SpriteKit calling function from class - swift

Question

This is from a simple game in SpriteKit with a Ball() class that has a function shieldOn() which, for the moment, simply replaces the texture of a single ball to that of a ball surrounded by a shield.
The ball is created like this in GameScene:
func getBall() {
let ball = Ball()
ball.createBall(parentNode: self)
}
Here is the Ball class
class Ball: SKSpriteNode {
func createBall(parentNode: SKNode) {
let ball = SKSpriteNode(texture: SKTexture(imageNamed: "ball2"))
ball.physicsBody = SKPhysicsBody(circleOfRadius: 25)
ball.name = "ball"
parentNode.addChild(ball)
ball.size = CGSize(width: 50, height: 50)
ball.position = CGPoint(x: 20, y: 200)
launch(spriteNode: ball, parentNode: parentNode)
}
private func launch(spriteNode: SKSpriteNode, parentNode: SKNode) {
spriteNode.physicsBody?.applyImpulse(CGVector(dx: 5, dy: 0))
}
func shieldOn() {
self.texture = SKTexture(imageNamed: "ballShield")
}
func shieldOff() {
self.texture = SKTexture(imageNamed: "ball2")
}
}
In the main section of my code (GameScene.swift) I don't have a reference to the ball. So I cycle through all of the nodes on the screen and try to cast the matching one as shown below. I crash with an error saying that it could not cast value of type SKSpriteNode to Ball.
for node in self.children {
if node.name == "ball" {
let ball = node as! Ball
ball.shieldOn()
}
}
I've tried a few variations with no luck. Am I at least working in the right direction? Thanks!

Answer 1

With the new information I think you want something like this:
Ball Class:
class Ball: SKSpriteNode{
init() {
let texture = SKTexture(imageNamed: "ball2")
let size = CGSize(width: 50, height: 50)
super.init(texture: texture, color: UIColor.clear, size: size)
self.name = "ball"
self.physicsBody = SKPhysicsBody(circleOfRadius: size.height/2)
self.physicsBody?.applyImpulse(CGVector(dx: 5, dy: 0))
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
func shieldOn() {
self.texture = SKTexture(imageNamed: "ballShield")
}
func shieldOff() {
self.texture = SKTexture(imageNamed: "ball2")
}
}
Then use this to create the ball:
func getBall() {
let ball = Ball()
ball.position = CGPoint(x: 20, y: 200)
scene?.addChild(ball)
}

Answer 2

Perhaps a better way to do this would be to keep an array of all Balls created and added to the scene. Then you could just iterate through your array and update their texture. You would not need to enumerate them on the screen, which can decrease performance if there are many moving sprites.
As far as your code goes, it looks like you might be affected by this bug:
https://forums.developer.apple.com/thread/26362