SpriteKit: creating a custom button

ButtonNode.swift

import SpriteKit

protocol ButtonNodeResponder: class {
    func buttonTriggered(button: ButtonNode)
}

class ButtonNode: SKSpriteNode {
    var identifier: String?
    var responder: ButtonNodeResponder?

    var defaultTexture: SKTexture
    var highlightedTexture: SKTexture?
    
    var isHighlighted = false {
        didSet {
            let highlightedTexture = (self.highlightedTexture != nil) ? self.highlightedTexture : defaultTexture
            texture = isHighlighted ? highlightedTexture : defaultTexture
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    init(defaultTexture: SKTexture, color: UIColor, size: CGSize) {
        self.defaultTexture = defaultTexture
        let color = UIColor.clearColor()
        super.init(texture: defaultTexture, color:color, size: size)
        self.userInteractionEnabled = true
    }
    
    convenience init(defaultTexture: SKTexture, size: CGSize) {
        let color = UIColor.clearColor()
        self.init(defaultTexture: defaultTexture, color:color, size: size)
    }
    
    convenience init(defaultTexture: SKTexture) {
        let color = UIColor.clearColor()
        let size = defaultTexture.size()
        self.init(defaultTexture: defaultTexture, color:color, size: size)
    }
    
    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
        super.touchesBegan(touches, withEvent: event)
        isHighlighted = true
    }
    
    override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
        super.touchesEnded(touches, withEvent: event)
        isHighlighted = false
        
        if containsTouches(touches) {
            buttonTriggered()
        }
    }
    
    override func touchesCancelled(touches: Set<UITouch>?, withEvent event: UIEvent?) {
        super.touchesCancelled(touches, withEvent: event)
        isHighlighted = false
    }
    
    private func containsTouches(touches: Set<UITouch>) -> Bool {
        guard let scene = scene else { fatalError("Button must be used within a scene.") }
        
        return touches.contains { touch in
            let touchPoint = touch.locationInNode(scene)
            let touchedNode = scene.nodeAtPoint(touchPoint)
            return touchedNode === self || touchedNode.inParentHierarchy(self)
        }
    }
    
    func buttonTriggered() {
        if userInteractionEnabled {
            if let responder = self.responder {
                responder.buttonTriggered(self)
            }
        }
    }
}