/swiftui-expert-skill

SwiftUI Animation Basics

Core animation concepts, implicit vs explicit animations, timing curves, and performance patterns.

Table of Contents

• Core Concepts
• Implicit Animations
• Explicit Animations
• Animation Placement
• Selective Animation
• Timing Curves
• Animation Performance
• Disabling Animations
• Debugging

Core Concepts

State changes trigger view updates. SwiftUI provides mechanisms to animate these changes.

Animation Process:

1. State change triggers view tree re-evaluation
2. SwiftUI compares new tree to current render tree
3. Animatable properties are identified and interpolated (~60 fps)

Key Characteristics:

• Animations are additive and cancelable
• Always start from current render tree state
• Blend smoothly when interrupted

Implicit Animations

Use .animation(_:value:) to animate when a specific value changes.

// GOOD - uses value parameter
Rectangle()
    .frame(width: isExpanded ? 200 : 100, height: 50)
    .animation(.spring, value: isExpanded)
    .onTapGesture { isExpanded.toggle() }

// BAD - deprecated, animates all changes unexpectedly
Rectangle()
    .frame(width: isExpanded ? 200 : 100, height: 50)
    .animation(.spring)  // Deprecated!

Explicit Animations

Use withAnimation for event-driven state changes.

// GOOD - explicit animation
Button("Toggle") {
    withAnimation(.spring) {
        isExpanded.toggle()
    }
}

// BAD - no animation context
Button("Toggle") {
    isExpanded.toggle()  // Abrupt change
}

When to use which:

• Implicit: Animations tied to specific value changes, precise view tree scope
• Explicit: Event-driven animations (button taps, gestures)

Animation Placement

Place animation modifiers after the properties they should animate.

// GOOD - animation after properties
Rectangle()
    .frame(width: isExpanded ? 200 : 100, height: 50)
    .foregroundStyle(isExpanded ? .blue : .red)
    .animation(.default, value: isExpanded)  // Animates both

// BAD - animation before properties
Rectangle()
    .animation(.default, value: isExpanded)  // Too early!
    .frame(width: isExpanded ? 200 : 100, height: 50)

Selective Animation

Animate only specific properties using multiple animation modifiers or scoped animations.

// GOOD - selective animation
Rectangle()
    .frame(width: isExpanded ? 200 : 100, height: 50)
    .animation(.spring, value: isExpanded)  // Animate size
    .foregroundStyle(isExpanded ? .blue : .red)
    .animation(nil, value: isExpanded)  // Don't animate color

// iOS 17+ scoped animation
Rectangle()
    .foregroundStyle(isExpanded ? .blue : .red)  // Not animated
    .animation(.spring) {
        $0.frame(width: isExpanded ? 200 : 100, height: 50)  // Animated
    }

Timing Curves

Built-in Curves

Modifiers

.animation(.default.speed(2.0), value: flag)  // 2x faster
.animation(.default.delay(0.5), value: flag)  // Delayed start
.animation(.default.repeatCount(3, autoreverses: true), value: flag)

Good vs Bad Timing

// GOOD - appropriate timing for interaction type
Button("Tap") {
    withAnimation(.spring(response: 0.3, dampingFraction: 0.7)) {
        isActive.toggle()
    }
}
.scaleEffect(isActive ? 0.95 : 1.0)

// BAD - too slow for button feedback
Button("Tap") {
    withAnimation(.easeInOut(duration: 1.0)) {  // Way too slow!
        isActive.toggle()
    }
}

// BAD - linear feels robotic
Rectangle()
    .animation(.linear(duration: 0.5), value: isActive)  // Mechanical

Animation Performance

Prefer Transforms Over Layout

// GOOD - GPU accelerated transforms
Rectangle()
    .frame(width: 100, height: 100)
    .scaleEffect(isActive ? 1.5 : 1.0)  // Fast
    .offset(x: isActive ? 50 : 0)        // Fast
    .rotationEffect(.degrees(isActive ? 45 : 0))  // Fast
    .animation(.spring, value: isActive)

// BAD - layout changes are expensive
Rectangle()
    .frame(width: isActive ? 150 : 100, height: isActive ? 150 : 100)  // Expensive
    .padding(isActive ? 50 : 0)  // Expensive

Narrow Animation Scope

// GOOD - animation scoped to specific subview
VStack {
    HeaderView()  // Not affected
    ExpandableContent(isExpanded: isExpanded)
        .animation(.spring, value: isExpanded)  // Only this
    FooterView()  // Not affected
}

// BAD - animation at root
VStack {
    HeaderView()
    ExpandableContent(isExpanded: isExpanded)
    FooterView()
}
.animation(.spring, value: isExpanded)  // Animates everything

Avoid Animation in Hot Paths

// GOOD - gate by threshold
.onPreferenceChange(ScrollOffsetKey.self) { offset in
    let shouldShow = offset.y < -50
    if shouldShow != showTitle {  // Only when crossing threshold
        withAnimation(.easeOut(duration: 0.2)) {
            showTitle = shouldShow
        }
    }
}

// BAD - animating every scroll change
.onPreferenceChange(ScrollOffsetKey.self) { offset in
    withAnimation {  // Fires constantly!
        self.offset = offset.y
    }
}

Disabling Animations

// GOOD - disable with transaction
Text("Count: \(count)")
    .transaction { $0.animation = nil }

// GOOD - disable from parent context
DataView()
    .transaction { $0.disablesAnimations = true }

// BAD - hacky zero duration
Text("Count: \(count)")
    .animation(.linear(duration: 0), value: count)  // Hacky

Debugging

// Slow down for inspection
#if DEBUG
.animation(.linear(duration: 3.0).speed(0.2), value: isExpanded)
#else
.animation(.spring, value: isExpanded)
#endif

// Debug modifier to log values
struct AnimationDebugModifier: ViewModifier, Animatable {
    var value: Double
    var animatableData: Double {
        get { value }
        set {
            value = newValue
            print("Animation: \(newValue)")
        }
    }
    func body(content: Content) -> some View {
        content.opacity(value)
    }
}

Quick Reference

Do

• Use .animation(_:value:) with value parameter
• Use withAnimation for event-driven animations
• Prefer transforms over layout changes
• Scope animations narrowly
• Choose appropriate timing curves

Don't

• Use deprecated .animation(_:) without value
• Animate layout properties in hot paths
• Apply broad animations at root level
• Use linear timing for UI (feels robotic)
• Animate on every frame in scroll handlers