/haptics

Haptic Design Principles

Apple's Causality-Harmony-Utility framework from WWDC 2021 for multimodal feedback design.

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.

Good vs Bad Timing

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

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

WWDC Example

• Ball hits wall -> haptic fires at collision moment (correct)
• Ball hits wall -> haptic fires 100ms later (confusing)

Key Insight

Fire haptic at the exact moment the visual change occurs. Even 100ms delay breaks the illusion of physical feedback.

Harmony: Senses Work Best When Coherent

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

Matching Intensity Across Senses

Code Pattern

func playImpact(for objectMass: CGFloat) {
    // Match haptic intensity to visual/audio characteristics
    let normalizedMass = min(max(objectMass / 100.0, 0.0), 1.0)

    let generator = UIImpactFeedbackGenerator(style: .medium)
    generator.impactOccurred(intensity: normalizedMass)

    // Audio pitch should also scale with mass
    playSound(pitch: 1.0 - (normalizedMass * 0.5))
}

WWDC Example: Shield Transformation

Initial attempt: 3 transient pulses (haptic) + progressive continuous sound (audio) - no harmony. Solution: Continuous haptic + continuous audio - unified 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

When NOT to Use Haptics

Decision Framework

func shouldPlayHaptic(for event: UserEvent) -> Bool {
    switch event {
    case .buttonTap(let importance):
        return importance == .high  // Only important buttons
    case .selectionChange:
        return true  // Picker-style selection feedback
    case .scroll:
        return false  // Never haptic on scroll
    case .success, .error:
        return true  // Always confirm outcomes
    }
}

Audio-Haptic Synchronization

Matching Animation Timing

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

    UIView.animate(withDuration: 0.5) {
        self.shieldView.transform = CGAffineTransform(scaleX: 1.2, y: 1.2)
    }
}

Coordinated Start

func playCoordinatedExperience() {
    impactGenerator.prepare()  // Reduce latency

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

Testing Checklist

• Does the haptic fire at the exact moment of visual change? (Causality)
• Do visual, audio, and haptic feel unified? (Harmony)
• Does this haptic provide clear value to the user? (Utility)
• Is the intensity appropriate for the action?
• Test with haptics disabled - does the UI still work?

Related WWDC Sessions

• Practice audio haptic design (WWDC 2021/10278)
• Introducing Core Haptics (WWDC 2019/520)
• Expanding the Sensory Experience (WWDC 2019/223)