Getting child SKSpriteNode Position

Question

I am currently attempting to create a game that will auto generate platforms on the scene when a platform leaves the scene.

I have created 5 SKSpriteNodes and added them as children to a Platform SKNode.

I then move them accross the screen with platformGroupNode.position.x-- inside my update function

However, I would like to detect when one of the children leaves the screen, so that I can generate a new platform.

I have tried the below, but it only seem to give me a position for that child inside the parent node

for child in shipGroupNode.children {
            println(child.position.x)
        }

Is there a better way to group Sprite Nodes to allow me to view their individual properties in relation to the scene and control there are a group?

Code ------------

mport SpriteKit

// Globals
let platformGroupNode = SKNode()
var platformX:CGFloat = 200

class GameScene: SKScene {
    override func didMoveToView(view: SKView) {

        platformGroupNode.position = CGPoint(x: 0, y: 0)
        addChild(platformGroupNode)

        // Add platforms
        addPlatform(platformX)

        // Add platforms
        for i in 1...4 {
            addPlatform(platformX)
            //println(platformX)
        }
    }

    override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
        /* Called when a touch begins */
    }

    override func update(currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */
        platformGroupNode.position.x--

        for child in platformGroupNode.children {
            println(child.position.x)
            println(convertPointToView(CGPoint(x: child.position.x, y: child.position.y)))
        }

    }

    func addPlatform(platformx:CGFloat) {
        let platform = SKSpriteNode(imageNamed:"Spaceship")

        platform.xScale = 0.5
        platform.yScale = 0.5
        platform.position = CGPoint(x: platformx, y: self.size.height/2)

        platformX = platformx + 200
        platformGroupNode.addChild(platform)
    }
}

Answer 1

You can use the intersectsNode method on SKScene to check whether the child nodes are visible; intersectsNode will return true whilst the children are visible.

for child in platformGroupNode.children {
    if let sprite = child as? SKSpriteNode where !intersectsNode(sprite) { 
        child.removeFromParent()

        // Generate new platform...
    }
}

An alternative to removing and then creating a new platform would be to reposition the platform that just moved offscreen, so it will move back onscreen. For example: if scrolling from right to left, when a platform moves off the left edge it will be repositioned on the right. This has the advantage of only needing to create a few platforms at the beginning.

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For this to work correctly it's essential that your SKScene's size is equal to its SKView size. For information on this have a look at SpriteKit Coordinates differ between iPad and iPhone