axiom-haptics

Haptics & Audio Feedback

Comprehensive guide to implementing haptic feedback on iOS. Every Apple Design Award winner uses excellent haptic feedback - Camera, Maps, Weather all use haptics masterfully to create delightful, responsive experiences.

Overview

Haptic feedback provides tactile confirmation of user actions and system events. When designed thoughtfully using the Causality-Harmony-Utility framework, axiom-haptics transform interfaces from functional to delightful.

This skill covers both simple haptics (UIFeedbackGenerator ) and advanced custom patterns (Core Haptics ), with real-world examples and audio-haptic synchronization techniques.

When to Use This Skill

Adding haptic feedback to user interactions
Choosing between UIFeedbackGenerator and Core Haptics
Designing audio-haptic experiences that feel unified
Creating custom haptic patterns with AHAP files
Synchronizing haptics with animations and audio
Debugging haptic issues (simulator vs device)
Optimizing haptic performance and battery impact

System Requirements

iOS 10+ for UIFeedbackGenerator
iOS 13+ for Core Haptics (CHHapticEngine)
iPhone 8+ for Core Haptics hardware support
Physical device required - haptics cannot be felt in Simulator

Part 1: Design Principles (WWDC 2021/10278)

Apple's audio and haptic design teams established three core principles for multimodal feedback:

Causality - Make it obvious what caused the feedback

Problem: User can't tell what triggered the haptic Solution: Haptic timing must match the visual/interaction moment

Example from WWDC:

✅ Ball hits wall → haptic fires at collision moment
❌ Ball hits wall → haptic fires 100ms later (confusing)

Code pattern:

// ✅ Immediate feedback on touch @objc func buttonTapped() { let generator = UIImpactFeedbackGenerator(style: .medium) generator.impactOccurred() // Fire immediately performAction() }

// ❌ Delayed feedback loses causality @objc func buttonTapped() { performAction() DispatchQueue.main.asyncAfter(deadline: .now() + 0.2) { let generator = UIImpactFeedbackGenerator(style: .medium) generator.impactOccurred() // Too late! } }

Harmony - Senses work best when coherent

Problem: Visual, audio, and haptic don't match Solution: All three senses should feel like a unified experience

Example from WWDC:

Small ball → light haptic + high-pitched sound
Large ball → heavy haptic + low-pitched sound
Shield transformation → continuous haptic + progressive audio

Key insight: A large object should feel heavy, sound low and resonant, and look substantial. All three senses reinforce the same experience.

Utility - Provide clear value

Problem: Haptics used everywhere "just because we can" Solution: Reserve haptics for significant moments that benefit the user

When to use haptics:

✅ Confirming an important action (payment completed)
✅ Alerting to critical events (low battery)
✅ Providing continuous feedback (scrubbing slider)
✅ Enhancing delight (app launch flourish)

When NOT to use haptics:

❌ Every single tap (overwhelming)
❌ Scrolling through long lists (battery drain)
❌ Background events user can't see (confusing)
❌ Decorative animations (no value)

Part 2: UIFeedbackGenerator (Simple Haptics)

For most apps, UIFeedbackGenerator provides 3 simple haptic types without custom patterns.

UIImpactFeedbackGenerator

Physical collision or impact sensation.

Styles (ordered light → heavy):

.light
Small, delicate tap
.medium
Standard tap (most common)
.heavy
Strong, solid impact
.rigid
Firm, precise tap
.soft
Gentle, cushioned tap

Usage pattern:

class MyViewController: UIViewController { let impactGenerator = UIImpactFeedbackGenerator(style: .medium)

override func viewDidLoad() {
    super.viewDidLoad()
    // Prepare reduces latency for next impact
    impactGenerator.prepare()
}

@objc func userDidTap() {
    impactGenerator.impactOccurred()
}

}

Intensity variation (iOS 13+):

// intensity: 0.0 (lightest) to 1.0 (strongest) impactGenerator.impactOccurred(intensity: 0.5)

Common use cases:

Button taps (.medium )
Toggle switches (.light )
Deleting items (.heavy )
Confirming selections (.rigid )

UISelectionFeedbackGenerator

Discrete selection changes (picker wheels, segmented controls).

Usage:

class PickerViewController: UIViewController { let selectionGenerator = UISelectionFeedbackGenerator()

func pickerView(_ picker: UIPickerView, didSelectRow row: Int,
                inComponent component: Int) {
    selectionGenerator.selectionChanged()
}

}

Feels like: Clicking a physical wheel with detents

Common use cases:

Picker wheels
Segmented controls
Page indicators
Step-through interfaces

UINotificationFeedbackGenerator

System-level success/warning/error feedback.

Types:

.success
Task completed successfully
.warning
Attention needed, but not critical
.error
Critical error occurred

Usage:

let notificationGenerator = UINotificationFeedbackGenerator()

func submitForm() { // Validate form if isValid { notificationGenerator.notificationOccurred(.success) saveData() } else { notificationGenerator.notificationOccurred(.error) showValidationErrors() } }

Best practice: Match haptic type to user outcome

✅ Payment succeeds → .success
✅ Form validation fails → .error
✅ Approaching storage limit → .warning

Performance: prepare()

Call prepare() before the haptic to reduce latency:

// ✅ Good - prepare before user action @IBAction func buttonTouchDown(_ sender: UIButton) { impactGenerator.prepare() // User's finger is down }

@IBAction func buttonTouchUpInside(_ sender: UIButton) { impactGenerator.impactOccurred() // Immediate haptic }

// ❌ Bad - unprepared haptic may lag @IBAction func buttonTapped(_ sender: UIButton) { let generator = UIImpactFeedbackGenerator() generator.impactOccurred() // May have 10-20ms delay }

Prepare timing: System keeps engine ready for ~1 second after prepare() .

Part 3: Core Haptics (Custom Haptics)

For apps needing custom patterns, Core Haptics provides full control over haptic waveforms.

Four Fundamental Elements

Engine (CHHapticEngine ) - Link to the phone's actuator
Player (CHHapticPatternPlayer ) - Playback control
Pattern (CHHapticPattern ) - Collection of events over time
Events (CHHapticEvent ) - Building blocks specifying the experience

CHHapticEngine Lifecycle

import CoreHaptics

class HapticManager { var engine: CHHapticEngine?

func initializeHaptics() {
    // Check device support
    guard CHHapticEngine.capabilitiesForHardware().supportsHaptics else {
        print("Device doesn't support haptics")
        return
    }

    do {
        // Create engine
        engine = try CHHapticEngine()

        // Handle interruptions (calls, Siri, etc.)
        engine?.stoppedHandler = { reason in
            print("Engine stopped: \(reason)")
            self.restartEngine()
        }

        // Handle reset (audio session changes)
        engine?.resetHandler = {
            print("Engine reset")
            self.restartEngine()
        }

        // Start engine
        try engine?.start()

    } catch {
        print("Failed to create haptic engine: \(error)")
    }
}

func restartEngine() {
    do {
        try engine?.start()
    } catch {
        print("Failed to restart engine: \(error)")
    }
}

}

Critical: Always set stoppedHandler and resetHandler to handle system interruptions.

CHHapticEvent Types

Transient Events

Short, discrete feedback (like a tap).

let intensity = CHHapticEventParameter( parameterID: .hapticIntensity, value: 1.0 // 0.0 to 1.0 )

let sharpness = CHHapticEventParameter( parameterID: .hapticSharpness, value: 0.5 // 0.0 (dull) to 1.0 (sharp) )

let event = CHHapticEvent( eventType: .hapticTransient, parameters: [intensity, sharpness], relativeTime: 0.0 // Seconds from pattern start )

Parameters:

hapticIntensity : Strength (0.0 = barely felt, 1.0 = maximum)
hapticSharpness : Character (0.0 = dull thud, 1.0 = crisp snap)

Continuous Events

Sustained feedback over time (like a vibration motor).

let intensity = CHHapticEventParameter( parameterID: .hapticIntensity, value: 0.8 )

let sharpness = CHHapticEventParameter( parameterID: .hapticSharpness, value: 0.3 )

let event = CHHapticEvent( eventType: .hapticContinuous, parameters: [intensity, sharpness], relativeTime: 0.0, duration: 2.0 // Seconds )

Use cases:

Rolling texture as object moves
Motor running
Charging progress
Long press feedback

Creating and Playing Patterns

func playCustomPattern() { // Create events let tap1 = CHHapticEvent( eventType: .hapticTransient, parameters: [ CHHapticEventParameter(parameterID: .hapticIntensity, value: 0.5), CHHapticEventParameter(parameterID: .hapticSharpness, value: 0.5) ], relativeTime: 0.0 )

let tap2 = CHHapticEvent(
    eventType: .hapticTransient,
    parameters: [
        CHHapticEventParameter(parameterID: .hapticIntensity, value: 0.7),
        CHHapticEventParameter(parameterID: .hapticSharpness, value: 0.7)
    ],
    relativeTime: 0.3
)

let tap3 = CHHapticEvent(
    eventType: .hapticTransient,
    parameters: [
        CHHapticEventParameter(parameterID: .hapticIntensity, value: 1.0),
        CHHapticEventParameter(parameterID: .hapticSharpness, value: 1.0)
    ],
    relativeTime: 0.6
)

do {
    // Create pattern from events
    let pattern = try CHHapticPattern(
        events: [tap1, tap2, tap3],
        parameters: []
    )

    // Create player
    let player = try engine?.makePlayer(with: pattern)

    // Play
    try player?.start(atTime: CHHapticTimeImmediate)

} catch {
    print("Failed to play pattern: \(error)")
}

}

CHHapticAdvancedPatternPlayer - Looping

For continuous feedback (rolling textures, motors), use advanced player:

func startRollingTexture() { let event = CHHapticEvent( eventType: .hapticContinuous, parameters: [ CHHapticEventParameter(parameterID: .hapticIntensity, value: 0.4), CHHapticEventParameter(parameterID: .hapticSharpness, value: 0.2) ], relativeTime: 0.0, duration: 0.5 )

do {
    let pattern = try CHHapticPattern(events: [event], parameters: [])

    // Use advanced player for looping
    let player = try engine?.makeAdvancedPlayer(with: pattern)

    // Enable looping
    try player?.loopEnabled = true

    // Start
    try player?.start(atTime: CHHapticTimeImmediate)

    // Update intensity dynamically based on ball speed
    updateTextureIntensity(player: player)

} catch {
    print("Failed to start texture: \(error)")
}

}

func updateTextureIntensity(player: CHHapticAdvancedPatternPlayer?) { let newIntensity = calculateIntensityFromBallSpeed()

let intensityParam = CHHapticDynamicParameter(
    parameterID: .hapticIntensityControl,
    value: newIntensity,
    relativeTime: 0
)

try? player?.sendParameters([intensityParam], atTime: CHHapticTimeImmediate)

}

Key difference: CHHapticPatternPlayer plays once, CHHapticAdvancedPatternPlayer supports looping and dynamic parameter updates.

Part 4: AHAP Files (Apple Haptic Audio Pattern)

AHAP (Apple Haptic Audio Pattern) files are JSON files combining haptic events and audio.

Basic AHAP Structure

{ "Version": 1.0, "Metadata": { "Project": "My App", "Created": "2024-01-15" }, "Pattern": [ { "Event": { "Time": 0.0, "EventType": "HapticTransient", "EventParameters": [ { "ParameterID": "HapticIntensity", "ParameterValue": 1.0 }, { "ParameterID": "HapticSharpness", "ParameterValue": 0.5 } ] } } ] }

Adding Audio to AHAP

{ "Version": 1.0, "Pattern": [ { "Event": { "Time": 0.0, "EventType": "AudioCustom", "EventParameters": [ { "ParameterID": "AudioVolume", "ParameterValue": 0.8 } ], "EventWaveformPath": "ShieldA.wav" } }, { "Event": { "Time": 0.0, "EventType": "HapticContinuous", "EventDuration": 0.5, "EventParameters": [ { "ParameterID": "HapticIntensity", "ParameterValue": 0.6 } ] } } ] }

Loading AHAP Files

func loadAHAPPattern(named name: String) -> CHHapticPattern? { guard let url = Bundle.main.url(forResource: name, withExtension: "ahap") else { print("AHAP file not found") return nil }

do {
    return try CHHapticPattern(contentsOf: url)
} catch {
    print("Failed to load AHAP: \(error)")
    return nil
}

}

// Usage if let pattern = loadAHAPPattern(named: "ShieldTransient") { let player = try? engine?.makePlayer(with: pattern) try? player?.start(atTime: CHHapticTimeImmediate) }

Design Workflow (WWDC Example)

Create visual animation (e.g., shield transformation, 500ms)
Design audio (convey energy gain and robustness)
Design haptic (feel the transformation)
Test harmony - Do all three senses work together?
Iterate - Swap AHAP assets until coherent
Implement - Update code to use final assets

Example iteration: Shield initially used 3 transient pulses (haptic) + progressive continuous sound (audio) → no harmony. Solution: Switch to continuous haptic + ShieldA.wav audio → unified experience.

Part 5: Audio-Haptic Synchronization

Matching Animation Timing

class ViewController: UIViewController { let animationDuration: TimeInterval = 0.5

func performShieldTransformation() {
    // Start haptic/audio simultaneously with animation
    playShieldPattern()

    UIView.animate(withDuration: animationDuration) {
        self.shieldView.transform = CGAffineTransform(scaleX: 1.2, y: 1.2)
        self.shieldView.alpha = 0.8
    }
}

func playShieldPattern() {
    if let pattern = loadAHAPPattern(named: "ShieldContinuous") {
        let player = try? engine?.makePlayer(with: pattern)
        try? player?.start(atTime: CHHapticTimeImmediate)
    }
}

}

Critical: Fire haptic at the exact moment the visual change occurs, not before or after.

Coordinating with Audio

import AVFoundation

class AudioHapticCoordinator { let audioPlayer: AVAudioPlayer let hapticEngine: CHHapticEngine

func playCoordinatedExperience() {
    // Prepare both systems
    hapticEngine.notifyWhenPlayersFinished { _ in
        return .stopEngine
    }

    // Start at exact same moment
    let startTime = CACurrentMediaTime() + 0.05  // Small delay for sync

    // Start audio
    audioPlayer.play(atTime: startTime)

    // Start haptic
    if let pattern = loadAHAPPattern(named: "CoordinatedPattern") {
        let player = try? hapticEngine.makePlayer(with: pattern)
        try? player?.start(atTime: CHHapticTimeImmediate)
    }
}

}

Part 6: Common Patterns

Button Tap

class HapticButton: UIButton { let impactGenerator = UIImpactFeedbackGenerator(style: .medium)

override func touchesBegan(_ touches: Set&#x3C;UITouch>, with event: UIEvent?) {
    super.touchesBegan(touches, with: event)
    impactGenerator.prepare()
}

override func touchesEnded(_ touches: Set&#x3C;UITouch>, with event: UIEvent?) {
    super.touchesEnded(touches, with: event)
    impactGenerator.impactOccurred()
}

}

Slider Scrubbing

class HapticSlider: UISlider { let selectionGenerator = UISelectionFeedbackGenerator() var lastValue: Float = 0

@objc func valueChanged() {
    let threshold: Float = 0.1

    if abs(value - lastValue) >= threshold {
        selectionGenerator.selectionChanged()
        lastValue = value
    }
}

}

Pull-to-Refresh

class PullToRefreshController: UIViewController { let impactGenerator = UIImpactFeedbackGenerator(style: .medium) var isRefreshing = false

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    let threshold: CGFloat = -100
    let offset = scrollView.contentOffset.y

    if offset &#x3C;= threshold &#x26;&#x26; !isRefreshing {
        impactGenerator.impactOccurred()
        isRefreshing = true
        beginRefresh()
    }
}

}

Success/Error Feedback

func handleServerResponse(_ result: Result<Data, Error>) { let notificationGenerator = UINotificationFeedbackGenerator()

switch result {
case .success:
    notificationGenerator.notificationOccurred(.success)
    showSuccessMessage()
case .failure:
    notificationGenerator.notificationOccurred(.error)
    showErrorAlert()
}

}

Part 7: Testing & Debugging

Simulator Limitations

Haptics DO NOT work in Simulator. You will see:

No haptic feedback
No warnings or errors
Code runs normally

Solution: Always test on physical device (iPhone 8 or newer).

Device Testing Checklist

Test with Haptics disabled in Settings → Sounds & Haptics
Test with Low Power Mode enabled
Test during incoming call (engine may stop)
Test with audio playing in background
Test with different intensity/sharpness values
Verify battery impact (Instruments Energy Log)

Debug Logging

func playHaptic() { #if DEBUG print("🔔 Playing haptic - Engine running: (engine?.currentTime ?? -1)") #endif

do {
    let player = try engine?.makePlayer(with: pattern)
    try player?.start(atTime: CHHapticTimeImmediate)

    #if DEBUG
    print("✅ Haptic started successfully")
    #endif
} catch {
    #if DEBUG
    print("❌ Haptic failed: \(error.localizedDescription)")
    #endif
}

}

Troubleshooting

Engine fails to start

Symptom: CHHapticEngine.start() throws error

Causes:

Device doesn't support Core Haptics (< iPhone 8)
Haptics disabled in Settings
Low Power Mode enabled

Solution:

func safelyStartEngine() { guard CHHapticEngine.capabilitiesForHardware().supportsHaptics else { print("Device doesn't support haptics") return }

do {
    try engine?.start()
} catch {
    print("Engine start failed: \(error)")
    // Fall back to UIFeedbackGenerator
    useFallbackHaptics()
}

}

Haptics not felt

Symptom: Code runs but no haptic felt on device

Debug steps:

Check Settings → Sounds & Haptics → System Haptics is ON
Check Low Power Mode is OFF
Verify device is iPhone 8 or newer
Check intensity > 0.3 (values below may be too subtle)
Test with UIFeedbackGenerator to isolate Core Haptics vs system issue

Audio out of sync with haptics

Symptom: Audio plays but haptic delayed or vice versa

Causes:

Not calling prepare() before haptic
Audio/haptic started at different times
Heavy main thread work blocking playback

Solution:

// ✅ Synchronized start func playCoordinated() { impactGenerator.prepare() // Reduce latency

// Start both simultaneously
audioPlayer.play()
impactGenerator.impactOccurred()

}

Audio file errors with AHAP

Symptom: AHAP pattern fails to load or play

Cause: Audio file > 4.2 MB or > 23 seconds

Solution: Keep audio files small and short. Use compressed formats (AAC) and trim to essential duration.

Resources

WWDC: 2021-10278, 2019-520, 2019-223

Docs: /corehaptics, /corehaptics/chhapticengine

Skills: axiom-swiftui-animation-ref, axiom-ui-testing, axiom-accessibility-diag

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