Liquid SwitchNew

// 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

/// Colors and type for `LiquidSwitch`. `.standard` is the house palette: the brand red on neutral glass.
public struct LiquidSwitchStyle: Sendable {
    /// The fill that floods the track while on.
    public var onTint: Color
    /// The thumb.
    public var thumb: Color
    /// The symbol in the thumb while on.
    public var symbol: Color
    /// The title.
    public var label: Color
    /// The title's font.
    public var font: Font
    /// Opacity of the switch while disabled with `.disabled(true)`.
    public var disabledOpacity: Double

    public init(
        onTint: Color = LiquidSwitchStyle.adaptive(light: 0xFF0000, dark: 0xFF0000),
        thumb: Color = LiquidSwitchStyle.adaptive(light: 0xFFFFFF, dark: 0xF4F3EF),
        symbol: Color = LiquidSwitchStyle.adaptive(light: 0x141414, dark: 0x141414),
        label: Color = LiquidSwitchStyle.adaptive(light: 0x141414, dark: 0xF4F3EF),
        font: Font = .body.weight(.semibold),
        disabledOpacity: Double = 0.45
    ) {
        self.onTint = onTint
        self.thumb = thumb
        self.symbol = symbol
        self.label = label
        self.font = font
        self.disabledOpacity = disabledOpacity
    }

    public static let standard = LiquidSwitchStyle()

    /// A color that follows the appearance, from two 0xRRGGBB values.
    public static func adaptive(light: UInt32, dark: UInt32) -> Color {
        Color(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)
        })
    }
}

/// A switch whose thumb moves like liquid: the edge facing where it is going leads and the far edge follows, so it
/// stretches across the track and gathers in, while the on tint floods the track out of it.
///
/// Use it like `Toggle`. With a title the whole row toggles on tap, the way a settings row does.
///
/// - Parameters:
///   - title: The label before the switch, read by VoiceOver. `nil` draws the switch alone; give it an `accessibilityLabel` then.
///   - isOn: The bound state. Changing it from outside moves the thumb the same way a tap does.
///   - symbol: An optional SF Symbol that fades into the thumb as it turns on, such as `"moon.fill"`.
///   - style: Colors and font. Defaults to the SwiftPieces house palette, adapting to light and dark.
public struct LiquidSwitch: View {
    public typealias Style = LiquidSwitchStyle

    @Environment(\.accessibilityReduceMotion) private var reduceMotion
    @Environment(\.isEnabled) private var isEnabled
    @Binding private var isOn: Bool
    /// The thumb's edges, in points from the track's leading end. Each is sprung on its own, so they can lead and trail.
    @State private var lower: CGFloat
    @State private var upper: CGFloat
    @State private var touching = false
    @State private var dragging = false
    @State private var flips = 0
    @GestureState private var inGesture = false

    private let title: String?
    private let symbol: String?
    private let style: Style

    private static let width: CGFloat = 52
    private static let height: CGFloat = 32
    private static let inset: CGFloat = 2
    private static var thumb: CGFloat { height - inset * 2 }
    /// How much wider the thumb grows under a finger, toward the middle.
    private static let swell: CGFloat = 6

    public init(_ title: String? = nil, isOn: Binding<Bool>, symbol: String? = nil, style: Style = .standard) {
        self.title = title
        self._isOn = isOn
        self.symbol = symbol
        self.style = style
        let rest = Self.rest(isOn.wrappedValue, pressed: false)
        self._lower = State(initialValue: rest.lower)
        self._upper = State(initialValue: rest.upper)
    }

    public var body: some View {
        HStack(spacing: 12) {
            if let title {
                Text(title)
                    .font(style.font)
                    .foregroundStyle(style.label)
                    .frame(maxWidth: .infinity, alignment: .leading)
            }
            track
        }
        .fontWeight(.semibold)
        .contentShape(.rect)
        .onTapGesture { if isEnabled { flip(to: !isOn) } }
        .opacity(isEnabled ? 1 : style.disabledOpacity)
        .animation(PieceMotion(reduceMotion: reduceMotion).morph, value: isEnabled)
        .sensoryFeedback(.impact(weight: .light), trigger: flips)
        .onChange(of: isOn) { _, on in
            // A drag owns the thumb until it lets go; the release lands it.
            if !dragging { flow(to: on) }
        }
        .accessibilityElement(children: .ignore)
        .accessibilityLabel(title ?? "")
        .accessibilityValue(isOn ? "On" : "Off")
        .accessibilityAddTraits(.isToggle)
        .accessibilityAction { if isEnabled { flip(to: !isOn) } }
    }

    private var track: some View {
        let width = Self.width, height = Self.height, inset = Self.inset, thumb = Self.thumb
        return LiquidSwitchEdge(value: lower) { lo in
            LiquidSwitchEdge(value: upper) { up in
                // How far on the thumb is, read from its leading edge so the tint arrives with it.
                let on = min(max((up - inset - thumb) / (width - inset * 2 - thumb), 0), 1)
                ZStack(alignment: .leading) {
                    Capsule()
                        .fill(style.onTint)
                        .frame(width: max(up + inset, height), height: height)
                        .opacity(on)
                    Capsule()
                        .fill(style.thumb)
                        .frame(width: max(up - lo, thumb * 0.6), height: thumb)
                        .shadow(color: .black.opacity(0.18), radius: 3, y: 1.5)
                        .overlay {
                            if let symbol {
                                Image(systemName: symbol)
                                    .font(.system(size: 13, weight: .semibold))
                                    .foregroundStyle(style.symbol)
                                    .scaleEffect(0.6 + 0.4 * on)
                                    .opacity(on)
                            }
                        }
                        .offset(x: lo)
                }
                .frame(width: width, height: height, alignment: .leading)
            }
        }
        .background {
            PieceLiquidGroup(lift: false) { Color.clear.pieceLiquid(.capsule, interactive: false) }
        }
        .frame(minHeight: 44)
        .contentShape(.rect)
        .highPriorityGesture(drag, including: isEnabled ? .all : .none)
        // A touch the system cancels (a scroll took over) never reaches onEnded: put the thumb back.
        .onChange(of: inGesture) { _, active in
            guard !active, touching else { return }
            touching = false
            dragging = false
            flow(to: isOn)
        }
    }

    // MARK: Interaction

    private var drag: some Gesture {
        let width = Self.width, inset = Self.inset, thumb = Self.thumb
        let start = isOn ? width - inset - thumb / 2 : inset + thumb / 2
        return DragGesture(minimumDistance: 0)
            .updating($inGesture) { _, inGesture, _ in inGesture = true }
            .onChanged { value in
                if !touching {
                    touching = true
                    let pressed = Self.rest(isOn, pressed: true)
                    withAnimation(PieceMotion(reduceMotion: reduceMotion).press) {
                        lower = pressed.lower
                        upper = pressed.upper
                    }
                }
                guard dragging || abs(value.translation.width) > 3 else { return }
                dragging = true
                // Directly under the finger, never through an animation, with a little give past either end.
                let center = PieceMotion.rubberBand(start + value.translation.width, in: (inset + thumb / 2)...(width - inset - thumb / 2), limit: 4)
                let half = (thumb + Self.swell) / 2
                quietly {
                    lower = center - half
                    upper = center + half
                }
            }
            .onEnded { value in
                defer { touching = false }
                guard dragging else {
                    if abs(value.translation.height) < 10 { flip(to: !isOn) }
                    else { flow(to: isOn) }
                    return
                }
                dragging = false
                // Land where the flick was heading, not just where the finger let go.
                let on = start + value.predictedEndTranslation.width > width / 2
                if on == isOn { flow(to: on, velocity: value.velocity.width) } else { flip(to: on) }
            }
    }

    private func flip(to on: Bool) {
        guard on != isOn else { return flow(to: on) }
        flips += 1
        isOn = on
    }

    /// Lands the thumb at rest for `on`. The edge facing the move leads on the tight spring and the far edge follows
    /// on the responsive one, so the thumb reaches across and gathers in with a little liquid give. A release passes
    /// the finger's speed to both. Under Reduce Motion both edges share one short spring and the thumb slides whole.
    private func flow(to on: Bool, velocity: CGFloat = 0) {
        let motion = PieceMotion(reduceMotion: reduceMotion)
        let target = Self.rest(on, pressed: false)
        guard !reduceMotion else {
            withAnimation(motion.snap) {
                lower = target.lower
                upper = target.upper
            }
            return
        }
        let forward = target.lower > lower
        withAnimation(motion.settle(velocity: velocity, from: upper, to: target.upper, spring: forward ? PieceMotion.tight : PieceMotion.responsive)) {
            upper = target.upper
        }
        withAnimation(motion.settle(velocity: velocity, from: lower, to: target.lower, spring: forward ? PieceMotion.responsive : PieceMotion.tight)) {
            lower = target.lower
        }
    }

    /// The thumb's edges at rest, swollen toward the middle while pressed.
    private static func rest(_ on: Bool, pressed: Bool) -> (lower: CGFloat, upper: CGFloat) {
        let grow = pressed ? swell : 0
        return on ? (width - inset - thumb - grow, width - inset) : (inset, inset + thumb + grow)
    }

    private func quietly(_ change: () -> Void) {
        var transaction = Transaction()
        transaction.disablesAnimations = true
        withTransaction(transaction, change)
    }
}

/// One edge of the thumb, animated on its own: SwiftUI interpolates `value` frame by frame and rebuilds the content
/// with it. Two nested give the two edges separate springs; one shape holding both would animate them as one value.
private struct LiquidSwitchEdge<Content: View>: View, Animatable {
    var value: CGFloat
    @ViewBuilder let content: (CGFloat) -> Content

    nonisolated var animatableData: CGFloat {
        get { value }
        set { value = newValue }
    }

    var body: some View { content(value) }
}

// MARK: - Example

/// A few settings rows, one with a symbol in its thumb.
private struct LiquidSwitchExample: View {
    @State private var assist = true
    @State private var dark = false
    @State private var receipts = true

    var body: some View {
        VStack(spacing: 20) {
            LiquidSwitch("Wi-Fi Assist", isOn: $assist)
            LiquidSwitch("Dark Appearance", isOn: $dark, symbol: "moon.fill")
            LiquidSwitch("Read Receipts", isOn: $receipts)
            LiquidSwitch("Location Services", isOn: .constant(false))
                .disabled(true)
        }
        .padding(24)
        .frame(maxWidth: 400)
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .background(LiquidSwitchStyle.adaptive(light: 0xF3F2EE, dark: 0x121212))
    }
}

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

#Preview("Dark") {
    LiquidSwitchExample()
        .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, momentum, rubberBand)

/// 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 {
    /// Where a flick at `velocity` (pt/s) coasts to. 0.998 coasts like a scroll view; 0.99 suits detents.
    nonisolated static func project(_ position: CGFloat, velocity: CGFloat, decelerationRate: CGFloat = 0.99) -> CGFloat {
        position + velocity / 1000 * decelerationRate / (1 - decelerationRate)
    }

    /// The candidate closest to `value`.
    nonisolated static func nearest(_ value: CGFloat, in candidates: [CGFloat]) -> CGFloat {
        candidates.min { abs($0 - value) < abs($1 - value) } ?? value
    }

    /// A settle that leaves at the finger's speed (pt/s). One per axis, each on its own `.offset(x:)` / `.offset(y:)`:
    /// a spring takes one velocity. SwiftUI's own velocity carry-over is unreliable; this always carries it.
    func settle(velocity: CGFloat, from current: CGFloat, to target: CGFloat, spring: Spring = PieceMotion.elastic) -> Animation {
        let spring = reduceMotion ? Spring(duration: 0.25, bounce: 0) : spring
        let distance = target - current
        guard abs(distance) >= 1 else { return .spring(spring) }
        // In whole distances per second, capped near the spring's frequency: a hard flick adds give, not a slingshot.
        // At exactly the frequency a critically damped spring cannot pass its target, so Reduce Motion stops there.
        let cap = 2 * Double.pi / spring.duration * (reduceMotion ? 1 : 1.5)
        let relative = min(max(Double(velocity / distance), -cap), cap)
        return .interpolatingSpring(spring, initialVelocity: relative)
    }
}

extension PieceMotion {
    /// A scroll view's edge resistance for a pull `overshoot` points past a limit; never reaches `limit`.
    /// About 24 to 40 for thumbs and toggles, 60 to 120 for cards and sheets. Band the total pull, not deltas.
    nonisolated static func rubberBand(_ overshoot: CGFloat, limit: CGFloat, coefficient: CGFloat = 0.55) -> CGFloat {
        guard limit > 0, overshoot != 0 else { return 0 }
        let banded = (1 - 1 / (abs(overshoot) * coefficient / limit + 1)) * limit
        return overshoot < 0 ? -banded : banded
    }

    /// `value` inside `range` passes through unchanged; past either end it moves with rubber-band resistance.
    nonisolated static func rubberBand(_ value: CGFloat, in range: ClosedRange<CGFloat>, limit: CGFloat, coefficient: CGFloat = 0.55) -> CGFloat {
        if value < range.lowerBound { return range.lowerBound + rubberBand(value - range.lowerBound, limit: limit, coefficient: coefficient) }
        if value > range.upperBound { return range.upperBound + rubberBand(value - range.upperBound, limit: limit, coefficient: coefficient) }
        return value
    }
}

// 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)

/// 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 }
    }
}

// swiftpieces-liquid: end