HowTo: Implement targeting or follow behavior for sprites with SpriteKit and SKConstraint in SWIFT

Welcome to Part 14 of my blog series about iOS game development.

At the developer conference WWDC, in June this year, Apple showed a new class in SpriteKit: SKConstraint. It can be used to define constraints for the orientation, the distance or the position of SpriteKit nodes. In my todays blog post I'll show how to use SKContraints to implement a follow and targeting behavior. I have updated this tutorial to XCode 6.1!

1. Create a new Project

2. Open the GameScene.swift file and remove the code for creating a label in didMoveToView:

    override func didMoveToView(view: SKView) {

        /* Setup your scene here */

    }

3. Create a global Sprite property:

    var sprite = SKSpriteNode()

    

    override func didMoveToView(view: SKView) {

        /* Setup your scene here */

    }

4. Create two sprites inside didMoveToView:

       self.backgroundColor = UIColor.blackColor()

        

        // Create the followed sprite

        sprite = SKSpriteNode(imageNamed:"Spaceship")

        sprite.xScale = 0.15

        sprite.yScale = 0.15

        sprite.position = CGPointMake(100, 100)

        self.addChild(sprite)

        

        // Create the follower sprite

        let followerSprite = SKSpriteNode(imageNamed:"Spaceship")

        followerSprite.xScale = 0.15

        followerSprite.yScale = 0.15

        followerSprite.position = CGPointMake(150, 150)

        followerSprite.color = UIColor.redColor()

        followerSprite.colorBlendFactor=0.8

        self.addChild(followerSprite)

5. Create two constraints inside didMoveToView: 

One to specify the distance between the two Sprites to implement the follow behavior:

        // Define Constraints for following behavior

        let rangeToSprite = SKRange(lowerLimit: 100.0, upperLimit: 150.0)

        let distanceConstraint = SKConstraint.distance(rangeToSprite, toNode: sprite)

One to specify the orientation of the follower Sprite to the followed Sprite: 

        // Define Constraints for orientation/targeting behavior

   let rangeForOrientation = SKRange(lowerLimit: CGFloat(M_2_PI*7), upperLimit: CGFloat(M_2_PI*7))

        let orientConstraint = SKConstraint.orientToNode(sprite, offset: rangeForOrientation)

Add the constraints to the follower Sprite:

        // Add constraints

        followerSprite.constraints = [orientConstraint, distanceConstraint]

6. Replace the code inside the touchesBegan method to implement the sprite movement:

    override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {

        /* Called when a touch begins */

        

        for touch: AnyObject in touches {

            let location = touch.locationInNode(self)

            let action = SKAction.moveTo(location, duration: 1)

            sprite.runAction(action)

        }

    }

That's all for today.

Cheers,
Stefan

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