HowTo: Implement targeting or follow behaviour with SpriteKit and SKConstraint in SWIFT

Welcome to Part 14 of my blog series about iOS game development: SpriteKit and SKConstraint

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 6.3 and SWIFT 1.2!

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:

```swift override func touchesBegan(touches: Set, withEvent event: UIEvent) { /* Called when a touch begins */ for touch in (touches as! Set) { let location = touch.locationInNode(self) let action = SKAction.moveTo(location, duration: 1) sprite.runAction(action) } }

7. Start the project:

The source code can be found in my GitHub repository.

That’s all for today.

Cheers,
Stefan