Permission Sheet

// Copyright (c) 2026 Saivion Hayes
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, and distribute the Software as part of
// an application, website, or product, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// Commons Clause Restriction
//
// You may use this Software, including for any commercial purpose, so long as
// you do not sell, sublicense, or redistribute the components themselves, whether
// alone, in a bundle, or as a ported version.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

import SwiftUI

/// Explains a permission, runs the real request, and reacts to the result.
///
/// - Parameters:
///   - systemImage: SF Symbol shown in the icon tile.
///   - title: Headline.
///   - message: One or two sentences of context.
///   - benefits: What the permission brings, previewed as a stack of notification banners above the title, each its symbol on a tinted tile and its line. Keep each line short, like a notification. Without benefits the sheet leads with the icon tile.
///   - allowTitle: Primary button title.
///   - deniedTitle: Headline shown after the request returns `false`.
///   - deniedMessage: Copy shown after a denial, above the "Open Settings" button.
///   - style: Colors and tile shape. Defaults to the SwiftPieces house palette, adapting to light and dark.
///   - request: Async permission request; return `true` when granted. The button spins and reads Requesting until it returns.
///   - onGranted: Called about a second after a grant, once the tile has turned into a check. Dismiss the sheet here.
///   - onSkip: Secondary handler. Omit to hide the "Not now" capsule.
public struct PermissionSheet: View {
    public struct Benefit: Identifiable {
        public let id = UUID()
        public var symbol: String
        public var text: String

        public init(symbol: String, text: String) {
            self.symbol = symbol
            self.text = text
        }
    }

    /// Colors and shape. `.standard` is the house palette. The sheet draws no background of its own; give its container `surface`.
    public struct Style: Sendable {
        /// Suggested sheet background, for `.presentationBackground(style.surface)`.
        public var surface: Color
        /// Headline and banner text, glyphs on clear banner tiles, and the request button's glass after a denial.
        public var label: Color
        /// Message, and the title of the "Not now" capsule.
        public var secondaryLabel: Color
        /// Glyphs and text on solid blocks and tinted glass.
        public var ink: Color
        /// The icon tile before a result.
        public var tile: Color
        /// The icon tile and the request button's glass after a grant.
        public var success: Color
        /// The icon tile after a denial.
        public var denied: Color
        /// The banners' app tiles, assigned in order and repeated: butter, sage and lilac by default. Pass `[.clear]` for neutral tiles.
        public var benefitTiles: [Color]
        /// The request button's glass tint.
        public var action: Color
        /// Corner radius of the big icon tile.
        public var cornerRadius: CGFloat

        /// Pass only what you want to change; `nil` keeps the house palette value.
        public init(surface: Color? = nil, label: Color? = nil, secondaryLabel: Color? = nil, ink: Color? = nil, tile: Color? = nil, success: Color? = nil, denied: Color? = nil, benefitTiles: [Color]? = nil, action: Color? = nil, cornerRadius: CGFloat = 26) {
            self.surface = surface ?? adaptive(light: 0xFFFFFF, dark: 0x1C1C1C)
            self.label = label ?? adaptive(light: 0x141414, dark: 0xF4F3EF)
            self.secondaryLabel = secondaryLabel ?? adaptive(light: 0x5C5A56, dark: 0xA6A49F)
            self.ink = ink ?? adaptive(light: 0x141414, dark: 0x141414)
            self.tile = tile ?? adaptive(light: 0x9CC2FF, dark: 0x9CC2FF)
            self.success = success ?? adaptive(light: 0xA9DCB7, dark: 0xA9DCB7)
            self.denied = denied ?? adaptive(light: 0xE9D5B3, dark: 0xE9D5B3)
            self.benefitTiles = (benefitTiles?.isEmpty == false ? benefitTiles : nil) ?? [adaptive(light: 0xFFD976, dark: 0xFFD976), adaptive(light: 0xA9DCB7, dark: 0xA9DCB7), adaptive(light: 0xCDB8FF, dark: 0xCDB8FF)]
            self.action = action ?? adaptive(light: 0xFF0000, dark: 0xFF0000)
            self.cornerRadius = cornerRadius
        }

        public static let standard = Style()
    }

    private enum Phase { case idle, requesting, granted, denied }

    @Environment(\.accessibilityReduceMotion) private var reduceMotion
    @Environment(\.openURL) private var openURL
    @ScaledMetric(relativeTo: .title) private var titleSize: CGFloat = 30
    @State private var phase: Phase = .idle
    @State private var revealed = false
    @State private var presentTick = 0
    @State private var grants = 0
    /// The Not now capsule, which melts into the request button on a grant.
    @State private var buds = PieceBuds()
    /// The benefit banner at the front of the stack.
    @State private var front = 0
    @State private var skipHeight: CGFloat = 56

    /// The sheet's rhythm. On present the rows follow the tile in one beat apart. When the answer comes back the
    /// card turns over from the top: the tile, then the old copy and rows leave, then the button, then the new copy.
    private let beat: Double = 0.07

    private let systemImage: String
    private let title: String
    private let message: String
    private let benefits: [Benefit]
    private let allowTitle: String
    private let deniedTitle: String
    private let deniedMessage: String
    private let style: Style
    private let request: () async -> Bool
    private let onGranted: () -> Void
    private let onSkip: (() -> Void)?

    public init(
        systemImage: String,
        title: String,
        message: String,
        benefits: [Benefit] = [],
        allowTitle: String = "Continue",
        deniedTitle: String = "Permission is off",
        deniedMessage: String = "You can turn it on any time in Settings.",
        style: Style = .standard,
        request: @escaping () async -> Bool,
        onGranted: @escaping () -> Void,
        onSkip: (() -> Void)? = nil
    ) {
        self.systemImage = systemImage
        self.title = title
        self.message = message
        self.benefits = benefits
        self.allowTitle = allowTitle
        self.deniedTitle = deniedTitle
        self.deniedMessage = deniedMessage
        self.style = style
        self.request = request
        self.onGranted = onGranted
        self.onSkip = onSkip
    }

    private var tileFill: Color { phase == .granted ? style.success : (phase == .denied ? style.denied : style.tile) }
    private var tileSymbol: String { phase == .granted ? "checkmark" : (phase == .denied ? "gearshape.fill" : systemImage) }
    private var buttonTitle: String {
        switch phase {
        case .idle: allowTitle
        case .requesting: String(localized: "Requesting", comment: "Permission sheet button while the system permission prompt is up")
        case .granted: String(localized: "Allowed", comment: "Permission sheet button once the permission is granted")
        case .denied: String(localized: "Open Settings", comment: "Permission sheet button after a denial, which opens the app's settings")
        }
    }
    private var buttonTint: Color { phase == .granted ? style.success : (phase == .denied ? style.label : style.action) }
    private var buttonInk: Color { phase == .denied ? style.surface : style.ink }

    public var body: some View {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        VStack(alignment: .center, spacing: 0) {
            // The lead: a preview of what the permission brings, as a stack of notification banners, or the icon tile
            // when there are no benefits to show.
            if benefits.isEmpty {
                tile
            } else {
                stack
            }

            ZStack(alignment: .top) {
                copy(title: title, message: message, shown: phase != .denied, away: -6)
                copy(title: deniedTitle, message: deniedMessage, shown: phase == .denied, away: 6)
            }
            .padding(.top, benefits.isEmpty ? 18 : 20)

            Spacer(minLength: 24)

            // Separate actions, so they rest apart: Not now only goos while it melts into the request button.
            PieceLiquidGroup {
                VStack(spacing: PieceLiquid.apart) {
                    // Above Not now, so Not now at home sits under it and melts in rather than over its label.
                    requestButton.zIndex(1)
                    if onSkip != nil { skipSlot }
                }
            }
        }
        .padding(24)
        .frame(maxWidth: .infinity)
        .fontWeight(.semibold)
        .sensoryFeedback(.success, trigger: phase) { _, new in new == .granted }
        .sensoryFeedback(.warning, trigger: phase) { _, new in new == .denied }
        .onAppear {
            if onSkip != nil { buds.place(["skip"]) }
        }
        .task {
            // A sheet is still sliding up when this appears, so the banners drop in once it has settled.
            try? await Task.sleep(for: .milliseconds(300))
            guard !Task.isCancelled else { return }
            presentTick += 1
            withAnimation(motion.reveal) { revealed = true }
        }
        .task(id: phase) {
            // While the sheet waits for an answer, the stack deals its next banner to the front every few seconds, so
            // each benefit gets read. Paused under Reduce Motion and once there is an answer.
            guard phase == .idle, benefits.count > 1, motion.allowsAmbient else { return }
            try? await Task.sleep(for: .seconds(1.2))
            while !Task.isCancelled {
                try? await Task.sleep(for: .seconds(2.6))
                guard !Task.isCancelled, phase == .idle else { return }
                withAnimation(motion.settle) { front = (front + 1) % benefits.count }
            }
        }
    }

    /// The icon tile, for a sheet without benefits. It turns to its next face the moment the answer comes back, and a
    /// grant lands with a pop on the haptic's beat. A denial only changes face: no shake, nothing celebratory.
    private var tile: some View {
        Image(systemName: tileSymbol)
            .font(.system(size: 30, weight: .semibold))
            .foregroundStyle(style.ink)
            .contentTransition(.symbolEffect(.replace))
            .symbolEffect(.bounce.down, options: reduceMotion ? .speed(100) : .default, value: presentTick)
            .frame(width: 72, height: 72)
            .background(tileFill, in: .rect(cornerRadius: style.cornerRadius, style: .continuous))
            .animation(PieceMotion(reduceMotion: reduceMotion).morph, value: phase)
            .piecePop(trigger: grants)
            .accessibilityHidden(true)
    }

    /// The benefits as a fanned stack of notification banners, the first in front. They drop in one after another as
    /// the sheet settles, the next is dealt to the front every few seconds, a grant pops the stack, and a denial greys
    /// it out: notifications off.
    private var stack: some View {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        let count = benefits.count
        let fan: CGFloat = 12
        return ZStack(alignment: .top) {
            ForEach(benefits.indices, id: \.self) { index in
                let rank = (index - front + count) % count
                banner(index)
                    // Behind the front banner, each is a little smaller and lower, so the edges of two show.
                    .scaleEffect(reduceMotion ? 1 : 1 - 0.06 * CGFloat(min(rank, 3)), anchor: .top)
                    .offset(y: revealed ? CGFloat(min(rank, 2)) * fan : (reduceMotion ? 0 : -28))
                    // The ones behind sit a shade darker, so the fanned edges read as separate cards.
                    .brightness(-0.035 * Double(min(rank, 2)))
                    .opacity(!revealed || rank > 2 ? 0 : 1)
                    .zIndex(Double(count - rank))
                    // Arriving, the front banner lands first and the rest follow it in behind; dealt, they settle with
                    // a little give.
                    .animation(motion.cascade(motion.reveal, index: rank, step: 0.09), value: revealed)
                    .animation(motion.settle, value: front)
            }
        }
        .padding(.bottom, CGFloat(min(count - 1, 2)) * fan)
        .saturation(phase == .denied ? 0 : 1)
        .opacity(phase == .denied ? 0.5 : 1)
        .animation(motion.cascade(motion.morph, index: 1, step: beat), value: phase)
        .piecePop(trigger: grants)
        .accessibilityElement(children: .ignore)
        .accessibilityLabel(Text(benefits.map(\.text).joined(separator: ". ")))
    }

    /// One benefit as a notification banner: a tinted app tile, the line, and "now", on a solid card.
    private func banner(_ index: Int) -> some View {
        let benefit = benefits[index]
        let tint = style.benefitTiles[index % style.benefitTiles.count]
        let clear = tint == .clear
        return HStack(alignment: .center, spacing: 12) {
            Image(systemName: benefit.symbol)
                .font(.system(size: 16, weight: .semibold))
                .foregroundStyle(clear ? style.label : style.ink)
                .frame(width: 38, height: 38)
                .background(clear ? style.label.opacity(0.08) : tint, in: .rect(cornerRadius: 10, style: .continuous))
            Text(benefit.text)
                .font(.subheadline.weight(.semibold))
                .foregroundStyle(style.label)
                .lineLimit(2)
                .frame(maxWidth: .infinity, alignment: .leading)
            Text("now", comment: "Time on the notification preview banners in the permission sheet")
                .font(.caption.weight(.semibold))
                .foregroundStyle(style.secondaryLabel)
        }
        .padding(12)
        .frame(minHeight: 64)
        // Solid, like a notification on the lock screen: the banners behind never show through the one in front.
        .background(bannerFill, in: RoundedRectangle(cornerRadius: 22, style: .continuous))
        .overlay { RoundedRectangle(cornerRadius: 22, style: .continuous).strokeBorder(style.label.opacity(0.06), lineWidth: 1) }
        .shadow(color: .black.opacity(0.06), radius: 10, y: 4)
    }

    /// The banners' card: a step off the sheet's surface, so it reads as its own layer in both appearances.
    private var bannerFill: Color { adaptive(light: 0xF6F5F2, dark: 0x2A2A2A) }

    /// The request on a capsule of glass. Its label morphs letter by letter to Requesting with a spinner, then to
    /// Allowed with a check as the glass turns sage, or to Open Settings on glass the label's color.
    private var requestButton: some View {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        return Button {
            if phase == .denied {
                if let url = URL(string: UIApplication.openSettingsURLString) { openURL(url) }
            } else {
                Task { await run() }
            }
        } label: {
            HStack(spacing: 8) {
                switch phase {
                case .requesting: ProgressView().tint(buttonInk).transition(motion.swap)
                case .granted: Image(systemName: "checkmark").transition(motion.swap)
                default: EmptyView()
                }
                PermissionSheetLabel(text: buttonTitle, font: .headline.weight(.semibold))
            }
            .font(.headline.weight(.semibold))
            .foregroundStyle(buttonInk)
            .padding(.horizontal, 20)
            .frame(maxWidth: .infinity, minHeight: 56)
            .contentShape(.capsule)
            .pieceLiquid(.capsule, tint: buttonTint, interactive: false)
            // A tap answers at once with the spinner. A result reaches the button's look two beats after the
            // tile. Only the look: its action and enabled state change at once, and its place moves with the
            // card, so it never trails its own container.
            .animation(phase == .requesting ? motion.morph : motion.cascade(motion.morph, index: 2, step: beat), value: phase)
        }
        .buttonStyle(PieceLiquidPressStyle())
        .disabled(phase == .requesting || phase == .granted)
        .accessibilityLabel(buttonTitle)
    }

    /// Not now's place under the request button. It keeps its size while Not now is inside the button, so the button
    /// just tapped never shifts.
    private var skipSlot: some View {
        skipLabel(out: true)
            .hidden()
            .onGeometryChange(for: CGFloat.self) { $0.size.height } action: { skipHeight = $0 }
            .overlay {
                if let onSkip, buds.contains("skip") { skipButton(onSkip) }
            }
    }

    private func skipLabel(out: Bool) -> some View {
        Text("Not now")
            .font(.headline.weight(.semibold))
            .foregroundStyle(style.secondaryLabel)
            .pieceBudContent(out: out)
            .padding(.horizontal, 20)
            .frame(maxWidth: .infinity, minHeight: 56)
    }

    /// A neutral glass capsule. On a grant it melts up into the request button, just inside its lower end, shrunk.
    private func skipButton(_ onSkip: @escaping () -> Void) -> some View {
        let out = buds.isOut("skip")
        return Button(action: onSkip) {
            skipLabel(out: out)
                .contentShape(.capsule)
                .pieceLiquid(.capsule, interactive: false)
        }
        .buttonStyle(PieceLiquidPressStyle())
        .disabled(phase == .requesting || phase == .granted)
        .accessibilityHidden(phase == .granted)
        .pieceBud(out: out, home: CGSize(width: 0, height: -(skipHeight / 2 + PieceLiquid.apart + skipHeight * PieceLiquid.homeScale / 2)))
    }

    /// Title and message. A denial turns the card over: the old copy lifts away through a blur a beat after the tile,
    /// and the denied copy rises into its place once the old one has mostly gone, so the two never blur together.
    /// Both stay in the stack, so the sheet keeps its height, and VoiceOver reads only the one shown.
    private func copy(title: String, message: String, shown: Bool, away: CGFloat) -> some View {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        return VStack(alignment: .center, spacing: 8) {
            Text(title)
                .font(.system(size: titleSize, weight: .semibold))
                .tracking(-0.8)
                .foregroundStyle(style.label)
            Text(message)
                .font(.body.weight(.semibold))
                .foregroundStyle(style.secondaryLabel)
        }
        .multilineTextAlignment(.center)
        .fixedSize(horizontal: false, vertical: true)
        .frame(maxWidth: .infinity, alignment: .center)
        // The delay is scoped to the look, so the copy never trails a card that moves or resizes around it.
        .animation(shown ? motion.cascade(motion.reveal, index: 3, step: beat) : motion.cascade(motion.dismiss, index: 1, step: beat)) { content in
            content
                .opacity(shown ? 1 : 0)
                .blur(radius: shown || reduceMotion ? 0 : 6)
                .offset(y: shown || reduceMotion ? 0 : away)
        }
        .accessibilityElement(children: .combine)
        .accessibilityHidden(!shown)
    }

    private func run() async {
        guard phase == .idle else { return }
        phase = .requesting
        let granted = await request()
        // The answer comes back: the haptic, the lead's new face and a grant's pop share this beat.
        withAnimation(PieceMotion(reduceMotion: reduceMotion).morph) {
            phase = granted ? .granted : .denied
            if granted { grants += 1 }
        }
        guard granted else { return }
        // Not now is no longer a choice, so it melts into the button that made it.
        if buds.contains("skip") {
            Task { await buds.gather(["skip"], reduceMotion: reduceMotion) }
        }
        try? await Task.sleep(for: .seconds(0.9))
        onGranted()
    }
}

/// A label that morphs letter by letter when it changes. At accessibility text sizes it is plain text that can wrap,
/// and blurs from the old words to the new instead.
private struct PermissionSheetLabel: View {
    let text: String
    let font: Font
    @Environment(\.dynamicTypeSize) private var typeSize
    @Environment(\.accessibilityReduceMotion) private var reduceMotion

    var body: some View {
        if typeSize.isAccessibilitySize {
            Text(text)
                .font(font)
                .multilineTextAlignment(.center)
                .id(text)
                .transition(PieceMotion(reduceMotion: reduceMotion).swap)
        } else {
            PieceMorphText(text: text, font: font)
        }
    }
}

/// A house-palette color that follows the interface style.
private func adaptive(light: UInt32, dark: UInt32) -> Color {
    Color(uiColor: UIColor { @Sendable traits in
        let hex = traits.userInterfaceStyle == .dark ? dark : light
        return UIColor(red: CGFloat((hex >> 16) & 0xFF) / 255, green: CGFloat((hex >> 8) & 0xFF) / 255, blue: CGFloat(hex & 0xFF) / 255, alpha: 1)
    })
}

// MARK: - Example

/// The sheet alone, presented over the plain ground. `granted: false` plays the denied answer.
private struct PermissionSheetExample: View {
    var granted = true
    @State private var showing = true
    @State private var height: CGFloat = 360

    var body: some View {
        Color.clear
            .frame(maxWidth: .infinity, maxHeight: .infinity)
            .background(adaptive(light: 0xF3F2EE, dark: 0x121212))
            .sheet(isPresented: $showing) {
                PermissionSheet(
                    systemImage: "bell.badge.fill",
                    title: "Turn on notifications",
                    message: "We only send what matters, and you can change this any time.",
                    benefits: [
                        .init(symbol: "clock.fill", text: "Design review in 30 minutes"),
                        .init(symbol: "person.2.fill", text: "Mara replied to Groceries"),
                        .init(symbol: "moon.fill", text: "Quiet hours from 10 PM to 7 AM"),
                    ],
                    allowTitle: "Allow notifications",
                    request: { try? await Task.sleep(for: .seconds(1)); return granted },
                    onGranted: { showing = false },
                    onSkip: { showing = false }
                )
                // Fitted to its content: the banner stack, the copy and the actions, not a full-height sheet.
                .fixedSize(horizontal: false, vertical: true)
                .onGeometryChange(for: CGFloat.self) { $0.size.height } action: { height = $0 }
                .presentationDetents([.height(height)])
                .presentationDragIndicator(.visible)
                .presentationCornerRadius(34)
                .presentationBackground(PermissionSheet.Style.standard.surface)
            }
    }
}

#Preview("Light") {
    PermissionSheetExample()
}

#Preview("Dark") {
    PermissionSheetExample()
        .preferredColorScheme(.dark)
}

#Preview("Denied") {
    PermissionSheetExample(granted: false)
}

// MARK: - Piece motion
//
// The SwiftPieces motion language: shared spring tokens and gesture physics, with the same values in every
// piece. Each piece carries a copy of only the parts it uses, so this file stands alone. Generated from
// registry/foundation/PieceMotion.swift in the SwiftPieces repo; edit it there, not here.
// swiftpieces-motion: 1.1.0 (core, follow, pressMath, pop)

/// The SwiftPieces motion language: five spring tiers, and a named role for every moment a piece moves,
/// each with its Reduce Motion substitute.
///
/// Build one from the environment, `PieceMotion(reduceMotion: reduceMotion)`, and pick the role that names
/// what just happened. Never animate the tracking of a finger: set gesture state directly in `onChanged`, so the
/// surface stays under the finger, and spring only the release.
private struct PieceMotion {
    var reduceMotion = false

    // Tiers. Overshoot and settle times are measured from rest with SwiftUI's Spring.
    /// No overshoot, 90% in about 90ms. A press arriving under the finger.
    static var tight: Spring { Spring(duration: 0.14, bounce: 0) }
    /// About 2.8% overshoot, 90% in about 140ms. Snapping to a detent, page or segment.
    static var responsive: Spring { Spring(duration: 0.32, bounce: 0.25) }
    /// About 8.4% overshoot. Visible give: a release, a return from past an edge, a drag settling home.
    static var elastic: Spring { Spring(duration: 0.42, bounce: 0.38) }
    /// About 15% overshoot. A resolved action landing. At most once per interaction.
    static var expressive: Spring { Spring(duration: 0.48, bounce: 0.48) }
    /// About 1.5% overshoot, unhurried. Opening large surfaces and ambient change.
    static var calm: Spring { Spring(duration: 0.5, bounce: 0.2) }

    /// Reduce Motion: settles become this short spring with no overshoot.
    private static var still: Animation { .spring(duration: 0.25, bounce: 0) }

    // Roles.
    /// Touch-down. Starts on the same frame and never bounces under the finger.
    var press: Animation { .spring(Self.tight) }
    /// The finger lifts off a pressed surface, which springs back through rest.
    var release: Animation { reduceMotion ? Self.still : .spring(Self.elastic) }
    /// A dragged thing comes to rest. With a gesture's velocity, use `settle(velocity:from:to:)` instead.
    var settle: Animation { reduceMotion ? Self.still : .spring(Self.elastic) }
    /// Lands on a detent, page or segment.
    var snap: Animation { reduceMotion ? Self.still : .spring(Self.responsive) }
    /// A number, a chart value or anything else people read moves to its new value. Never overshoots, so it
    /// never shows a value that isn't true.
    var value: Animation { reduceMotion ? Self.still : .spring(duration: 0.35, bounce: 0) }
    /// Comes back from past a limit: a pull beyond the edge, a value pushed against its bound.
    var rebound: Animation { reduceMotion ? Self.still : .spring(Self.elastic) }
    /// A shape or container changes size, corner radius or form, with a little give at the end.
    var morph: Animation { reduceMotion ? Self.still : .spring(duration: 0.4, bounce: 0.2) }
    /// Something appears, opens or expands. Opening is a little slower than closing.
    var reveal: Animation { reduceMotion ? .easeOut(duration: 0.2) : .spring(Self.calm) }
    /// Something leaves, closes or collapses. Quick and firm, out of the way.
    var dismiss: Animation { reduceMotion ? .easeIn(duration: 0.18) : .spring(duration: 0.3, bounce: 0.08) }
    /// A resolved action lands: a check, a sent state, a reaction.
    var success: Animation { reduceMotion ? .easeOut(duration: 0.24) : .spring(Self.expressive) }
    /// A refused action. Firm, no wobble; `pieceShake` adds the movement.
    var error: Animation { reduceMotion ? .easeOut(duration: 0.2) : .spring(Self.responsive) }
    /// Slow ambient change. Loops themselves stop under Reduce Motion: check `allowsAmbient`.
    var ambient: Animation { reduceMotion ? .easeInOut(duration: 0.3) : .spring(Self.calm) }

    /// Loops, drifts, idle breathing and parallax run only when this is true.
    var allowsAmbient: Bool { !reduceMotion }

    /// A moving transition, or a plain fade under Reduce Motion.
    func transition(_ transition: AnyTransition) -> AnyTransition {
        reduceMotion ? .opacity : transition
    }

    /// For content replaced inside a container that stays put (a label, a glyph, a count): the old content
    /// blurs out as the new one sharpens in. A fade under Reduce Motion.
    @MainActor var swap: AnyTransition {
        reduceMotion ? .opacity : AnyTransition(.blurReplace)
    }
}

extension PieceMotion {
    /// Follow-through: rank 0 leads, each later rank arrives a beat later on a slightly looser spring. Safe to reverse.
    func follow(_ spring: Spring = PieceMotion.elastic, rank: Int) -> Animation {
        guard !reduceMotion else { return .spring(duration: 0.25, bounce: 0) }
        let k = Double(min(max(rank, 0), 6))
        return .spring(duration: spring.duration + 0.04 * k, bounce: min(spring.bounce + 0.02 * k, 0.55))
    }

    /// One-shot entrances only: item `index` waits 30ms per place, capped at the seventh. Exits go together.
    func cascade(_ animation: Animation, index: Int, step: Double = 0.03) -> Animation {
        guard !reduceMotion, index > 0 else { return animation }
        return animation.delay(step * Double(min(index, 7)))
    }
}

extension PieceMotion {
    /// About `depth` points per edge, not a fixed percentage: an icon sinks to 0.92, a pill 0.95, a card 0.985.
    nonisolated static func pressScale(for size: CGSize, depth: CGFloat = 2.5) -> CGFloat {
        let side = (max(size.width, 1) * max(size.height, 1)).squareRoot()
        return min(max(1 - depth * 2 / side, 0.92), 0.985)
    }

    /// An anchor partway from the center toward the touch, so the press leans into the finger without tipping.
    nonisolated static func pressAnchor(touch: CGPoint?, in size: CGSize, lean: CGFloat = 0.6) -> UnitPoint {
        guard let touch, size.width > 0, size.height > 0 else { return .center }
        let x = min(max(touch.x / size.width, 0), 1)
        let y = min(max(touch.y / size.height, 0), 1)
        return UnitPoint(x: 0.5 + (x - 0.5) * lean, y: 0.5 + (y - 0.5) * lean)
    }
}

/// Anticipation, overshoot, settle: dips, swells past full size and lands each time `trigger` changes. Under
/// Reduce Motion it stays still (same view, no identity change) and the color or symbol carries the meaning.
private struct PiecePop: ViewModifier {
    let trigger: AnyHashable
    var amount: CGFloat = 0.08
    @Environment(\.accessibilityReduceMotion) private var reduceMotion

    func body(content: Content) -> some View {
        let amount = reduceMotion ? 0 : amount
        content.keyframeAnimator(initialValue: CGFloat(1), trigger: trigger) { view, scale in
            view.scaleEffect(scale)
        } keyframes: { _ in
            KeyframeTrack {
                CubicKeyframe(1 - amount * 0.4, duration: 0.08)
                SpringKeyframe(1 + amount, duration: 0.14, spring: Spring(duration: 0.18, bounce: 0))
                SpringKeyframe(1, duration: 0.42, spring: PieceMotion.expressive)
            }
        }
    }
}

private extension View {
    /// Pops each time `trigger` changes. Use a counter, never a Bool that can flip back before it fires.
    func piecePop(trigger: some Hashable & Sendable, amount: CGFloat = 0.08) -> some View {
        modifier(PiecePop(trigger: AnyHashable(trigger), amount: amount))
    }
}

// swiftpieces-motion: end

// MARK: - Piece liquid
//
// The SwiftPieces liquid glass language: glass shapes that merge through a neck, bubbles that bud out of
// and melt back into each other, and the frosted fallback before iOS 26. Each piece carries a copy of only
// the parts it uses, so this file stands alone. Generated from registry/foundation/PieceLiquid.swift in the
// SwiftPieces repo; edit it there, not here. The rules are in LIQUID_GLASS.md.
// swiftpieces-liquid: 1.8.0 (liquid, liquidPress, bud, morphText)

/// The liquid glass language: one merge distance, two rest gaps, and the springs a bubble leaves and comes home on.
///
/// Glass shapes inside one `PieceLiquidGroup` melt into each other through a neck when they come within `merge`
/// points. Parts of one control rest `joined`, inside that distance, so the neck holds; separate actions rest
/// `apart`, outside it, so they only goo while one buds out of, or melts back into, another.
private enum PieceLiquid {
    /// Glass shapes closer than this share a neck.
    static let merge: CGFloat = 20
    /// The gap between parts of one control (a stepper's buttons, a progress pill and its stop): the neck holds,
    /// short and smooth, about two thirds of the shapes' height at its waist. Joined parts read best at one height.
    static let joined: CGFloat = 4
    /// The gap between separate actions (menu items, confirm and cancel, chips): they rest as their own bubbles.
    static let apart: CGFloat = 26
    /// How far a bubble shrinks while it is home inside its parent.
    static let homeScale: CGFloat = 0.72

    /// A bubble leaving its parent: slow enough that the neck's stretch and snap read.
    static func split(reduceMotion: Bool) -> Animation {
        reduceMotion ? .easeOut(duration: 0.2) : .spring(duration: 0.62, bounce: 0.22)
    }

    /// A bubble going home. No bounce: a bounce would carry it out through the far side of its parent.
    static func home(reduceMotion: Bool) -> Animation {
        reduceMotion ? .easeIn(duration: 0.18) : .spring(duration: 0.5, bounce: 0)
    }
}

/// A group of glass shapes that merge into one liquid surface. On iOS 26 it is a `GlassEffectContainer`; before
/// that, and under Reduce Transparency, the shapes draw on their own and simply don't merge. `lift` adds the soft
/// shadow liquid glass floats on in light mode.
private struct PieceLiquidGroup<Content: View>: View {
    var spacing: CGFloat = PieceLiquid.merge
    var lift = true
    @ViewBuilder var content: Content
    @Environment(\.accessibilityReduceTransparency) private var reduceTransparency
    @Environment(\.colorScheme) private var colorScheme

    var body: some View {
        container
            .shadow(color: .black.opacity(lift && colorScheme == .light ? 0.07 : 0), radius: 18, y: 8)
    }

    @ViewBuilder private var container: some View {
        #if compiler(>=6.2)
        if #available(iOS 26, *), !reduceTransparency {
            GlassEffectContainer(spacing: spacing) { content }
        } else {
            content
        }
        #else
        content
        #endif
    }
}

/// One liquid glass shape: Liquid Glass on iOS 26, carrying `tint` as a solid colour and swelling under a press when
/// `interactive`; a frosted Material with a light rim and a soft shadow before that; a solid fill under Reduce
/// Transparency.
///
/// The tint is painted inside clear glass rather than tinting the glass. Tinted glass in a group bleeds its colour
/// through every neck as a smear, so a red button would glow into the white pill it is joined to; painted inside,
/// the colour stays crisp to the shape's edge, the necks between shapes are clear glass, and a tint change animates
/// like any colour (tinted glass snaps).
private struct PieceLiquidSurface<S: Shape>: ViewModifier {
    var shape: S
    var tint: Color?
    var interactive: Bool
    @Environment(\.accessibilityReduceTransparency) private var reduceTransparency
    @Environment(\.colorScheme) private var colorScheme
    @Environment(\.pieceLiquidScale) private var scale

    func body(content: Content) -> some View {
        #if compiler(>=6.2)
        if #available(iOS 26, *), !reduceTransparency {
            // Scaled as two parts, content and outline, about the same centre. A scaleEffect on a glass view inside a
            // GlassEffectContainer shrinks the glass but leaves what it carries full size, pinned to its corner.
            content
                .background { shape.fill(tint ?? .clear) }
                .scaleEffect(scale)
                .glassEffect(glass, in: shape.scale(scale))
        } else {
            fallback(content).scaleEffect(scale)
        }
        #else
        fallback(content).scaleEffect(scale)
        #endif
    }

    #if compiler(>=6.2)
    @available(iOS 26, *)
    private var glass: Glass {
        interactive ? Glass.regular.interactive() : .regular
    }
    #endif

    private func fallback(_ content: Content) -> some View {
        let dark = colorScheme == .dark
        return content
            .background {
                if reduceTransparency {
                    shape.fill(tint ?? (dark ? Color(white: 0.17) : Color(white: 0.97)))
                } else {
                    ZStack {
                        shape.fill(.regularMaterial)
                        if let tint { shape.fill(tint.opacity(0.88)) }
                    }
                }
            }
            .overlay { shape.stroke(Color.white.opacity(dark ? 0.14 : 0.7), lineWidth: 0.5) }
            .shadow(color: .black.opacity(dark ? 0.32 : 0.08), radius: 10, y: 5)
    }
}

private struct PieceLiquidScaleKey: EnvironmentKey {
    static let defaultValue: CGFloat = 1
}

private extension EnvironmentValues {
    /// How much the liquid shapes below are scaled, about their own centres. Nested scales multiply.
    var pieceLiquidScale: CGFloat {
        get { self[PieceLiquidScaleKey.self] }
        set { self[PieceLiquidScaleKey.self] = newValue }
    }
}

private extension View {
    /// Draws this view on a liquid glass `shape`. Put it inside a `PieceLiquidGroup` so it can merge with its neighbours.
    func pieceLiquid<S: Shape>(_ shape: S, tint: Color? = nil, interactive: Bool = true) -> some View {
        modifier(PieceLiquidSurface(shape: shape, tint: tint, interactive: interactive))
    }

    /// Scales the liquid glass shapes in this view, content and outline together, about their own centres. Use it
    /// instead of `scaleEffect` on a glass view (a press, a lift, a swell): inside a group a plain `scaleEffect` shrinks
    /// the glass but leaves its content full size and off centre. Animates like any other value.
    func pieceLiquidScale(_ scale: CGFloat) -> some View {
        transformEnvironment(\.pieceLiquidScale) { $0 *= scale }
    }
}

/// The press for a glass control: the same size-aware sink and lean as `piecePress`, applied through
/// `pieceLiquidScale` so the glass and what it carries sink together (a plain scaleEffect on glass leaves the content
/// behind). Put the glass inside what it presses: the label of a button, the view this modifies. Under Reduce Motion
/// it shades instead of moving.
private struct PieceLiquidPress: ViewModifier {
    let pressed: Bool
    var touch: CGPoint?
    var depth: CGFloat = 2.5
    @Environment(\.accessibilityReduceMotion) private var reduceMotion
    @Environment(\.colorScheme) private var colorScheme
    @State private var size: CGSize = .zero
    @State private var anchor: UnitPoint = .center

    func body(content: Content) -> some View {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        let scale = pressed && !reduceMotion ? PieceMotion.pressScale(for: size, depth: depth) : 1
        // A scale about `anchor` is a scale about the centre plus this shift toward the anchor.
        let lean = CGSize(width: (anchor.x - 0.5) * size.width * (1 - scale), height: (anchor.y - 0.5) * size.height * (1 - scale))
        content
            .onGeometryChange(for: CGSize.self) { $0.size } action: { size = $0 }
            .brightness(pressed && reduceMotion ? (colorScheme == .dark ? 0.1 : -0.08) : 0)
            .pieceLiquidScale(scale)
            .offset(lean)
            .animation(pressed ? motion.press : motion.release, value: pressed)
            .onChange(of: pressed) { _, isPressed in
                if isPressed { anchor = PieceMotion.pressAnchor(touch: touch, in: size) }
            }
            .onChange(of: touch) { _, touch in
                if pressed, let touch { anchor = PieceMotion.pressAnchor(touch: touch, in: size) }
            }
    }
}

/// `PiecePressStyle` for glass buttons: the label (with its `.pieceLiquid` inside) sinks as one.
private struct PieceLiquidPressStyle: ButtonStyle {
    var depth: CGFloat = 2.5

    func makeBody(configuration: Configuration) -> some View {
        configuration.label.pieceLiquidPress(configuration.isPressed, depth: depth)
    }
}

private extension View {
    /// Sinks this view's glass while `pressed`, leaning toward `touch` (in this view's coordinates) when given.
    func pieceLiquidPress(_ pressed: Bool, touch: CGPoint? = nil, depth: CGFloat = 2.5) -> some View {
        modifier(PieceLiquidPress(pressed: pressed, touch: touch, depth: depth))
    }
}

/// The bud: how a bubble leaves and rejoins its parent, driven explicitly so every bubble shows the whole cycle.
///
/// A bubble is born at `home`, inside its parent, where the two glass shapes are one. It springs out to `rest`, and
/// while it is inside the merge distance a neck holds it to the parent, thinning as it goes, until it snaps free.
/// Going home it springs back on a spring with no bounce, the neck reaches out and re-forms, and only once it has
/// melted all the way in is it removed. Both offsets are relative to where the bubble is laid out. Under Reduce
/// Motion it stays at `rest`: its content fades and its glass closes in place.
private struct PieceBud: ViewModifier {
    var out: Bool
    var rest: CGSize
    var home: CGSize
    @Environment(\.accessibilityReduceMotion) private var reduceMotion

    func body(content: Content) -> some View {
        if reduceMotion {
            // No travel. Inside a group glass ignores opacity, so the glass closes to nothing in place on the short
            // Reduce Motion ease while its content fades; the opacity covers a bubble outside a group.
            content
                .pieceLiquidScale(out ? 1 : 0.001)
                .opacity(out ? 1 : 0)
                .offset(rest)
        } else {
            // The glass shrinks through `pieceLiquidScale`, never a plain scaleEffect (see there), then moves.
            content
                .pieceLiquidScale(out ? 1 : PieceLiquid.homeScale)
                .offset(out ? rest : home)
        }
    }
}

/// A bubble's own content, on its own clock: gone the moment the bubble heads home, so it never rides over the
/// parent's content, and arriving just after the bubble leaves.
private struct PieceBudContent: ViewModifier {
    var out: Bool

    func body(content: Content) -> some View {
        content
            .blur(radius: out ? 0 : 6)
            .opacity(out ? 1 : 0)
            .animation(out ? .easeOut(duration: 0.3).delay(0.1) : .easeOut(duration: 0.14), value: out)
    }
}

private extension View {
    /// Places a bubble at `rest` while `out`, and at `home` (inside its parent, shrunk) while not.
    func pieceBud(out: Bool, rest: CGSize = .zero, home: CGSize) -> some View {
        modifier(PieceBud(out: out, rest: rest, home: home))
    }

    /// Hides a bubble's icon or label while it is home. Put it on the content, inside the glass.
    func pieceBudContent(out: Bool) -> some View {
        modifier(PieceBudContent(out: out))
    }
}

/// Which bubbles exist and which are out. A bubble is added home with no animation, sent out on the next frame,
/// and called home before it is removed, so it always melts in rather than fading. Keep one in `@State`.
@MainActor @Observable
private final class PieceBuds {
    private(set) var present: [String] = []
    private(set) var out: Set<String> = []
    /// The latest call for each bubble. A bloom or gather that has been overtaken (a bubble sent home while it was
    /// still waiting to go out, or called out again while melting) leaves that bubble alone.
    @ObservationIgnored private var turn: [String: Int] = [:]

    func contains(_ id: String) -> Bool { present.contains(id) }
    func isOut(_ id: String) -> Bool { out.contains(id) }

    private func claim(_ ids: [String]) -> [String: Int] {
        var mine: [String: Int] = [:]
        for id in ids {
            let next = (turn[id] ?? 0) + 1
            turn[id] = next
            mine[id] = next
        }
        return mine
    }

    /// Puts bubbles straight out at rest with no motion: a view's first frame, or a state restored.
    func place(_ ids: [String]) {
        _ = claim(ids)
        var quiet = Transaction()
        quiet.disablesAnimations = true
        withTransaction(quiet) {
            for id in ids where !present.contains(id) { present.append(id) }
            out.formUnion(ids)
        }
    }

    /// Adds bubbles home, then sends each out, `stagger` seconds apart, after an optional `delay`.
    func bloom(_ ids: [String], reduceMotion: Bool, stagger: Double = 0.05, delay: Double = 0) async {
        let mine = claim(ids)
        var quiet = Transaction()
        quiet.disablesAnimations = true
        withTransaction(quiet) {
            for id in ids where !present.contains(id) { present.append(id) }
        }
        try? await Task.sleep(for: .milliseconds(24 + Int(max(delay, 0) * 1000)))
        let split = PieceLiquid.split(reduceMotion: reduceMotion)
        for (i, id) in ids.enumerated() where turn[id] == mine[id] {
            withAnimation(split.delay(reduceMotion ? 0 : Double(i) * stagger)) { _ = out.insert(id) }
        }
    }

    /// Calls bubbles home, last first, then removes them once they have melted in.
    func gather(_ ids: [String], reduceMotion: Bool, stagger: Double = 0.04) async {
        let mine = claim(ids)
        let home = PieceLiquid.home(reduceMotion: reduceMotion)
        for (i, id) in ids.reversed().enumerated() {
            withAnimation(home.delay(reduceMotion ? 0 : Double(i) * stagger)) { _ = out.remove(id) }
        }
        try? await Task.sleep(for: .milliseconds(Int((0.52 + Double(ids.count) * stagger) * 1000)))
        var quiet = Transaction()
        quiet.disablesAnimations = true
        withTransaction(quiet) { present.removeAll { ids.contains($0) && !out.contains($0) && turn[$0] == mine[$0] } }
    }
}

/// A label that changes letter by letter: letters both strings share hold still, the rest blur out and the new ones
/// blur in a few milliseconds apart. Under Reduce Motion it cross-fades. VoiceOver reads the whole string.
///
/// Only text whose letters stand on their own is cut into letters: Latin, Greek, Cyrillic, CJK, digits and symbols.
/// Those letters are laid out left to right whatever the layout direction, the order they read in, so "1.5×" stays
/// "1.5×" in an Arabic or Hebrew interface instead of coming out backwards. Text in a right-to-left script, or in one
/// whose letters join or reorder (Arabic, Hebrew, Devanagari, Thai and their neighbours), can't be cut apart without
/// breaking it, so it changes as one: the old string blurs out where it stood while the new one blurs in.
private struct PieceMorphText: View {
    var text: String
    var font: Font = .body.weight(.semibold)
    @Environment(\.accessibilityReduceMotion) private var reduceMotion

    var body: some View {
        // The morphing label at its full size when it fits. When it doesn't (a long label at an accessibility text
        // size, a narrow screen), the same words wrap onto a second line and shrink a little instead of running past
        // the screen's edge; that one changes as a whole rather than letter by letter. A single word never splits
        // across lines: it stays on one and shrinks further.
        let oneWord = !text.contains(where: \.isWhitespace)
        ViewThatFits(in: .horizontal) {
            morphing.fixedSize()
            Text(text)
                .lineLimit(oneWord ? 1 : 2)
                .multilineTextAlignment(.center)
                .minimumScaleFactor(oneWord ? 0.5 : 0.75)
                .fixedSize(horizontal: false, vertical: true)
                .id(text)
                .transition(transition(0, scale: 0.9))
        }
        .font(font)
        .accessibilityElement(children: .ignore)
        .accessibilityLabel(text)
    }

    @ViewBuilder private var morphing: some View {
        if Self.splits(text) {
            let glyphs = Array(text)
            HStack(spacing: 0) {
                ForEach(glyphs.indices, id: \.self) { i in
                    Text(String(glyphs[i]))
                        .id("\(i)\(glyphs[i])")
                        .transition(transition(i, scale: 0.6))
                }
            }
            // The letters' reading order, not the layout's: an HStack lays its children out backwards under a
            // right-to-left layout.
            .environment(\.layoutDirection, .leftToRight)
        } else {
            ZStack(alignment: .leading) {
                Text(text)
                    .id(text)
                    .transition(transition(0, scale: 0.9))
            }
        }
    }

    private func transition(_ i: Int, scale: CGFloat) -> AnyTransition {
        guard !reduceMotion else { return .opacity }
        return AnyTransition(.blurReplace(.downUp)).combined(with: .scale(scale: scale, anchor: .bottom))
            .animation(.spring(duration: 0.42, bounce: 0.3).delay(Double(i) * 0.022))
    }

    /// Whether `text` can be cut into letters that each read left to right on their own: no right-to-left script, no
    /// script that joins or reorders its letters, and no bidirectional control characters.
    private static func splits(_ text: String) -> Bool {
        !text.unicodeScalars.contains { scalar in
            switch scalar.value {
            case 0x0590...0x08FF, // Hebrew, Arabic, Syriac, Thaana, NKo, Samaritan, Mandaic
                 0x0900...0x0DFF, // Devanagari to Sinhala
                 0x0E00...0x0FFF, // Thai, Lao, Tibetan
                 0x1000...0x109F, // Myanmar
                 0x1780...0x17FF, // Khmer
                 0xA8E0...0xA8FF, // Devanagari Extended
                 0xFB1D...0xFDFF, // Hebrew and Arabic presentation forms
                 0xFE70...0xFEFF, // Arabic presentation forms B
                 0x200E...0x200F, 0x202A...0x202E, 0x2066...0x2069, // bidirectional marks, embeddings and isolates
                 0x10800...0x10FFF, // historic right-to-left scripts
                 0x1E800...0x1EFFF: // Adlam and other right-to-left scripts
                return true
            default:
                return false
            }
        }
    }
}

// swiftpieces-liquid: end