Unexpectedly found nil while unwrapping an Optional value - boxNode.addChildNode(importedBox!) - ios

Question

I have some code that runs a for loop to create 2 objects and stuffs them into the boxArray then adds them to the scene. The first time through the code works fine to create one box but during the addBox(num: Int) function to create the second box it crashes with the error in the subject line. Its this line that is responsible "boxNode.addChildNode(importedBox!)" but Im at a loss as to why since it works the first time.
importedSceneBox = SCNScene(named: "art.scnassets/boxObject.dae")
for var i = 0; i <= 1; ++i {
let boxNode = self.addBox(i)
boxArray.append(boxNode)
theScene.rootNode.addChildNode(boxArray[i])
}
func addBox(num: Int) -> SCNNode{
let boxNode = SCNNode()
let importedBox = importedSceneBox.rootNode.childNodeWithName("pCube4", recursively: false)
importedBox?.scale = SCNVector3Make(70, 70, 70)
boxNode.addChildNode(importedBox!)
boxNode.position = SCNVector3Make(5, 100, 3)
let collisionBox = SCNBox(width: 5.0, height: 5.0, length: 5.0, chamferRadius: 0)
boxNode.physicsBody?.physicsShape = SCNPhysicsShape(geometry: collisionBox, options: nil)
boxNode.physicsBody = SCNPhysicsBody.dynamicBody()
boxNode.physicsBody?.mass = 1
boxNode.physicsBody?.restitution = 0.8
boxNode.physicsBody?.damping = 0.5
boxNode.name = "dice" + String(num)
boxNode.physicsBody?.allowsResting = true
return boxNode
}

Answer 1

hi your importedBox is an optional meaning it can have a value or it can be nil.
you are using "!"(IUO implicitly unwrapped optional) behind importedBox which means that your telling the compiler that your sure its not be nil. How ever in this case it WAS nil thus the crash "found nil when unwrapping optional"
So recap importedBox is nil but you marked it with ! so the compiler think it will never have a nil value but a nil was found thus the crash.
to fix this you can use guard or optional binding
Guard example
func addBox(num: Int) -> SCNNode{
let boxNode = SCNNode()
let importedBox = importedSceneBox.rootNode.childNodeWithName("pCube4", recursively: false)
importedBox?.scale = SCNVector3Make(70, 70, 70)
guard let unwrapBox = importedBox else {
//do your error handling here
return SCNNode()
}
boxNode.addChildNode(unwrapBox)
boxNode.position = SCNVector3Make(5, 100, 3)
let collisionBox = SCNBox(width: 5.0, height: 5.0, length: 5.0, chamferRadius: 0)
boxNode.physicsBody?.physicsShape = SCNPhysicsShape(geometry: collisionBox, options: nil)
boxNode.physicsBody = SCNPhysicsBody.dynamicBody()
boxNode.physicsBody?.mass = 1
boxNode.physicsBody?.restitution = 0.8
boxNode.physicsBody?.damping = 0.5
boxNode.name = "dice" + String(num)
boxNode.physicsBody?.allowsResting = true
return boxNode
}
optional binding example
func addBox(num: Int) -> SCNNode{
let boxNode = SCNNode()
let importedBox = importedSceneBox.rootNode.childNodeWithName("pCube4", recursively: false)
importedBox?.scale = SCNVector3Make(70, 70, 70)
if let unwrapBox = importedBox else {
boxNode.addChildNode(unwrapBox)
boxNode.position = SCNVector3Make(5, 100, 3)
let collisionBox = SCNBox(width: 5.0, height: 5.0, length: 5.0, chamferRadius: 0)
boxNode.physicsBody?.physicsShape = SCNPhysicsShape(geometry: collisionBox, options: nil)
boxNode.physicsBody = SCNPhysicsBody.dynamicBody()
boxNode.physicsBody?.mass = 1
boxNode.physicsBody?.restitution = 0.8
boxNode.physicsBody?.damping = 0.5
boxNode.name = "dice" + String(num)
boxNode.physicsBody?.allowsResting = true
return boxNode
}else{
//do your error handling here
return SCNNode()
}
}
also as a side note in the error handling cases we are creating a default value since your function returns a SCNNode and not a SCNNode optional.
Alternatively you can return nil if you change your function signature to be
func addBox(num: Int) -> SCNNode? {
if let unwrappedBox = importedBox {
//do stuff
}else{
return nil
}
}