axiom-spritekit

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Translated

Use when building SpriteKit games, implementing physics, actions, scene management, or debugging game performance. Covers scene graph, physics engine, actions system, game loop, rendering optimization.

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Sourcecharleswiltgen/axiom

Added on2026-02-07

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SKILL.md Content

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SpriteKit Game Development Guide

Purpose: Build reliable SpriteKit games by mastering the scene graph, physics engine, action system, and rendering pipeline iOS Version: iOS 13+ (SwiftUI integration), iOS 11+ (SKRenderer) Xcode: Xcode 15+

When to Use This Skill

Use this skill when:

Building a new SpriteKit game or interactive simulation
Implementing physics (collisions, contacts, forces, joints)
Setting up game architecture (scenes, layers, cameras)
Optimizing frame rate or reducing draw calls
Implementing touch/input handling in a game
Managing scene transitions and data passing
Integrating SpriteKit with SwiftUI or Metal
Debugging physics contacts that don't fire
Fixing coordinate system confusion

Do NOT use this skill for:

SceneKit 3D rendering (
```
axiom-scenekit
```
)
GameplayKit entity-component systems
Metal shader programming (
```
axiom-metal-migration-ref
```
)
General SwiftUI layout (
```
axiom-swiftui-layout
```
)

1. Mental Model

Coordinate System

SpriteKit uses a bottom-left origin with Y pointing up. This differs from UIKit (top-left, Y down).

SpriteKit:          UIKit:
┌─────────┐         ┌─────────┐
│    +Y    │         │  (0,0)  │
│    ↑     │         │    ↓    │
│    │     │         │    +Y   │
│(0,0)──→+X│        │    │    │
└─────────┘         └─────────┘

Anchor Points define which point on a sprite maps to its

position

. Default is

(0.5, 0.5)

(center).

swift

// Common anchor point trap:
// Anchor (0, 0) = bottom-left of sprite is at position
// Anchor (0.5, 0.5) = center of sprite is at position (DEFAULT)
// Anchor (0.5, 0) = bottom-center (useful for characters standing on ground)
sprite.anchorPoint = CGPoint(x: 0.5, y: 0)

Scene anchor point maps the view's frame to scene coordinates:

```
(0, 0)
```
— scene origin at bottom-left of view (default)
```
(0.5, 0.5)
```
— scene origin at center of view

Node Tree

Everything in SpriteKit is an

SKNode

in a tree hierarchy. Parent transforms propagate to children.

SKScene
├── SKCameraNode (viewport control)
├── SKNode "world" (game content layer)
│   ├── SKSpriteNode "player"
│   ├── SKSpriteNode "enemy"
│   └── SKNode "platforms"
│       ├── SKSpriteNode "platform1"
│       └── SKSpriteNode "platform2"
└── SKNode "hud" (UI layer, attached to camera)
    ├── SKLabelNode "score"
    └── SKSpriteNode "healthBar"

Z-Ordering

zPosition

controls draw order. Higher values render on top. Nodes at the same

zPosition

render in child array order (unless

ignoresSiblingOrder

true

swift

// Establish clear z-order layers
enum ZLayer {
    static let background: CGFloat = -100
    static let platforms: CGFloat = 0
    static let items: CGFloat = 10
    static let player: CGFloat = 20
    static let effects: CGFloat = 30
    static let hud: CGFloat = 100
}

2. Scene Architecture

Scale Mode Decision

Mode	Behavior	Use When
`.aspectFill`	Fills view, crops edges	Full-bleed games (most games)
`.aspectFit`	Fits in view, letterboxes	Puzzle games needing exact layout
`.resizeFill`	Stretches to fill	Almost never — distorts
`.fill`	Matches view size exactly	Scene adapts to any ratio

swift

class GameScene: SKScene {
    override func sceneDidLoad() {
        scaleMode = .aspectFill
        // Design for a reference size, let aspectFill crop edges
    }
}

Camera Node Pattern

Always use

SKCameraNode

for viewport control. Attach HUD elements to the camera so they don't scroll.

swift

let camera = SKCameraNode()
camera.name = "mainCamera"
addChild(camera)
self.camera = camera

// HUD follows camera automatically
let scoreLabel = SKLabelNode(text: "Score: 0")
scoreLabel.position = CGPoint(x: 0, y: size.height / 2 - 50)
camera.addChild(scoreLabel)

// Move camera to follow player
let follow = SKConstraint.distance(SKRange(constantValue: 0), to: playerNode)
camera.constraints = [follow]

Layer Organization

swift

// Create layer nodes for organization
let worldNode = SKNode()
worldNode.name = "world"
addChild(worldNode)

let hudNode = SKNode()
hudNode.name = "hud"
camera?.addChild(hudNode)

// All gameplay objects go in worldNode
worldNode.addChild(playerSprite)
worldNode.addChild(enemySprite)

// All UI goes in hudNode (moves with camera)
hudNode.addChild(scoreLabel)

Scene Transitions

swift

// Preload next scene for smooth transitions
guard let nextScene = LevelScene(fileNamed: "Level2") else { return }
nextScene.scaleMode = .aspectFill

let transition = SKTransition.fade(withDuration: 0.5)
view?.presentScene(nextScene, transition: transition)

Data passing between scenes: Use a shared game state object, not node properties.

swift

class GameState {
    static let shared = GameState()
    var score = 0
    var currentLevel = 1
    var playerHealth = 100
}

// In scene transition:
let nextScene = LevelScene(size: size)
// GameState.shared is already accessible
view?.presentScene(nextScene, transition: .fade(withDuration: 0.5))

Note: A singleton works for simple games. For larger projects with testing needs, consider passing a

GameState

instance through scene initializers to avoid hidden global state.

Cleanup in
willMove(from:)
:

swift

override func willMove(from view: SKView) {
    removeAllActions()
    removeAllChildren()
    physicsWorld.contactDelegate = nil
}

3. Physics Engine

Bitmask Discipline

This is the #1 source of SpriteKit bugs. Physics bitmasks use a 32-bit system where each bit represents a category.

swift

struct PhysicsCategory {
    static let none:       UInt32 = 0
    static let player:     UInt32 = 0b0001  // 1
    static let enemy:      UInt32 = 0b0010  // 2
    static let ground:     UInt32 = 0b0100  // 4
    static let projectile: UInt32 = 0b1000  // 8
    static let powerUp:    UInt32 = 0b10000 // 16
}

Three bitmask properties (all default to

0xFFFFFFFF

— everything):

Property	Purpose	Default
`categoryBitMask`	What this body IS	`0xFFFFFFFF`
`collisionBitMask`	What it BOUNCES off	`0xFFFFFFFF`
`contactTestBitMask`	What TRIGGERS delegate	`0x00000000`

The default
collisionBitMask
of
0xFFFFFFFF
means everything collides with everything. This is the most common source of unexpected physics behavior.

swift

// CORRECT: Explicit bitmask setup
player.physicsBody?.categoryBitMask = PhysicsCategory.player
player.physicsBody?.collisionBitMask = PhysicsCategory.ground | PhysicsCategory.enemy
player.physicsBody?.contactTestBitMask = PhysicsCategory.enemy | PhysicsCategory.powerUp

enemy.physicsBody?.categoryBitMask = PhysicsCategory.enemy
enemy.physicsBody?.collisionBitMask = PhysicsCategory.ground | PhysicsCategory.player
enemy.physicsBody?.contactTestBitMask = PhysicsCategory.player | PhysicsCategory.projectile

Bitmask Checklist

For every physics body, verify:

```
categoryBitMask
```
set to exactly one category
```
collisionBitMask
```
set to only categories it should bounce off (NOT
```
0xFFFFFFFF
```
)
```
contactTestBitMask
```
set to categories that should trigger delegate callbacks
Delegate is assigned:
```
physicsWorld.contactDelegate = self
```

Contact Detection

swift

class GameScene: SKScene, SKPhysicsContactDelegate {
    override func didMove(to view: SKView) {
        physicsWorld.contactDelegate = self
    }

    func didBegin(_ contact: SKPhysicsContact) {
        // Sort bodies so bodyA has the lower category
        let (first, second): (SKPhysicsBody, SKPhysicsBody)
        if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
            (first, second) = (contact.bodyA, contact.bodyB)
        } else {
            (first, second) = (contact.bodyB, contact.bodyA)
        }

        // Now dispatch based on categories
        if first.categoryBitMask == PhysicsCategory.player &&
           second.categoryBitMask == PhysicsCategory.enemy {
            guard let playerNode = first.node, let enemyNode = second.node else { return }
            playerHitEnemy(player: playerNode, enemy: enemyNode)
        }
    }
}

Modification rule: You cannot modify the physics world inside

didBegin

didEnd

. Set flags and apply changes in

update(_:)

swift

var enemiesToRemove: [SKNode] = []

func didBegin(_ contact: SKPhysicsContact) {
    // Flag for removal — don't remove here
    if let enemy = contact.bodyB.node {
        enemiesToRemove.append(enemy)
    }
}

override func update(_ currentTime: TimeInterval) {
    for enemy in enemiesToRemove {
        enemy.removeFromParent()
    }
    enemiesToRemove.removeAll()
}

Body Types

Type	Created With	Responds to Forces	Use For
Dynamic volume	`init(circleOfRadius:)` , `init(rectangleOf:)` , `init(texture:size:)`	Yes	Players, enemies, projectiles
Static volume	Dynamic body + `isDynamic = false`	No (but collides)	Platforms, walls
Edge	`init(edgeLoopFrom:)` , `init(edgeFrom:to:)`	No (boundary only)	Screen boundaries, terrain

swift

// Screen boundary using edge loop
physicsBody = SKPhysicsBody(edgeLoopFrom: frame)

// Texture-based body for irregular shapes
guard let texture = enemy.texture else { return }
enemy.physicsBody = SKPhysicsBody(texture: texture, size: enemy.size)

// Circle for performance (cheapest collision detection)
bullet.physicsBody = SKPhysicsBody(circleOfRadius: 5)

Tunneling Prevention

Fast-moving objects can pass through thin walls. Fix:

swift

// Enable precise collision detection for fast objects
bullet.physicsBody?.usesPreciseCollisionDetection = true

// Make walls thick enough (at least as wide as fastest object moves per frame)
// At 60fps, an object at velocity 600pt/s moves 10pt/frame

Forces vs Impulses

swift

// Force: continuous (applied per frame, accumulates)
body.applyForce(CGVector(dx: 0, dy: 100))

// Impulse: instant velocity change (one-time, like a jump)
body.applyImpulse(CGVector(dx: 0, dy: 50))

// Torque: continuous rotation
body.applyTorque(0.5)

// Angular impulse: instant rotation change
body.applyAngularImpulse(1.0)

4. Actions System

Core Patterns

swift

// Movement
let move = SKAction.move(to: CGPoint(x: 200, y: 300), duration: 1.0)
let moveBy = SKAction.moveBy(x: 100, y: 0, duration: 0.5)

// Rotation
let rotate = SKAction.rotate(byAngle: .pi * 2, duration: 1.0)

// Scale
let scale = SKAction.scale(to: 2.0, duration: 0.3)

// Fade
let fadeOut = SKAction.fadeOut(withDuration: 0.5)
let fadeIn = SKAction.fadeIn(withDuration: 0.5)

Sequencing and Grouping

swift

// Sequence: one after another
let moveAndFade = SKAction.sequence([
    SKAction.move(to: target, duration: 1.0),
    SKAction.fadeOut(withDuration: 0.3),
    SKAction.removeFromParent()
])

// Group: all at once
let spinAndGrow = SKAction.group([
    SKAction.rotate(byAngle: .pi * 2, duration: 1.0),
    SKAction.scale(to: 2.0, duration: 1.0)
])

// Repeat
let pulse = SKAction.repeatForever(SKAction.sequence([
    SKAction.scale(to: 1.2, duration: 0.3),
    SKAction.scale(to: 1.0, duration: 0.3)
]))

Named Actions (Critical for Management)

swift

// Use named actions so you can cancel/replace them
node.run(pulse, withKey: "pulse")

// Later, stop the pulse:
node.removeAction(forKey: "pulse")

// Check if running:
if node.action(forKey: "pulse") != nil {
    // Still pulsing
}

Custom Actions with Weak Self

swift

// WRONG: Retain cycle risk
node.run(SKAction.run {
    self.score += 1  // Strong capture of self
})

// CORRECT: Weak capture
node.run(SKAction.run { [weak self] in
    self?.score += 1
})

// For repeating actions, always use weak self
let spawn = SKAction.repeatForever(SKAction.sequence([
    SKAction.run { [weak self] in self?.spawnEnemy() },
    SKAction.wait(forDuration: 2.0)
]))
scene.run(spawn, withKey: "enemySpawner")

Timing Modes

swift

action.timingMode = .linear     // Constant speed (default)
action.timingMode = .easeIn     // Accelerate from rest
action.timingMode = .easeOut    // Decelerate to rest
action.timingMode = .easeInEaseOut  // Smooth start and end

Actions vs Physics

Never use actions to move physics-controlled nodes. Actions override the physics simulation, causing jittering and missed collisions.

swift

// WRONG: Action fights physics
playerNode.run(SKAction.moveTo(x: 200, duration: 0.5))

// CORRECT: Use forces/impulses for physics bodies
playerNode.physicsBody?.applyImpulse(CGVector(dx: 50, dy: 0))

// CORRECT: Use actions for non-physics nodes (UI, effects, decorations)
hudLabel.run(SKAction.scale(to: 1.5, duration: 0.2))

5. Input Handling

Touch Handling

swift

// CRITICAL: isUserInteractionEnabled must be true on the responding node
// SKScene has it true by default; other nodes default to false

class Player: SKSpriteNode {
    init() {
        super.init(texture: SKTexture(imageNamed: "player"), color: .clear, size: CGSize(width: 50, height: 50))
        isUserInteractionEnabled = true  // Required!
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        // Handle touch on this specific node
    }
}

Coordinate Space Conversion

swift

// Touch location in SCENE coordinates (most common)
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    guard let touch = touches.first else { return }
    let locationInScene = touch.location(in: self)

    // Touch location in a SPECIFIC NODE's coordinates
    let locationInWorld = touch.location(in: worldNode)

    // Hit test: what node was touched?
    let touchedNodes = nodes(at: locationInScene)
}

Common mistake: Using

touch.location(in: self.view)

returns UIKit coordinates (Y-flipped). Always use

touch.location(in: self)

for scene coordinates.

Game Controller Support

swift

import GameController

func setupControllers() {
    NotificationCenter.default.addObserver(
        self, selector: #selector(controllerConnected),
        name: .GCControllerDidConnect, object: nil
    )

    // Check already-connected controllers
    for controller in GCController.controllers() {
        configureController(controller)
    }
}

6. Performance

Performance Priorities

For detailed performance diagnosis, see

axiom-spritekit-diag

Symptom 3. Key priorities:

Node count — Remove offscreen nodes, use object pooling
Draw calls — Use texture atlases, replace SKShapeNode with pre-rendered textures
Physics cost — Prefer simple body shapes, limit
```
usesPreciseCollisionDetection
```
Particles — Limit birth rate, set finite emission counts

Debug Overlays (Always Enable During Development)

swift

if let view = self.view as? SKView {
    view.showsFPS = true
    view.showsNodeCount = true
    view.showsDrawCount = true
    view.showsPhysics = true  // Shows physics body outlines

    // Performance: render order optimization
    view.ignoresSiblingOrder = true
}

Texture Atlas Batching

Sprites using textures from the same atlas render in a single draw call.

swift

// Create atlas in Xcode: Assets → New Sprite Atlas
// Or use .atlas folder in project

let atlas = SKTextureAtlas(named: "Characters")
let texture = atlas.textureNamed("player_idle")
let sprite = SKSpriteNode(texture: texture)

// Preload atlas to avoid frame drops
SKTextureAtlas.preloadTextureAtlases([atlas]) {
    // Atlas ready — present scene
}

SKShapeNode Trap

SKShapeNode generates one draw call per instance. It cannot be batched. Use it for prototyping and debug visualization only.

swift

// WRONG: 100 SKShapeNodes = 100 draw calls
for _ in 0..<100 {
    let dot = SKShapeNode(circleOfRadius: 5)
    addChild(dot)
}

// CORRECT: Pre-render to texture, use SKSpriteNode
let shape = SKShapeNode(circleOfRadius: 5)
shape.fillColor = .red
guard let texture = view?.texture(from: shape) else { return }
for _ in 0..<100 {
    let dot = SKSpriteNode(texture: texture)
    addChild(dot)
}

Object Pooling

For frequently spawned/destroyed objects (bullets, particles, enemies):

swift

class BulletPool {
    private var available: [SKSpriteNode] = []
    private let texture: SKTexture

    init(texture: SKTexture, initialSize: Int = 20) {
        self.texture = texture
        for _ in 0..<initialSize {
            available.append(createBullet())
        }
    }

    private func createBullet() -> SKSpriteNode {
        let bullet = SKSpriteNode(texture: texture)
        bullet.physicsBody = SKPhysicsBody(circleOfRadius: 3)
        bullet.physicsBody?.categoryBitMask = PhysicsCategory.projectile
        bullet.physicsBody?.collisionBitMask = PhysicsCategory.none
        bullet.physicsBody?.contactTestBitMask = PhysicsCategory.enemy
        return bullet
    }

    func spawn() -> SKSpriteNode {
        if available.isEmpty {
            available.append(createBullet())
        }
        let bullet = available.removeLast()
        bullet.isHidden = false
        bullet.physicsBody?.isDynamic = true
        return bullet
    }

    func recycle(_ bullet: SKSpriteNode) {
        bullet.removeAllActions()
        bullet.removeFromParent()
        bullet.physicsBody?.isDynamic = false
        bullet.physicsBody?.velocity = .zero
        bullet.isHidden = true
        available.append(bullet)
    }
}

Offscreen Node Removal

swift

// Manual removal is faster than shouldCullNonVisibleNodes
override func update(_ currentTime: TimeInterval) {
    enumerateChildNodes(withName: "bullet") { node, _ in
        if !self.frame.intersects(node.frame) {
            self.bulletPool.recycle(node as! SKSpriteNode)
        }
    }
}

7. Game Loop

Frame Cycle (8 Phases)

1. update(_:)              ← Your game logic here
2. didEvaluateActions()    ← Actions completed
3. [Physics simulation]    ← SpriteKit runs physics
4. didSimulatePhysics()    ← Physics done, adjust results
5. [Constraint evaluation] ← SKConstraints applied
6. didApplyConstraints()   ← Constraints done
7. didFinishUpdate()       ← Last chance before render
8. [Rendering]             ← Frame drawn

Delta Time

swift

private var lastUpdateTime: TimeInterval = 0

override func update(_ currentTime: TimeInterval) {
    let dt: TimeInterval
    if lastUpdateTime == 0 {
        dt = 0
    } else {
        dt = currentTime - lastUpdateTime
    }
    lastUpdateTime = currentTime

    // Clamp delta time to prevent spiral of death
    // (when app returns from background, dt can be huge)
    let clampedDt = min(dt, 1.0 / 30.0)

    updatePlayer(deltaTime: clampedDt)
    updateEnemies(deltaTime: clampedDt)
}

Pause Handling

swift

// Pause the scene (stops actions, physics, update loop)
scene.isPaused = true

// Pause specific subtree only
worldNode.isPaused = true  // Game paused but HUD still animates

// Handle app backgrounding
NotificationCenter.default.addObserver(
    self, selector: #selector(pauseGame),
    name: UIApplication.willResignActiveNotification, object: nil
)

8. Particle Effects

Emitter Best Practices

swift

// Load from .sks file (designed in Xcode Particle Editor)
guard let emitter = SKEmitterNode(fileNamed: "Explosion") else { return }
emitter.position = explosionPoint
addChild(emitter)

// CRITICAL: Auto-remove after emission completes
let duration = TimeInterval(emitter.numParticlesToEmit) / TimeInterval(emitter.particleBirthRate)
    + TimeInterval(emitter.particleLifetime + emitter.particleLifetimeRange / 2)
emitter.run(SKAction.sequence([
    SKAction.wait(forDuration: duration),
    SKAction.removeFromParent()
]))

Target Node for Trails

Without

targetNode

, particles move with the emitter. For trails (like rocket exhaust), set

targetNode

to the scene:

swift

let trail = SKEmitterNode(fileNamed: "RocketTrail")!
trail.targetNode = scene  // Particles stay where emitted
rocketNode.addChild(trail)

Infinite Emitter Cleanup

swift

// WRONG: Infinite emitter never cleaned up
let fire = SKEmitterNode(fileNamed: "Fire")!
fire.numParticlesToEmit = 0  // 0 = infinite
addChild(fire)
// Memory leak — particles accumulate forever

// CORRECT: Set emission limit or remove when done
fire.numParticlesToEmit = 200  // Stops after 200 particles

// Or manually stop and remove:
fire.particleBirthRate = 0  // Stop new particles
fire.run(SKAction.sequence([
    SKAction.wait(forDuration: TimeInterval(fire.particleLifetime)),
    SKAction.removeFromParent()
]))

9. SwiftUI Integration

SpriteView (Recommended, iOS 14+)

The simplest way to embed SpriteKit in SwiftUI. Use this unless you need custom SKView configuration.

swift

import SpriteKit
import SwiftUI

struct GameView: View {
    var body: some View {
        SpriteView(scene: {
            let scene = GameScene(size: CGSize(width: 390, height: 844))
            scene.scaleMode = .aspectFill
            return scene
        }(), debugOptions: [.showsFPS, .showsNodeCount])
        .ignoresSafeArea()
    }
}

UIViewRepresentable (Advanced)

Use when you need full control over SKView configuration (custom frame rate, transparency, or multiple scenes).

swift

import SwiftUI
import SpriteKit

struct SpriteKitView: UIViewRepresentable {
    let scene: SKScene

    func makeUIView(context: Context) -> SKView {
        let view = SKView()
        view.showsFPS = true
        view.showsNodeCount = true
        view.ignoresSiblingOrder = true
        return view
    }

    func updateUIView(_ view: SKView, context: Context) {
        if view.scene == nil {
            view.presentScene(scene)
        }
    }
}

SKRenderer for Metal Hybrid

Use

SKRenderer

when SpriteKit is one layer in a Metal pipeline:

swift

let renderer = SKRenderer(device: metalDevice)
renderer.scene = gameScene

// In your Metal render loop:
renderer.update(atTime: currentTime)
renderer.render(
    withViewport: viewport,
    commandBuffer: commandBuffer,
    renderPassDescriptor: renderPassDescriptor
)

10. Anti-Patterns

Anti-Pattern 1: Default Bitmasks

Time cost: 30-120 minutes debugging phantom collisions

swift

// WRONG: Default collisionBitMask is 0xFFFFFFFF
let body = SKPhysicsBody(circleOfRadius: 10)
node.physicsBody = body
// Collides with EVERYTHING — even things it shouldn't

// CORRECT: Always set all three masks explicitly
body.categoryBitMask = PhysicsCategory.player
body.collisionBitMask = PhysicsCategory.ground
body.contactTestBitMask = PhysicsCategory.enemy

Anti-Pattern 2: Missing contactTestBitMask

Time cost: 30-60 minutes wondering why didBegin never fires

swift

// WRONG: contactTestBitMask defaults to 0 — no contacts ever fire
player.physicsBody?.categoryBitMask = PhysicsCategory.player
// Forgot contactTestBitMask!

// CORRECT: Both bodies need compatible masks
player.physicsBody?.contactTestBitMask = PhysicsCategory.enemy
enemy.physicsBody?.categoryBitMask = PhysicsCategory.enemy

Anti-Pattern 3: Actions on Physics Bodies

Time cost: 1-3 hours of jittering and missed collisions

swift

// WRONG: SKAction.move overrides physics position each frame
playerNode.run(SKAction.moveTo(x: 200, duration: 1.0))
// Physics body position is set by action, ignoring forces/collisions

// CORRECT: Use physics for physics-controlled nodes
playerNode.physicsBody?.applyForce(CGVector(dx: 100, dy: 0))

Anti-Pattern 4: SKShapeNode for Gameplay

Time cost: Hours diagnosing frame drops

Each SKShapeNode is a separate draw call that cannot be batched. 50 shape nodes = 50 draw calls. See the pre-render-to-texture pattern in Section 6 (SKShapeNode Trap) for the fix.

Anti-Pattern 5: Strong Self in Action Closures

Time cost: Memory leaks, eventual crash

swift

// WRONG: Strong capture in repeating action
node.run(SKAction.repeatForever(SKAction.sequence([
    SKAction.run { self.spawnEnemy() },
    SKAction.wait(forDuration: 2.0)
])))

// CORRECT: Weak capture
node.run(SKAction.repeatForever(SKAction.sequence([
    SKAction.run { [weak self] in self?.spawnEnemy() },
    SKAction.wait(forDuration: 2.0)
])))

11. Code Review Checklist

Physics

Every physics body has explicit
```
categoryBitMask
```
(not default)
Every physics body has explicit
```
collisionBitMask
```
(not
```
0xFFFFFFFF
```
)
Bodies needing contact detection have
```
contactTestBitMask
```
set
```
physicsWorld.contactDelegate
```
is assigned
No world modifications inside
```
didBegin
```
/
```
didEnd
```
callbacks
Fast objects use
```
usesPreciseCollisionDetection
```

Actions

No
```
SKAction.move
```
/
```
rotate
```
on physics-controlled nodes
Repeating actions use
```
withKey:
```
for cancellation
```
SKAction.run
```
closures use
```
[weak self]
```
One-shot emitters are removed after emission

Performance

Debug overlays enabled during development
```
ignoresSiblingOrder = true
```
on SKView
No SKShapeNode in gameplay sprites (use pre-rendered textures)
Texture atlases used for related sprites
Offscreen nodes removed manually

Scene Management

```
willMove(from:)
```
cleans up actions, children, delegates
Scene data passed via shared state, not node properties
Camera used for viewport control

12. Pressure Scenarios

Scenario 1: "Physics Contacts Don't Work — Ship Tonight"

Pressure: Deadline pressure to skip systematic debugging

Wrong approach: Randomly changing bitmask values, adding

0xFFFFFFFF

everywhere, or disabling physics

Correct approach (2-5 minutes):

Enable
```
showsPhysics
```
— verify bodies exist and overlap
Print all three bitmasks for both bodies
Verify
```
contactTestBitMask
```
on body A includes category of body B (or vice versa)
Verify
```
physicsWorld.contactDelegate
```
is set
Verify you're not modifying the world inside the callback

Push-back template: "Let me run the 5-step bitmask checklist. It takes 2 minutes and catches 90% of contact issues. Random changes will make it worse."

Scenario 2: "Frame Rate Is Fine on My Device"

Pressure: Authority says "it runs at 60fps for me, ship it"

Wrong approach: Shipping without profiling on minimum-spec device

Correct approach:

Enable
```
showsFPS
```
,
```
showsNodeCount
```
,
```
showsDrawCount
```
Test on oldest supported device
If >200 nodes or >30 draw calls, investigate
Check for SKShapeNode in gameplay
Verify offscreen nodes are being removed

Push-back template: "Performance varies by device. Let me check node count and draw calls — takes 30 seconds with debug overlays. If counts are low, we're safe to ship."

Scenario 3: "Just Use SKShapeNode, It's Faster to Code"

Pressure: Sunk cost — already built with SKShapeNode, don't want to redo

Wrong approach: Shipping with 100+ SKShapeNodes causing frame drops

Correct approach:

Check
```
showsDrawCount
```
— each SKShapeNode adds a draw call
If >20 shape nodes in gameplay, pre-render to textures
Use
```
view.texture(from:)
```
to convert once, reuse as SKSpriteNode
Keep SKShapeNode only for debug visualization

Push-back template: "Each SKShapeNode is a separate draw call. Converting to pre-rendered textures is a 15-minute refactor that can double frame rate. SKSpriteNode from atlas = 1 draw call for all of them."

Resources

WWDC: 2014-608, 2016-610, 2017-609, 2013-502

Docs: /spritekit, /spritekit/skscene, /spritekit/skphysicsbody, /spritekit/maximizing-node-drawing-performance

Skills: axiom-spritekit-ref, axiom-spritekit-diag

axiom-spritekit

NPX Install

Tags

SKILL.md Content

SpriteKit Game Development Guide

When to Use This Skill

1. Mental Model

Coordinate System

Node Tree

Z-Ordering

2. Scene Architecture

Scale Mode Decision

Camera Node Pattern

Layer Organization

Scene Transitions

3. Physics Engine

Bitmask Discipline

Bitmask Checklist

Contact Detection

Body Types

Tunneling Prevention

Forces vs Impulses

4. Actions System

Core Patterns

Sequencing and Grouping

Named Actions (Critical for Management)

Custom Actions with Weak Self

Timing Modes

Actions vs Physics

5. Input Handling

Touch Handling

Coordinate Space Conversion

Game Controller Support

6. Performance

Performance Priorities

Debug Overlays (Always Enable During Development)

Texture Atlas Batching

SKShapeNode Trap

Object Pooling

Offscreen Node Removal

7. Game Loop

Frame Cycle (8 Phases)

Delta Time

Pause Handling

8. Particle Effects

Emitter Best Practices

Target Node for Trails

Infinite Emitter Cleanup

9. SwiftUI Integration

SpriteView (Recommended, iOS 14+)

UIViewRepresentable (Advanced)

SKRenderer for Metal Hybrid

10. Anti-Patterns

Anti-Pattern 1: Default Bitmasks

Anti-Pattern 2: Missing contactTestBitMask

Anti-Pattern 3: Actions on Physics Bodies

Anti-Pattern 4: SKShapeNode for Gameplay

Anti-Pattern 5: Strong Self in Action Closures

11. Code Review Checklist

Physics

Actions

Performance

Scene Management

12. Pressure Scenarios

Scenario 1: "Physics Contacts Don't Work — Ship Tonight"

Scenario 2: "Frame Rate Is Fine on My Device"

Scenario 3: "Just Use SKShapeNode, It's Faster to Code"

Resources