cannot change material - ios

Question

I added one line of code (where it says //LINE CHANGED) to change the material to a blue color. However, when I load the game, the default ship stays as the normal texture. Any ideas as to why this is happening? I can change the material just fine on a cube I make but for some reason the ships material is not changing.
override func viewDidLoad() {
super.viewDidLoad()
// create a new scene
let scene = SCNScene(named: "art.scnassets/ship.dae")!
// create and add a camera to the scene
let cameraNode = SCNNode()
cameraNode.camera = SCNCamera()
scene.rootNode.addChildNode(cameraNode)
// place the camera
cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)
// create and add a light to the scene
let lightNode = SCNNode()
lightNode.light = SCNLight()
lightNode.light!.type = SCNLightTypeOmni
lightNode.position = SCNVector3(x: 0, y: 10, z: 10)
scene.rootNode.addChildNode(lightNode)
// create and add an ambient light to the scene
let ambientLightNode = SCNNode()
ambientLightNode.light = SCNLight()
ambientLightNode.light!.type = SCNLightTypeAmbient
ambientLightNode.light!.color = UIColor.darkGrayColor()
scene.rootNode.addChildNode(ambientLightNode)
// retrieve the ship node
let ship = scene.rootNode.childNodeWithName("ship", recursively: true)!
//LINE CHANGED
ship.geometry?.firstMaterial?.diffuse.contents = UIColor.blueColor()
//LINE CHANGED
// animate the 3d object\
ship.runAction(SCNAction.repeatActionForever(SCNAction.rotateByX(0, y: 2, z: 0, duration: 1)))
// retrieve the SCNView
let scnView = self.view as SCNView
// set the scene to the view
scnView.scene = scene
// allows the user to manipulate the camera
scnView.allowsCameraControl = true
// show statistics such as fps and timing information
scnView.showsStatistics = true
// configure the view
scnView.backgroundColor = UIColor.blackColor()
// add a tap gesture recognizer
let tapGesture = UITapGestureRecognizer(target: self, action: "handleTap:")
let gestureRecognizers = NSMutableArray()
gestureRecognizers.addObject(tapGesture)
if let existingGestureRecognizers = scnView.gestureRecognizers {
gestureRecognizers.addObjectsFromArray(existingGestureRecognizers)
}
scnView.gestureRecognizers = gestureRecognizers
}

Answer 1

ship.geometry is nil. So the rest of the line will have no effect.
If you inspect the file "ship.dae" in xcode you will see that the node named "ship" has no geometry attached but it has a child node named "shipMesh" that owns the geometry.

Answer 2

Toyos already covered the answer pretty well, but I wanted to add some advice that's too long for a comment...
There's an inherent danger to optional chaining in Swift (or messaging things that can be nil in ObjC, for that matter) — if it fails, it'll do so silently. You might say that's less bad than a crash, but it's harder to debug.
Instead of writing a longChain?.of?.optionalCalls(), consider using optional binding (if let) for parts of it so you can see where the chain is breaking due to a nil value. It's more code, but it lets you check each step of the chain and handle failures in a way that makes the most sense to your app. This can even be a debugging strategy — if you have a line of optional-chained calls that seems to be doing nothing, replace it with an equivalent nesting of if let statements until you find the problem. (Then go back to optional chaining once you've fixed it, if you like.)
Lots of people harsh on implicitly unwrapped optionals, but there are times and places where they can be useful. I'd say this is one of them — the chain of optionals you need to get through is something you can assure at app build time (just not at source compile time), because it's the configuration of a DAE shipped in your app. (This is basically the same rationale as that behind IBOutlets typically being IUOs.) Writing ship.geometry!.firstMaterial!.contents would alert you to the mismatch between what your DAE has and what you think it has, because you'd crash dereferencing geometry.