Detect collision between two objects in Swift

Question

Collision detection is pretty simple in Swift - yet on this particular project, body collision is not triggering the didBeginContact event as usual.

Here is my checklist for two bodies colliding (using soldiers and bullets):

1. Add SKPhysicsContactDelegate to the class.
2. set the physicsWorld appropriately, usually: self.physicsWorld.contactDelegate = self
3. Create categories for each group of nodes you will have. E.g. let bulletCategory = 0x1 << 0
4. Create SKNodes for each bullet and soldier.
5. Give each bullet and soldier a physics body with a matching shape.
6. Set each bullet's categoryBitMask to the bulletCategory (soldiers are set to soldierCategory).
7. Set each bullet's contactBitMask to the soldierCategory (soldiers are set to bulletCategory).
8. Define didBeginContact() handler.

Below is my complete code. It is a minimal ~20 line "Hello World" for collision.

If you create a new "Game" and copy paste this into the GameScene.swift, it will run, just no collision events will be fired.

import SpriteKit

class GameScene: SKScene, SKPhysicsContactDelegate {
    var soldier = SKShapeNode(circleOfRadius: 40)

    let soldierCategory:UInt32 = 0x1 << 0;
    let bulletCategory:UInt32 = 0x1 << 1;

    override func didMoveToView(view: SKView) {
        self.physicsWorld.contactDelegate = self

        // THE soldier
        soldier.fillColor = SKColor.redColor()
        soldier.position = CGPoint(x: CGRectGetMidX(self.frame), y: 40)
        soldier.physicsBody = SKPhysicsBody(circleOfRadius: 20)
        soldier.physicsBody!.dynamic = false
        soldier.physicsBody!.categoryBitMask = soldierCategory
        soldier.physicsBody!.contactTestBitMask = bulletCategory
        soldier.physicsBody!.collisionBitMask = 0


        // bulletS
        var timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("makeBullet"), userInfo: nil, repeats: true)

        self.addChild(soldier)
    }

    func makeBullet() {
        var bullet = SKShapeNode(rect: CGRect(x: CGRectGetMidX(self.frame), y: self.frame.height, width: 10, height: 40), cornerRadius: CGFloat(0))
        bullet.fillColor = SKColor.redColor()

        bullet.physicsBody = SKPhysicsBody(rectangleOfSize: CGSize(width: 10, height: 40))
        bullet.physicsBody?.dynamic = false
        bullet.physicsBody?.categoryBitMask = bulletCategory
        bullet.physicsBody?.contactTestBitMask = soldierCategory
        bullet.physicsBody?.collisionBitMask = soldierCategory

        var movebullet = SKAction.moveByX(0, y: CGFloat(-400), duration: 1)
        var movebulletForever = SKAction.repeatActionForever(movebullet)
        bullet.runAction(movebulletForever)

        self.addChild(bullet)
    }

    func didBeginContact(contact: SKPhysicsContact) {
        print("CONTACT")
    }

    override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
        /* Called when a touch begins */
    }

    override func update(currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */
    }
}

Answer 1

Here are a few items missing from your checklist:

1. One of the physics bodies must be dynamic
2. The physics bodies must be moved by a force/impulse or by setting their velocities
3. The positions of the physics bodies should match their corresponding sprite/shape nodes

For (2), you are moving each bullet by changing its position over time with an SKAction.

For (3), the position of each bullet starts at (0, 0), while the shape is drawn at the top-center of the scene. Since the physics body is centered at the shape node's position, there is a mismatch between the locations of the shape and the physics body; the bullet's physics body never makes contact with the soldier. To resolve this, try

    var bullet = SKShapeNode(rectOfSize: CGSizeMake(10, 40))
    bullet.position = CGPointMake(self.size.width/2.0, self.size.height)

Also, the physics body of the soldier is half the radius of its shape.