SKPhysicsContact body, can't change properties

Question

I have a SpriteKit physics contact firing in didBegin(contact:). I grab the physics body for the instance of the Dot object that I want to move offscreen, but when I try to change its position like so, nothing happens:

First Approach

/* In GameScene.swift */

func didBegin(_ contact: SKPhysicsContact) {
    let dotBody: SKPhysicsBody
    if contact.bodyA.categoryBitMask == 0b1 {
        dotBody = contact.bodyB
    } else {
        dotBody = contact.bodyA
    }

    if let dot = dotBody.node as? Dot {
        dot.position.x = 10000
    }
}

However, if I instead call a method in my Dot class, passing in that body, the position gets set correctly:

Second Approach

/* In GameScene.swift */

func didBegin(_ contact: SKPhysicsContact) {
    let dotBody: SKPhysicsBody
    if contact.bodyA.categoryBitMask == 0b1 {
        dotBody = contact.bodyB
    } else {
        dotBody = contact.bodyA
    }

    if let dot = dotBody.node as? Dot {
        dot.move()
    }        
}

Here is the Dot class:

import SpriteKit
class Dot : SKSpriteNode {
    let dotTex = SKTexture(imageNamed: "dot")
    init() {
        super.init(texture: dotTex, color: .clear, size: dotTex.size())
        self.physicsBody = SKPhysicsBody(circleOfRadius: size.width / 2)
        self.physicsBody?.categoryBitMask = 0b1 << 1
        self.physicsBody?.contactTestBitMask = 0b1
    }

    func move() {
        let reset = SKAction.run {
            self.position.x = 10000
        }
        self.run(reset)
    }

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

Can anyone explain why the position change in the second approach works but it does not work in my first approach?

Here's a link to the project on Github.

Answer 1

func didBegin(_ contact: SKPhysicsContact) {
  let hitDot = contact.bodyB.node! as! SKSpriteNode
  let hitDot1 = contact.bodyA.node! as! SKSpriteNode
    if hitDot.name? == "dot" || hitDot1.name? == "dot" {
       reset = true
    }        
}

   override func update(_ currentTime: TimeInterval) {
    if reset {
        newDot.position.x = 0
        reset = false
    }
}

There might be small errors in the code coz i just edited here. But hope this gives u the idea. This should work without errors.

Answer 2

EDIT:

I think it happens because u are creating a subclass of SKSpriteNode rather than creating instance of a SKSpriteNode. Unless u are going to specialize it for something else I don't see a reason to do that. Instead I used these

import Foundation
import SpriteKit

class Dot {

let dotSprite: SKSpriteNode

init() {
    dotSprite = SKSpriteNode(imageNamed: "Spaceship")
//        dotSprite.physicsBody = SKPhysicsBody(circleOfRadius: dotSprite.size.width/2)
//        dotSprite.physicsBody?.categoryBitMask = 0b1 << 1
//        dotSprite.physicsBody?.contactTestBitMask = 0b1

}

func move() {
    self.dotSprite.position.x = 0
}

func moveAnim(){
    let reset = SKAction.moveTo(x: 0, duration: 3)
    dotSprite.run(reset)
}

}

And in my SKScene

import SpriteKit
import GameplayKit

class GameScene: SKScene {

let newDot = Dot()


override func didMove(to view: SKView) {

    self.addChild(newDot.dotSprite)
    newDot.dotSprite.setScale(0.5)
    newDot.dotSprite.position = CGPoint(x: frame.midX, y: frame.midY)

}


override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {

    newDot.dotSprite.position.x = 200
    newDot.moveAnim()

}

}

Note that I have disable physics body coz I haven't created another body for it to land on (so it would keep on falling). Also I used the Spaceship as a sprite.

So now u can have any custom methods in the dot class. Also u can manipulate directly the properties of the dot SpriteNode. Also create multiple instances. And now you can change it directly when the contact happens or call a method in the dot class to move. Hope this was helpful. :)