What It Does

SwiftUI makes it easy to animate simple interface updates, but it quickly gets complicated when you want to update custom parameters during animations.

Animatable ViewModifiers make it possible to smoothly change a single VectorArithmetic conforming parameter. Chained AnimatablePairs can handle more, but it’s messy.

Transitionable allows you to create any changes you want to a set of parameters as an animation proceeds from 0-1.

How To Use It

Create a Transitionable conforming struct containing the parameters you want to transition:

struct FontDescriptor: Transitionable
    let style: BlockFontStyle
    let size: CGFloat
    let weight: CGFloat
    let lineSpacing: CGFloat

Implement transition(to:, progress:), defining how your parameters should change over the course of an animation – progress will go from 0 to 1:

func transition(_ other: FontDescriptor, progress: CGFloat) -> FontDescriptor {
        let progress = progress * progress
        return FontDescriptor(
            style: progress < 0.5 ? style :,
            size: progress.lerp(min: size, max: other.size),
            weight: progress.lerp(min: weight, max: other.weight),
            lineSpacing: progress.lerp(min: lineSpacing, max: other.lineSpacing))

Add an EnvironmentKey for your parameters:

private struct FontDescriptorKey: EnvironmentKey {
    static let defaultValue = Style.fontDescriptor(for: defaultState)

extension EnvironmentValues {
    var fontDescriptor: FontDescriptor {
        get { self[FontDescriptorKey.self] }
        set { self[FontDescriptorKey.self] = newValue }

Pass your parameters to child views using the View extension:

    .environmentTransition(\.fontDescriptor, Style.fontDescriptor(for: someState))

Use the parameters as you wish in your View:

struct MyTextView: View
    @Environment(\.fontDescriptor) var fontDescriptor: FontDescriptor
    var body: some View {


View Github