Smart KeyPaths in Swift 4 not working properly - swift

Question

I am trying to create an custom action block for an SKSpriteNode, I have the following code:
let sprite = SKSpriteNode(color: SKColor.red, size: CGSize(width: 50, height: 50))
sprite.position = CGPoint(x: 320, y: 240)
self.addChild(sprite)
let animation = SKAction.customAction(withDuration: 0, actionBlock: {
(node, elapsedTime) in
var initialValue : CGFloat?
initialValue = node[keyPath: \SKSpriteNode.position.x] //Extraneous argument label 'keyPath:' in subscript
node[keyPath: \SKSpriteNode.position.x] = 10 //Ambiguous reference to member 'subscript'
})
sprite.run(animation)
I am getting two errors, the first is that the compiler is thinking I have an Extraneous argument of 'keyPath', which is not the case, because if I were to remove it like it suggests, I get this error:
Cannot convert value of type 'ReferenceWritableKeyPath' to expected argument type 'String'
The second error I get is:
Ambiguous reference to member 'subscript'
I am not exactly sure why I am getting all of these errors, and I am not sure exactly what the errors mean. If somebody could explain them to me and propose a solution, that would be great. Thanks in advance.

Answer 1

The keyPath isn't working because node has type SKNode and not SKSpriteNode. You can use a conditional cast to establish that the node is an SKSpriteNode:
let animation = SKAction.customAction(withDuration: 0, actionBlock: {
(node, elapsedTime) in
var initialValue : CGFloat?
if let spritenode = node as? SKSpriteNode {
initialValue = spritenode[keyPath: \SKSpriteNode.position.x]
spritenode[keyPath: \SKSpriteNode.position.x] = 10
}
})