Craig Armitage Craig Armitage - 3 months ago 34
Swift Question

Casting custom SKSpriteNode class after collision

I've got a couple of custom classes derived from SKSpriteNode and when they collide I want to call a method from the custom "Treat" class but it crashes on impact with the error.. " Unexpectedly found Nil when unwrapping an Optional value"

This is the Treat class...

import UIKit
import SpriteKit

class Treat: SKSpriteNode {

var isActive: Bool!

override init(texture: SKTexture!, color: SKColor, size: CGSize) {
self.isActive = true

super.init(texture: texture, color: color, size: size)

self.texture = SKTexture(imageNamed: "treat1")
self.zPosition = 3
self.size = CGSize(width: 25, height: 25)
self.physicsBody = SKPhysicsBody(rectangleOf: self.size)

self.physicsBody?.friction = 0.1
self.physicsBody?.restitution = 0.8
self.physicsBody?.mass = 0.01
self.physicsBody?.affectedByGravity = true
self.physicsBody?.allowsRotation = true
self.physicsBody?.isDynamic = true
self.physicsBody?.categoryBitMask = PhysicsCategory.Treat
self.physicsBody?.contactTestBitMask = PhysicsCategory.Player
self.physicsBody?.collisionBitMask = PhysicsCategory.Ground
self.physicsBody?.usesPreciseCollisionDetection = true
self.name = "Treat";

let rotateDuration = Double(arc4random()) / 0xFFFFFFFF
let rotateAction = SKAction.rotate(byAngle: CGFloat(M_PI), duration: rotateDuration)
let rotateActionRepeatingForever = SKAction.repeatForever(rotateAction)
self.run(rotateActionRepeatingForever)

}

convenience init(color: SKColor, isActive: Bool = false) {
let size = CGSize(width:0, height: 0);
self.init(texture:nil, color: color, size: size)
self.isActive = isActive
}

required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}

func throwTreat() {
NSLog("Throwing Treat")
let arc4randoMax:Double = 0x100000000
let upper = 2.0
let lower = 4.4
let throwableAngle = Float32((Double(arc4random()) / arc4randoMax) * (upper - lower) + lower)

self.physicsBody?.applyImpulse(CGVector(dx: CGFloat(-throwableAngle),dy: CGFloat(throwableAngle)))
}

func removeTreat(showEffect: Bool) {
if self.isActive != false {
self.isActive = false
if showEffect != false {

}
self.removeFromParent()
}

}

func onImpactWithPlayer() {
self.removeFromParent()
}


}


And this is the physics collision detection..

func didBegin(_ contact: SKPhysicsContact) {

var firstBody: SKPhysicsBody
var secondBody: SKPhysicsBody

if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
firstBody = contact.bodyA
secondBody = contact.bodyB
} else {
firstBody = contact.bodyB
secondBody = contact.bodyA
}

if ((firstBody.categoryBitMask & PhysicsCategory.Player != 0) &&
(secondBody.categoryBitMask & PhysicsCategory.Ground != 0)) {
self.playerNode.isJumping = false
}


if ((firstBody.categoryBitMask & PhysicsCategory.Player != 0) &&
(secondBody.categoryBitMask & PhysicsCategory.Treat != 0)) {
levelScore = levelScore + 1
(secondBody.node as! Treat).onImpactWithPlayer() // This line crashes

}

}


I'm new to swift and Object Oriented Programming so its not making a lot of sense at the moment. I know its a simple fix and I'm being stupid...

Answer

Your code seems mostly correct.

What I believe is happening here

When 2 physics bodies collide, the didBegin(_ contact: SKPhysicsContact) can be called several times over the same frame.

Since here

(secondBody.node as! Treat).onImpactWithPlayer()

you are removing Treat from the scene graph, the next time didBegin(_ contact:) is invoked over the same frame you get a crash because the node has been removed.

Solution

Just replace this

(secondBody.node as! Treat).onImpactWithPlayer()

with this

 (secondBody.node as? Treat)?.onImpactWithPlayer()
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