Glass Mode Bar

// 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
import UIKit

/// One liquid glass body that becomes the controls of the mode you pick, without leaving its place.
///
/// At rest a capsule spans the width offered, its mode icons spread evenly across it. Tapping a mode buds a signal
/// anchor out of the capsule's leading end, carrying that mode's icon, while the capsule draws its leading edge back
/// to make room and trades its icons for the mode's controls, which you build. The two rest joined, one body. Tapping
/// the anchor melts it home into the capsule as the icons return. The outer edges never move.
///
/// ```swift
/// @State private var mode: String?
///
/// GlassModeBar(modes: [
///     .init(id: "voice", systemImage: "mic", title: "Voice"),
///     .init(id: "timer", systemImage: "timer", title: "Timer"),
/// ], selection: $mode) { mode in
///     switch mode.id {
///     case "voice": RecordingControls()
///     default: TimerChips()
///     }
/// }
/// ```
///
/// - Parameters:
///   - modes: The modes, in order across the capsule.
///   - selection: The open mode's `id`, or `nil` at rest. Set it yourself to open or close a mode from your code.
///   - style: Colors. Defaults to the SwiftPieces house palette, adapting to light and dark.
///   - panel: The controls for a mode, laid out in the capsule beside the anchor. They fill the capsule's height (56pt)
///     and the width left after the anchor. Keep them plain content: the capsule is already the glass.
public struct GlassModeBar<Panel: View>: View {
    /// One mode. The same type as `GlassModeBarMode`, so `.init(id:systemImage:title:)` works at the call site.
    public typealias Mode = GlassModeBarMode
    /// Colors. The same type as `GlassModeBarStyle`.
    public typealias Style = GlassModeBarStyle

    @Environment(\.accessibilityReduceMotion) private var reduceMotion
    /// The anchor bubble: home inside the capsule's leading end until a mode opens, then out beside it.
    @State private var buds = PieceBuds()
    /// The mode the body is showing. It trails `selection` while the anchor melts home, so the panel stays until the
    /// anchor is inside the capsule.
    @State private var shown: String?
    /// The mode the anchor carries. It outlives `shown` while the anchor melts home, so the anchor never loses its
    /// glyph on the way in.
    @State private var anchored: String?
    @State private var openTick = 0
    @State private var closeTick = 0

    private let modes: [Mode]
    private let selection: Binding<String?>
    private let style: Style
    private let panel: (Mode) -> Panel

    /// The capsule's height and the anchor's diameter: the house size for round controls.
    private let height: CGFloat = 56

    public init(modes: [Mode], selection: Binding<String?>, style: Style = .standard, @ViewBuilder panel: @escaping (Mode) -> Panel) {
        self.modes = modes
        self.selection = selection
        self.style = style
        self.panel = panel
    }

    private var motion: PieceMotion { PieceMotion(reduceMotion: reduceMotion) }
    private var open: Mode? { shown.flatMap { id in modes.first { $0.id == id } } }
    private var anchorMode: Mode? { anchored.flatMap { id in modes.first { $0.id == id } } }

    public var body: some View {
        // Read here as well as in the anchor's builder, so a bud change always redraws the body.
        let _ = (buds.present, buds.out)
        PieceLiquidGroup {
            ZStack(alignment: .leading) {
                // The anchor first, so while it is home it sits under the capsule and melts into it.
                if buds.contains("anchor"), let anchorMode {
                    anchor(anchorMode)
                }
                capsule
                    // Open, the capsule draws its leading edge back to make room for the anchor, which rests joined.
                    .padding(.leading, open == nil ? 0 : height + PieceLiquid.joined)
            }
        }
        .fontWeight(.semibold)
        .animation(motion.morph, value: shown)
        .sensoryFeedback(.selection, trigger: openTick)
        .sensoryFeedback(.impact(flexibility: .soft, intensity: 0.5), trigger: closeTick)
        .onChange(of: selection.wrappedValue) { _, id in apply(id) }
        .onAppear {
            // A mode passed in already open shows straight away, with no motion.
            if let id = selection.wrappedValue, modes.contains(where: { $0.id == id }) {
                shown = id
                anchored = id
                buds.place(["anchor"])
            }
        }
    }

    // MARK: Capsule

    /// The glass body: the mode icons at rest, the open mode's controls beside the anchor.
    private var capsule: some View {
        ZStack {
            if let open {
                panel(open)
                    .padding(.horizontal, 14)
                    .frame(maxWidth: .infinity, alignment: .leading)
                    .transition(motion.swap)
            } else {
                HStack(spacing: 0) {
                    ForEach(modes) { mode in
                        Button { selection.wrappedValue = mode.id } label: {
                            Image(systemName: mode.systemImage)
                                .font(.title3.weight(.semibold))
                                .foregroundStyle(style.label)
                                .frame(maxWidth: .infinity, minHeight: height)
                                .contentShape(.rect)
                        }
                        .buttonStyle(.plain)
                        .accessibilityLabel(mode.title)
                    }
                }
                .padding(.horizontal, 6)
                .transition(motion.swap)
            }
        }
        .frame(maxWidth: .infinity, minHeight: height)
        .pieceLiquid(Capsule(), interactive: false)
    }

    // MARK: Anchor

    /// The signal bubble carrying the open mode's icon. Born just inside the capsule's leading end, shrunk, and budded
    /// out to rest joined beside it; tapping it closes the mode.
    private func anchor(_ mode: Mode) -> some View {
        let out = buds.isOut("anchor")
        return Button { selection.wrappedValue = nil } label: {
            Image(systemName: mode.systemImage)
                .font(.title3.weight(.semibold))
                .foregroundStyle(style.ink)
                .pieceBudContent(out: out)
                .frame(width: height, height: height)
                .pieceLiquid(.circle, tint: out ? style.anchor : nil)
        }
        .buttonStyle(PieceLiquidPressStyle())
        .accessibilityLabel(Text("Close \(mode.title)"))
        .pieceBud(out: out, home: CGSize(width: height / 2 + PieceLiquid.joined + height * PieceLiquid.homeScale / 2, height: 0))
    }

    // MARK: Opening and closing

    private func apply(_ id: String?) {
        if let id, modes.contains(where: { $0.id == id }) {
            guard shown != id else { return }
            if shown != nil {
                // Switching modes from your code: the panel swaps in place and the anchor's glyph with it.
                shown = id
                anchored = id
                return
            }
            // One beat: the capsule draws back and trades its icons for the panel while the anchor buds out, so the
            // body is never empty between the two.
            shown = id
            anchored = id
            openTick += 1
            Task { await buds.bloom(["anchor"], reduceMotion: reduceMotion) }
        } else if shown != nil {
            closeTick += 1
            Task {
                // The anchor melts home while the capsule reaches back for it and the icons return, overlapping, so
                // there is no blank beat between the anchor going in and the icons coming back.
                async let home: Void = buds.gather(["anchor"], reduceMotion: reduceMotion)
                try? await Task.sleep(for: .milliseconds(reduceMotion ? 0 : 160))
                shown = nil
                await home
                anchored = nil
            }
        }
    }
}

/// One mode of a `GlassModeBar`: its icon in the capsule and on the anchor, and its name for VoiceOver.
public struct GlassModeBarMode: Identifiable, Hashable, Sendable {
    public var id: String
    public var systemImage: String
    public var title: String

    public init(id: String, systemImage: String, title: String) {
        self.id = id
        self.systemImage = systemImage
        self.title = title
    }
}

/// Colors of a `GlassModeBar`. `.standard` is the house palette.
public struct GlassModeBarStyle: Sendable {
    /// The mode icons on the glass.
    public var label: Color
    /// The glyph on the tinted anchor.
    public var ink: Color
    /// The anchor's tint: signal, the brand red.
    public var anchor: Color

    /// Pass only what you want to change; `nil` keeps the house value.
    public init(label: Color? = nil, ink: Color? = nil, anchor: Color? = nil) {
        self.label = label ?? glassModeBarColor(light: 0x141414, dark: 0xF4F3EF)
        self.ink = ink ?? Color(red: 0x14 / 255, green: 0x14 / 255, blue: 0x14 / 255)
        self.anchor = anchor ?? Color(red: 1, green: 0, blue: 0)
    }

    public static let standard = GlassModeBarStyle()
}

/// A house-palette colour that follows the interface style.
private func glassModeBarColor(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

/// A capture bar with four modes. Voice becomes a live waveform and a running time, Photo a row of recent shots,
/// Timer a set of durations, Text the format controls. It opens and closes on its own so the morph can be watched.
private struct GlassModeBarExample: View {
    @Environment(\.accessibilityReduceMotion) private var reduceMotion
    @State private var mode: String?
    @State private var minutes = 15

    private let modes: [GlassModeBarMode] = [
        .init(id: "text", systemImage: "textformat", title: "Text"),
        .init(id: "voice", systemImage: "mic", title: "Voice"),
        .init(id: "photo", systemImage: "photo", title: "Photo"),
        .init(id: "timer", systemImage: "timer", title: "Timer"),
    ]

    var body: some View {
        GlassModeBar(modes: modes, selection: $mode) { mode in
            AnyView(panel(mode.id))
        }
        .padding(.horizontal, 20)
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .background(glassModeBarColor(light: 0xF3F2EE, dark: 0x121212))
        .task {
            while !Task.isCancelled {
                for next in ["voice", "timer", "photo", "text"] {
                    try? await Task.sleep(for: .seconds(0.9))
                    mode = next
                    try? await Task.sleep(for: .seconds(2.2))
                    mode = nil
                }
            }
        }
    }

    /// Each mode's controls span the whole inside of the capsule.
    @ViewBuilder private func panel(_ id: String) -> some View {
        let ink = glassModeBarColor(light: 0x141414, dark: 0xF4F3EF)
        let soft = ink.opacity(0.08)
        switch id {
        case "voice":
            HStack(spacing: 12) {
                TimelineView(.animation(paused: reduceMotion)) { context in
                    let t = context.date.timeIntervalSinceReferenceDate
                    // As many bars as fit, spread across the full width.
                    HStack(spacing: 0) {
                        ForEach(0..<28, id: \.self) { i in
                            Capsule()
                                .fill(Color(red: 1, green: 0, blue: 0))
                                .frame(width: 3, height: 6 + 18 * abs(sin(t * 3 + Double(i) * 0.6)))
                                .frame(maxWidth: .infinity)
                        }
                    }
                }
                .frame(height: 28)
                Text("0:07").monospacedDigit().foregroundStyle(ink)
            }
            .accessibilityElement(children: .ignore)
            .accessibilityLabel("Recording, 7 seconds")
        case "photo":
            HStack(spacing: 6) {
                ForEach([0x9CC2FF, 0xFFD976, 0xA9DCB7, 0xCDB8FF, 0xE9D5B3], id: \.self) { hex in
                    RoundedRectangle(cornerRadius: 8, style: .continuous)
                        .fill(glassModeBarColor(light: UInt32(hex), dark: UInt32(hex)))
                        .frame(maxWidth: .infinity)
                        .frame(height: 38)
                }
            }
            .accessibilityElement(children: .ignore)
            .accessibilityLabel("Recent photos")
        case "timer":
            HStack(spacing: 8) {
                ForEach([5, 15, 25, 45], id: \.self) { value in
                    Button { minutes = value } label: {
                        Text("\(value)m")
                            .foregroundStyle(ink)
                            .frame(maxWidth: .infinity)
                            .frame(height: 34)
                            .background(minutes == value ? ink.opacity(0.18) : soft, in: .capsule)
                    }
                    .buttonStyle(.plain)
                }
            }
        default:
            HStack(spacing: 0) {
                ForEach(["bold", "italic", "underline", "strikethrough", "list.bullet"], id: \.self) { symbol in
                    Image(systemName: symbol).frame(maxWidth: .infinity)
                }
            }
            .font(.body.weight(.semibold))
            .foregroundStyle(ink)
        }
    }
}

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

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

// 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, pressMath)

/// 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 {
    /// 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)
    }
}

// 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: 1.7.1 (liquid, liquidPress, bud)

/// 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] } }
    }
}

// swiftpieces-liquid: end