How to detect different types of collisions in SpriteKit - ios

Question

I have two different types of collisions I want to detect in SpriteKit:
When ballOne hits goalOne and when ballTwo hits goalTwo:
So far I've added my different physics categories:
struct physicsCategory {
static let ballOne :UInt32 = 0x1 << 0
static let goalOne :UInt32 = 0x1 << 1
static let ballTwo :UInt32 = 0x1 << 2
static let goalTwo :UInt32 = 0x1 << 3
}
And here's the relevant parts of my class:
class gameScene: SKScene, SKPhysicsContactDelegate {
var borderBody:SKPhysicsBody!
var myscore:Int = 0
var opponentScore:Int = 10
override func sceneDidLoad() {
super.sceneDidLoad()
physicsWorld.contactDelegate = self
}
func didBegin(_ contact: SKPhysicsContact) {
let firstBody = contact.bodyA.node as! SKSpriteNode
let secondBody = contact.bodyB.node as! SKSpriteNode
if ((firstBody.name == "ballOne") && (secondBody.name == "goalOne")) {
collisionGoalOne(BallOne: firstBody, GoalOne: secondBody)
}
else {
}
}
func collisionGoalOne(BallOne: SKSpriteNode, GoalOne: SKSpriteNode) {
addScore()
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
let touch = touches.first
let touchLocation = touch!.location(in: self)
let ballOne = childNode(withName: "ballOne") as! SKSpriteNode
ballOne.physicsBody?.categoryBitMask = physicsCategory.ballOne
ballOne.physicsBody?.collisionBitMask = physicsCategory.goalOne
ballOne.physicsBody?.contactTestBitMask = physicsCategory.goalOne
ballOne.physicsBody?.isDynamic = true
let goalOne = childNode(withName: "goalOne") as! SKSpriteNode
goalOne.physicsBody?.categoryBitMask = physicsCategory.goalOne
goalOne.physicsBody?.collisionBitMask = physicsCategory.ballOne
goalOne.physicsBody?.contactTestBitMask = physicsCategory.ballOne
goalOne.physicsBody?.isDynamic = false
}
}
Right now the app crashes when the ball collides with the wall on
let firstBody = contact.bodyA.node as! SKSpriteNode
with "Could not cast value of type 'appName.gameScene' (0x105320) to 'SKSpriteNode' (0x1877b5c)."
Would love to understand why this crash is happening and if this is even a good approach to collision handling. Thank you.

Answer 1

FirstBody might not be able to be cast to SKSpriteNode, so only create that variable after you check which nodes collide.
Note that ballOne could be either bodyA or bodyB so change your code to check for both possibilities.
I changed your code a bit. Hope this helps
if ((contact.bodyA.node?.name == "ballOne") && (contact.bodyB.node?.name == "goalOne")) {
let firstBody = contact.bodyA.node as! SKSpriteNode
let secondBody = contact.bodyB.node as! SKSpriteNode
collisionGoalOne(BallOne: firstBody, GoalOne: secondBody)
} else if ((contact.bodyA.node?.name == "goalOne") && (contact.bodyB.node?.name == "ballOne")) {
let firstBody = contact.bodyA.node as! SKSpriteNode
let secondBody = contact.bodyB.node as! SKSpriteNode
collisionGoalOne(BallOne: secondBody, GoalOne: firstBody)
}

Answer 2

I think there are a couple of changes you might want to make.
When your ball node contacts the wall, I believe your didBegin(_ contact: SKPhysicsContact) method is invoked with one body as the ball (SKNode) and the other body as the scene border (SKScene).
Forcing a cast of SKScene to SKSpriteNode with let firstBody = contact.bodyA.node as! SKSpriteNode crashes, as RT5754 points out.
To fix this you might consider the following:
Add a physics category for the wall/border. Something like:
static let sceneBorder :UInt32 = 0x1 << 4
Update the border and the ball nodes to have collision physics:
ballNode.physicsBody?.collisionBitMask = physicsCategory.sceneBorder
borderNode.physicsBody?.collisionBitMask = physicsCategory.ballOne + physicsCategory.ballTwo
When handling other contact logic, my pattern for
didBegin(_ contact: SKPhysicsContact)
is like this:
func didBegin(_ contact: SKPhysicsContact) {
var firstBody: SKPhysicsBody = contact.bodyA
var secondBody: SKPhysicsBody = contact.bodyB
//in did begin contact, order the bodies by physics category
if firstBody.categoryBitMask > secondBody.categoryBitMask {
let temp = secondBody
secondBody = firstBody
firstBody = temp
}
//logic to handle contact is based on physicsCategory
if secondBody.categoryBitMask == physicsCategory.whatever {
methodToHandleWhatever(thingOne: firstBody, thingTwo: secondBody)
}