iOS

Control Sprites with SKAction and SKConstraint


How to implement a space shooter with SpriteKit and SWIFT – Part 2:

Adding enemies, bullets and shooting with SKAction and SKConstraint

 

Tutorial Overview: How to implement a space shooter with SpriteKit and SWIFT

  • Part 1: Initial project setup, sprite creation and movement using SKAction and SKConstraint
  • Part 2: Adding enemies, bullets and shooting with SKAction and SKConstraint
  • Part 3: Adding a HUD with SKLabelNode and SKSpriteNode
  • Part 4: Adding basic game logic and collision detection
  • Part 5: Adding particles and sound 
  • Part 6: GameCenter integration
  • Part 7: iAd integration
  • Part 8: In-App Purchases

Add the enemies:

I’ll add several enemy sprites. These will automatically follow and orient to the hero sprite.  You can download the code from Part 1 here.
part2-1
 
 

1. Add a new class EnemySpriteController:

part2-2
 
 
 
part2-3

2. Import SpriteKit, define the class and add an array which stores all enemies:

import SpriteKit
 
// Controller class for:
// – creating/destroying enemies,
// – shooting
// – animitaions
class EnemySpriteController {
var enemySprites: [SKSpriteNode] = []
 
}

3. Add a new method spawnEnemy to EnemySpriteController:

Nothing magic here. Just create a SKSpriteNode and add it to the enemy collection. Targeting and orientation behavior is implemented with SKConstraints. For details check my post: HowTo: Implement targeting or follow behavior for sprites with SpriteKit and SKConstraint.
 
    // Return a new enemy sprite which follows the targetSprite node
func spawnEnemy(targetSprite: SKNode) -> SKSpriteNode {
 
        // create a new enemy sprite
let newEnemy = SKSpriteNode(imageNamed:„Spaceship“)
enemySprites.append(newEnemy)
        newEnemy.xScale = 0.08
        newEnemy.yScale = 0.08
        newEnemy.color = UIColor.redColor()
        newEnemy.colorBlendFactor=0.4
 
        // position new sprite at a random position on the screen
        var sizeRect = UIScreen.mainScreen().applicationFrame;
        var posX = arc4random_uniform(UInt32(sizeRect.size.width))
        var posY = arc4random_uniform(UInt32(sizeRect.size.height))
        newEnemy.position = CGPoint(x: CGFloat(posX), y: CGFloat(posY))
 
        // Define Constraints for orientation/targeting behavior
        let i = enemySprites.count-1
let rangeForOrientation = SKRange(constantValue:CGFloat(M_2_PI*7))
let orientConstraint = SKConstraint.orientToNode(targetSprite, offset: rangeForOrientation)
let rangeToSprite = SKRange(lowerLimit: 80, upperLimit: 90)
var distanceConstraint: SKConstraint
 
        // First enemy has to follow spriteToFollow, second enemy has to follow first enemy, …
if enemySprites.count-1 == 0 {
            distanceConstraint = SKConstraint.distance(rangeToSprite, toNode: targetSprite)
        } else {
            distanceConstraint = SKConstraint.distance(rangeToSprite, toNode: enemySprites[i-1])
        }
        newEnemy.constraints = [orientConstraint, distanceConstraint]
 
return newEnemy
    }

4. Create a property for the EnemySpriteController object inside GameScene.swift:

var enemySprites = EnemySpriteController()

5. Create some enemies at the end of didMoveToView method inside GameScene.swift:

// Add enemy sprites
for(var i=0; i<3;i++){
self.addChild(enemySprites.spawnEnemy(heroSprite))
}

Result are three red enemy sprites which will follow the white spaceship. Next steps are adding bullets and shooting.

 
 SKConstraint Tutorial 1
 

6. Add a shoot method inside EnemySpriteController.swift:

The shoot method iterates over each enemy sprite, creates a bullet, determines a vector to the target object and starts a SKAction which moves the bullet.
 
 
// Shoot in direction of spriteToShoot
func shoot(targetSprite: SKNode) {
 
for enemy in enemySprites {
 
// Create the bullet sprite
let bullet = SKSpriteNode()
    bullet.color = UIColor.greenColor()
    bullet.size = CGSize(width: 5,height: 5)
    bullet.position = CGPointMake(enemy.position.x, enemy.position.y)
    targetSprite.parent?.addChild(bullet)
 
// Determine vector to targetSprite
let vector = CGVectorMake((targetSprite.position.x-enemy.position.x), targetSprite.position.y-enemy.position.y)
 
// Create the action to move the bullet. Don’t forget to remove the bullet!
let bulletAction = SKAction.sequence([SKAction.repeatAction(SKAction.moveBy(vector, duration: 1), count: 10) ,  SKAction.waitForDuration(30.0/60.0), SKAction.removeFromParent()])
    bullet.runAction(bulletAction)
 
  }
}
 

7. Call shoot inside the update method of GameScene.swift:

SpriteKit cannot guarantee in which time intervals the update method is called. To ensure that the enemies shoot every second, I’ll store the time interval when shoot was called in a global property.  
 
var _dLastShootTime: CFTimeInterval = 1
 
override func update(currentTime: CFTimeInterval) {
  /* Called before each frame is rendered */
 
if currentTime – _dLastShootTime >= 1 {
enemySprites.shoot(heroSprite)
_dLastShootTime=currentTime
  }
}
SKConstraint Tutorial
 

That’s all for today. In my next part I’ll add a HUD, implement a basic game loging and add collision detection. You can download the code from GitHub: Part 2 or the latest version here. You can also download my prototyping App for this tutorial series:

 
 

Cheers,

Stefan

 
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