = self.frame.width
bg.zPosition = -2
bg.run(moveBGForever)
self.addChild(bg)
i += 1
}
I now want a new background to come onto the screen after x amount of time to give the feel the player is moving into a different part of the game.
Could I put this code into a function and trigger it with NSTimer after say 20 seconds but change the start position of the new bg to be off screen?

Answer 1

The trouble with repeatForever actions is you don't know where they are at a certain moment. NSTimers are not as precise as you'd like, so using a timer may miss the right time or jump in too early depending on rendering speeds and frame rate.
I would suggest replacing your moveBGForever with a bgAnimation as a sequence of your move & shift actions. Then, when you run bgAnimation action, you run it with a completion block of { self.cycleComplete = true }. cycleComplete would be a boolean variable that indicates whether the action sequence is done or not. In your scene update method you can check if this variable is true and if it is, you can run the sequence action once again. Don't forget to reset the cycleComplete var to false.
Perhaps it sounds more complex but gives you control of whether you want to run one more cycle or not. If not, then you can change the texture and run the cycle again.
Alternatively you may leave it as it is and only change the texture(s) after making sure the sprite is outside the visible area, e.g. its Y position is > view size height.
In SpriteKit you can use wait actions with completion blocks. This is more straightforward than using a NSTimer.
So, to answer your question - when using actions for moving the sprites on-screen, you should not change the sprite's position at any time - this is what the actions do. You only need to make sure that you update the texture when the position is off-screen. When the time comes, obviously some of the sprites will be displayed, so you can't change the texture of all 3 at the same time. For that you may need a helper variable to check in your update cycle (as I suggested above) and replace the textures when the time is right (sprite Y pos is off-screen).