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

Adding enemies, bullets and shooting with SKAction and SKConstraint

Tutorial Overview:

• 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.

1. Add a new class EnemySpriteController:

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.

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

  }

}

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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