frust_widgets/

slider.rs

//! The `Slider` interactive widget: a horizontal track with a thumb
//! that reports a `0.0..=1.0` value as it is dragged.
//!
//! [`slider`] produces a [`SliderView`] carrying the current `value` and an
//! `on_change` closure. A `Down` on the track captures the pointer and reports
//! the value at that x-position; each captured `Move` reports the updated value
//! (continuous drag); `Up`/`Cancel` release. Like [`crate::Checkbox`] it is not
//! its own source of truth — it fires `on_change` and the next rebuild feeds the
//! new value back in. Horizontal only for v1.
//!
//! # Design-language branch
//!
//! `paint` reads [`Theme::design_language`] and re-skins the knob only when it
//! is [`DesignLanguage::Cupertino`] — groove/fill geometry, colors, event
//! handling, and the entire `Material3`/unthemed path are **untouched**: a
//! Material-tagged or unthemed paint runs the exact same sequence as before
//! this branch was added (same `fill_rounded_rect` call count/order/values),
//! so Material rendering stays byte-identical. The kit's mined data
//! (`kit-colors-type-metrics.json`) has **no `Sliders` size record at all**
//! (only shadow recipes for the knob in `glass-recipes.json`, with no
//! width/height fields). Per the same evidence rule this repo applies to any
//! unsourced platform metric,
//! [`CUPERTINO_THUMB`] is therefore **community-approximate**, not kit-cited
//! (see its doc comment); the groove thickness ([`TRACK_H`]) is shared with
//! the Material path unchanged, since there is no evidence either way to
//! diverge it. The knob's reflective treatment (drop shadow + specular
//! gradient) reads the theme's own `glass.control` tier (buttons/toggles/
//! sliders — the "control" glass tier covers interactive controls generally,
//! see `frust-theme/src/glass.rs`'s module docs), so the recipe is whatever
//! the active design system authored, never a value named here. iOS 26.2's
//! "Liquid Glass
//! slider" claims (a user-facing OS tint-adjustment control) are **not**
//! what this branch re-skins — this is Frust's `Slider` widget, a different
//! control; this widget ships a visual re-skin only, no behavior changes.

use std::rc::Rc;

use frust_core::accesskit::Role;
use frust_core::{
    BoxConstraints, BuildCtx, ChangeFlags, EventCtx, EventResult, InputEvent, LayoutCtx, PaintCtx,
    PaintScene, PointerPhase, SemanticsCtx, View, Widget,
};
use frust_theme::{DesignLanguage, GlassMaterial, GlassScale, Theme};
use kurbo::{Point, Size};
use peniko::{Brush, Color, Gradient};

use crate::authoring::presses;

/// Slider control height, in logical px.
const HEIGHT: f64 = 24.0;
/// Default track width when the incoming constraints are unbounded.
const DEFAULT_WIDTH: f64 = 200.0;
/// Track (groove) thickness, in logical px. Shared by both design-language
/// paths (see the module docs' Design-language branch section — no kit or
/// community evidence to diverge it for Cupertino).
const TRACK_H: f64 = 4.0;
/// Thumb diameter, in logical px — the Material path only (see [`THUMB_FILL`]
/// for its color; [`CUPERTINO_THUMB`] is the Cupertino-branch equivalent).
const THUMB: f64 = 18.0;
/// Cupertino knob diameter, in logical px.
///
/// **Community-approximate**, not kit-cited: the kit's mined data has no
/// `Sliders` size record at all (see the module docs) — 28pt is the
/// widely-cited default `UISlider` thumb diameter community reimplementations
/// converge on, distinctly larger than the Material path's 18pt (M3's own
/// slider spec).
const CUPERTINO_THUMB: f64 = 28.0;
/// The Cupertino knob's fill — plain white, mirroring
/// [`crate::cupertino::switch`]'s knob and `UISlider`'s stock white-circle
/// thumb (its `minimumTrackTintColor`/active-fill carries the tint instead;
/// see [`resolve_colors`]'s `fill`, reused unchanged for the active track on
/// both design-language paths).
const CUPERTINO_THUMB_FILL: Color = Color::from_rgb8(0xFF, 0xFF, 0xFF);
/// Peak alpha of the Cupertino knob's specular-highlight gradient at its
/// center — see [`crate::cupertino::switch`]'s constant of the same name and
/// role (this module mirrors that treatment exactly).
const SPECULAR_PEAK_SCALE: f32 = 1.8;
/// Unfilled (inactive) track color (unthemed fallback; a theme resolves this
/// from `colors.surface_container_highest`, per the M3 slider spec).
const TRACK: Color = Color::from_rgb8(0xD1, 0xD5, 0xDB);
/// Filled (active) track color (unthemed fallback; a theme resolves this from
/// `colors.primary`).
const FILL: Color = Color::from_rgb8(0x3B, 0x82, 0xF6);
/// Thumb color (unthemed fallback; a theme resolves this from `colors.primary`).
const THUMB_FILL: Color = Color::from_rgb8(0x1D, 0x4E, 0xD8);

/// The resolved slider paint colors: `(inactive_track, active_track, thumb)`.
/// Themed per the Material 3 slider spec: the inactive track is
/// `surface_container_highest`, the active track and thumb are `primary`.
/// Unthemed: the [`TRACK`]/[`FILL`]/[`THUMB_FILL`] constants exactly, so a
/// pre-theme app renders unchanged.
fn resolve_colors(theme: Option<&Theme>) -> (Color, Color, Color) {
    match theme {
        Some(theme) => {
            let scheme = theme.scheme();
            (
                scheme.surface_container_highest,
                scheme.primary,
                scheme.primary,
            )
        }
        None => (TRACK, FILL, THUMB_FILL),
    }
}

/// `true` when `theme` is tagged [`DesignLanguage::Cupertino`] — the sole
/// gate on the Cupertino knob branch (see the module docs). An unthemed paint
/// (`None`) and a `Material3`-tagged one both return `false`, running the
/// Material path unchanged.
fn is_cupertino(theme: Option<&Theme>) -> bool {
    theme.map(|t| t.design_language) == Some(DesignLanguage::Cupertino)
}

/// The `control` glass tier (buttons/toggles/sliders) the Cupertino knob's
/// shadow/highlight reads: `theme.glass.control`.
///
/// The unthemed arm is **unreachable by construction** — the sole call site
/// sits inside the `is_cupertino(theme)` guard in `paint`, and
/// `is_cupertino(None)` is `false`, so `theme` is always `Some` here. It
/// therefore resolves to the language-free [`GlassScale::opaque_material`]
/// tier rather than naming a design language's glass recipe: a fallback no
/// paint pass can observe must not pin this baseline widget to a catalog's
/// token table.
fn resolve_control_glass(theme: Option<&Theme>) -> GlassMaterial {
    theme
        .map(|t| t.glass.control.clone())
        .unwrap_or_else(|| GlassScale::opaque_material().control)
}

/// Return `color` with its alpha channel replaced by `alpha` (mirrors the
/// per-module helper of the same shape used across `frust-widgets`).
fn with_alpha(color: Color, alpha: f32) -> Color {
    let c = color.components;
    Color::new([c[0], c[1], c[2], alpha])
}

/// A view-held, typed change callback (erased on build).
type OnChange<State> = Rc<dyn Fn(&mut State, f64)>;

/// A declarative slider. See the [module docs](self).
pub struct SliderView<State: 'static> {
    value: f64,
    on_change: OnChange<State>,
}

/// Create a slider at `value` (clamped to `0.0..=1.0`) that fires
/// `on_change(state, new_value)` as it is dragged.
pub fn slider<State: 'static, F: Fn(&mut State, f64) + 'static>(
    value: f64,
    on_change: F,
) -> SliderView<State> {
    SliderView {
        value: value.clamp(0.0, 1.0),
        on_change: Rc::new(on_change),
    }
}

/// PascalCase alias for [`slider`].
#[allow(non_snake_case)]
pub fn Slider<State: 'static, F: Fn(&mut State, f64) + 'static>(
    value: f64,
    on_change: F,
) -> SliderView<State> {
    slider(value, on_change)
}

/// The retained widget for a [`SliderView`].
pub struct SliderWidget {
    value: f64,
    /// Armed by a `Down` (alongside `capture_pointer`), cleared on `Up`/`Cancel`.
    /// Gates `Move` so a hover `Move` (no prior press) never fires `on_change`;
    /// a `Down` still jumps to and reports the tapped value regardless.
    captured: bool,
    on_change: crate::authoring::ErasedArgCallback<f64>,
}

/// Map a widget-local x (in `0..=width`) to a `0.0..=1.0` value, clamped.
fn value_from_x(x: f64, width: f64) -> f64 {
    if width <= 0.0 {
        0.0
    } else {
        (x / width).clamp(0.0, 1.0)
    }
}

impl<State: 'static> View<State> for SliderView<State> {
    type Element = SliderWidget;

    fn build(&self, _ctx: &mut BuildCtx<'_>) -> SliderWidget {
        SliderWidget {
            value: self.value,
            captured: false,
            on_change: crate::authoring::erase_callback_arg(&self.on_change),
        }
    }

    fn rebuild(
        &self,
        prev: &Self,
        element: &mut SliderWidget,
        _ctx: &mut BuildCtx<'_>,
    ) -> ChangeFlags {
        element.on_change = crate::authoring::erase_callback_arg(&self.on_change);
        if prev.value != self.value {
            element.value = self.value;
            ChangeFlags::PAINT
        } else {
            ChangeFlags::NONE
        }
    }
}

impl Widget for SliderWidget {
    fn layout(&mut self, _ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
        let width = if bc.max().width.is_finite() {
            bc.max().width
        } else {
            DEFAULT_WIDTH
        };
        bc.constrain(Size::new(width, HEIGHT))
    }

    fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut dyn PaintScene) {
        let theme = Theme::from_paint_ctx(ctx);
        let (track, fill, thumb_fill) = resolve_colors(theme);
        let o = ctx.origin();
        let w = ctx.size().width;
        let mid_y = o.y + ctx.size().height / 2.0;
        // Groove.
        scene.fill_rounded_rect(
            Point::new(o.x, mid_y - TRACK_H / 2.0),
            Size::new(w, TRACK_H),
            TRACK_H / 2.0,
            track,
        );
        // Filled portion up to the thumb.
        let thumb_x = o.x + self.value.clamp(0.0, 1.0) * w;
        scene.fill_rounded_rect(
            Point::new(o.x, mid_y - TRACK_H / 2.0),
            Size::new((thumb_x - o.x).max(0.0), TRACK_H),
            TRACK_H / 2.0,
            fill,
        );

        if is_cupertino(theme) {
            // Cupertino re-skin: larger reflective knob — see
            // the module docs' Design-language branch section.
            let diam = CUPERTINO_THUMB;
            let thumb_origin = Point::new(thumb_x - diam / 2.0, mid_y - diam / 2.0);
            let thumb_size = Size::new(diam, diam);
            let glass_control = resolve_control_glass(theme);
            scene.draw_shadow(
                Point::new(
                    thumb_origin.x,
                    thumb_origin.y + glass_control.shadow.y_offset,
                ),
                thumb_size,
                diam / 2.0,
                glass_control.shadow.blur_std_dev,
                with_alpha(Color::BLACK, glass_control.shadow.color_alpha),
            );
            scene.fill_rounded_rect(thumb_origin, thumb_size, diam / 2.0, CUPERTINO_THUMB_FILL);
            let highlight_center = Point::new(thumb_x - diam * 0.18, mid_y - diam * 0.28);
            let peak_alpha = (glass_control.hairline_alpha * SPECULAR_PEAK_SCALE).min(1.0);
            let highlight = Gradient::new_radial(highlight_center, (diam * 0.65) as f32)
                .with_stops([
                    (0.0f32, with_alpha(CUPERTINO_THUMB_FILL, peak_alpha)),
                    (1.0f32, with_alpha(CUPERTINO_THUMB_FILL, 0.0)),
                ]);
            scene.fill_rounded_rect_brush(
                thumb_origin,
                thumb_size,
                diam / 2.0,
                &Brush::Gradient(highlight),
            );
        } else {
            // Material path (and unthemed): byte-identical to the pre-Cupertino
            // behavior — thumb (rounded-rect stand-in for a circle in v1).
            scene.fill_rounded_rect(
                Point::new(thumb_x - THUMB / 2.0, mid_y - THUMB / 2.0),
                Size::new(THUMB, THUMB),
                THUMB / 2.0,
                thumb_fill,
            );
        }
    }

    fn event(&mut self, ctx: &mut EventCtx, event: &InputEvent) -> EventResult {
        let InputEvent::Pointer(p) = event else {
            return EventResult::Ignored;
        };
        match p.phase {
            PointerPhase::Down => {
                if !presses(p) {
                    return EventResult::Ignored;
                }
                self.captured = true;
                ctx.capture_pointer();
                let v = value_from_x(p.position.x, ctx.size().width);
                (self.on_change)(ctx, v);
                ctx.request_redraw();
                EventResult::Handled
            }
            PointerPhase::Move => {
                // A hover `Move` (no prior press) never reports a value.
                if !self.captured {
                    return EventResult::Ignored;
                }
                let v = value_from_x(p.position.x, ctx.size().width);
                (self.on_change)(ctx, v);
                ctx.request_redraw();
                EventResult::Handled
            }
            PointerPhase::Up | PointerPhase::Cancel => {
                if !self.captured {
                    return EventResult::Ignored;
                }
                self.captured = false;
                ctx.request_redraw();
                EventResult::Handled
            }
        }
    }

    fn semantics(&self, ctx: &mut SemanticsCtx) {
        // A Slider node exposing its normalized `0.0..=1.0` value and range.
        ctx.push_node(Role::Slider, |node| {
            node.set_numeric_value(self.value);
            node.set_min_numeric_value(0.0);
            node.set_max_numeric_value(1.0);
        });
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::any::Any;

    #[derive(Default)]
    struct Val {
        value: f64,
        changes: u32,
    }

    fn widget(value: f64) -> SliderWidget {
        let view = slider::<Val, _>(value, |s: &mut Val, v: f64| {
            s.value = v;
            s.changes += 1;
        });
        let mut counter = 0u64;
        View::<Val>::build(&view, &mut BuildCtx::new(&mut counter))
    }

    fn ev(phase: PointerPhase, x: f64) -> InputEvent {
        InputEvent::Pointer(frust_core::PointerEvent {
            phase,
            position: Point::new(x, 12.0),
            button: frust_core::PointerButton::Primary,
        })
    }

    /// The same event on the secondary (right) button.
    fn secondary_ev(phase: PointerPhase, x: f64) -> InputEvent {
        InputEvent::Pointer(frust_core::PointerEvent {
            phase,
            position: Point::new(x, 12.0),
            button: frust_core::PointerButton::Secondary,
        })
    }

    fn dispatch(w: &mut SliderWidget, state: &mut Val, event: &InputEvent) {
        let state_any: &mut dyn Any = state;
        let mut ctx = EventCtx::new(state_any, Point::ZERO, Size::new(200.0, HEIGHT));
        w.event(&mut ctx, event);
    }

    #[test]
    fn a_secondary_press_never_captures_or_reports_a_value() {
        let mut w = widget(0.0);
        let mut state = Val::default();
        dispatch(&mut w, &mut state, &secondary_ev(PointerPhase::Down, 50.0));
        assert!(!w.captured, "no capture opened");
        assert_eq!(state.changes, 0, "and no value reported");
        // A move after it is an ordinary uncaptured pass, not a drag.
        dispatch(&mut w, &mut state, &ev(PointerPhase::Move, 120.0));
        assert_eq!(state.changes, 0);

        // The primary press still reports.
        dispatch(&mut w, &mut state, &ev(PointerPhase::Down, 50.0));
        assert!(w.captured);
        assert_eq!(state.value, 0.25);
    }

    #[test]
    fn value_math_maps_position_to_fraction() {
        assert_eq!(value_from_x(0.0, 200.0), 0.0);
        assert_eq!(value_from_x(100.0, 200.0), 0.5);
        assert_eq!(value_from_x(200.0, 200.0), 1.0);
        // Clamps outside the track.
        assert_eq!(value_from_x(-40.0, 200.0), 0.0);
        assert_eq!(value_from_x(9000.0, 200.0), 1.0);
        // Degenerate zero-width track.
        assert_eq!(value_from_x(10.0, 0.0), 0.0);
    }

    #[test]
    fn down_reports_value_at_position() {
        let mut w = widget(0.0);
        let mut state = Val::default();
        dispatch(&mut w, &mut state, &ev(PointerPhase::Down, 50.0));
        assert_eq!(state.value, 0.25);
        assert_eq!(state.changes, 1);
    }

    #[test]
    fn drag_reports_continuously_and_clamps() {
        let mut w = widget(0.0);
        let mut state = Val::default();
        dispatch(&mut w, &mut state, &ev(PointerPhase::Down, 100.0));
        dispatch(&mut w, &mut state, &ev(PointerPhase::Move, 300.0)); // past the end
        assert_eq!(state.value, 1.0);
        dispatch(&mut w, &mut state, &ev(PointerPhase::Move, -20.0)); // before the start
        assert_eq!(state.value, 0.0);
        assert_eq!(state.changes, 3);
        // Up does not report a value.
        dispatch(&mut w, &mut state, &ev(PointerPhase::Up, 0.0));
        assert_eq!(state.changes, 3);
    }

    #[test]
    fn hover_move_without_down_is_ignored_noop() {
        let mut w = widget(0.3);
        let mut state = Val::default();
        let state_any: &mut dyn Any = &mut state;
        let mut ctx = EventCtx::new(state_any, Point::ZERO, Size::new(200.0, HEIGHT));
        let result = w.event(&mut ctx, &ev(PointerPhase::Move, 100.0));
        assert!(matches!(result, EventResult::Ignored));
        assert!(!ctx.needs_redraw(), "hover must not request a redraw");
        assert_eq!(state.changes, 0, "hover must not report a value");
    }

    #[test]
    fn cancel_clears_captured_state() {
        let mut w = widget(0.0);
        let mut state = Val::default();
        dispatch(&mut w, &mut state, &ev(PointerPhase::Down, 100.0));
        assert!(w.captured);
        dispatch(&mut w, &mut state, &ev(PointerPhase::Cancel, 100.0));
        assert!(!w.captured, "Cancel disarms the drag");
        // A follow-up hover Move must not report.
        let before = state.changes;
        dispatch(&mut w, &mut state, &ev(PointerPhase::Move, 40.0));
        assert_eq!(state.changes, before, "hover after Cancel does not report");
    }

    /// Records rounded-rect fill colors in paint order (track, fill, thumb).
    /// `shadows`/`brushes` separately record the Cupertino knob's reflective-
    /// treatment calls (`draw_shadow`/`fill_rounded_rect_brush`), which never
    /// fire on the Material path (see `material_and_unthemed_paint_have_no_reflective_treatment`).
    #[derive(Default)]
    struct TrackRecorder {
        rrects: Vec<Color>,
        shadows: Vec<(Point, Size, f64, f64, Color)>,
        brushes: Vec<(Point, Size, f64, Brush)>,
    }

    impl PaintScene for TrackRecorder {
        fn fill_rect(&mut self, _o: Point, _s: Size, _c: Color) {}
        fn draw_text(&mut self, _o: Point, _t: &str) {}
        fn fill_rounded_rect(&mut self, _o: Point, _s: Size, _r: f64, color: Color) {
            self.rrects.push(color);
        }
        fn draw_shadow(&mut self, o: Point, s: Size, radius: f64, std_dev: f64, color: Color) {
            self.shadows.push((o, s, radius, std_dev, color));
        }
        fn fill_rounded_rect_brush(&mut self, o: Point, s: Size, radius: f64, brush: &Brush) {
            self.brushes.push((o, s, radius, brush.clone()));
        }
    }

    fn paint_colors(value: f64, theme: Option<&Theme>) -> Vec<Color> {
        let mut w = widget(value);
        let mut rec = TrackRecorder::default();
        let mut ctx = match theme {
            Some(t) => PaintCtx::new(Point::ZERO, Size::new(200.0, HEIGHT)).with_theme(t),
            None => PaintCtx::new(Point::ZERO, Size::new(200.0, HEIGHT)),
        };
        w.paint(&mut ctx, &mut rec);
        rec.rrects
    }

    #[test]
    fn unthemed_paint_uses_fallback_constants() {
        assert_eq!(paint_colors(0.5, None), vec![TRACK, FILL, THUMB_FILL]);
    }

    #[test]
    fn themed_paint_resolves_the_scheme_s_slider_roles() {
        let theme = Theme::neutral();
        let scheme = theme.scheme();
        assert_eq!(
            paint_colors(0.5, Some(&theme)),
            vec![
                scheme.surface_container_highest,
                scheme.primary,
                scheme.primary
            ],
        );
    }

    #[test]
    fn non_cupertino_and_unthemed_paint_have_no_reflective_treatment() {
        // No draw_shadow/fill_rounded_rect_brush call leaks into the default
        // (or unthemed) path — only a `Cupertino`-tagged theme takes the
        // reflective branch.
        let mut unthemed = widget(0.5);
        let mut rec = TrackRecorder::default();
        let mut ctx = PaintCtx::new(Point::ZERO, Size::new(200.0, HEIGHT));
        unthemed.paint(&mut ctx, &mut rec);
        assert_eq!(rec.rrects.len(), 3, "track + fill + thumb, nothing else");
        assert!(rec.shadows.is_empty());
        assert!(rec.brushes.is_empty());

        let theme = Theme::neutral();
        assert_ne!(
            theme.design_language,
            DesignLanguage::Cupertino,
            "the default path under test must not be the Cupertino one"
        );
        let mut themed = widget(0.5);
        let mut rec = TrackRecorder::default();
        let mut ctx = PaintCtx::new(Point::ZERO, Size::new(200.0, HEIGHT)).with_theme(&theme);
        themed.paint(&mut ctx, &mut rec);
        assert_eq!(rec.rrects.len(), 3);
        assert!(rec.shadows.is_empty());
        assert!(rec.brushes.is_empty());
    }

    /// A `Cupertino`-tagged theme carrying a translucent `control` glass tier
    /// — the shape a Cupertino design system installs, built inline here
    /// because no design language ships in this crate. The shadow/hairline
    /// values are arbitrary-but-distinctive: the branch under test reads them
    /// straight off the theme, so what matters is that the assertions below
    /// track THESE numbers rather than a hardcoded recipe.
    fn cupertino_theme() -> Theme {
        Theme::builder(Theme::neutral())
            .design_language(DesignLanguage::Cupertino)
            .map_glass(|mut g| {
                g.control = GlassMaterial {
                    blur_radius_intent: 15.0,
                    fills_light: Vec::new(),
                    fills_dark: Vec::new(),
                    hairline_alpha: 0.3,
                    shadow: frust_theme::ShadowSpec {
                        y_offset: 2.0,
                        blur_std_dev: 4.0,
                        color_alpha: 0.12,
                    },
                };
                g
            })
            .build()
    }

    #[test]
    fn cupertino_themed_paint_uses_the_larger_reflective_knob() {
        let theme = cupertino_theme();
        assert_eq!(theme.design_language, DesignLanguage::Cupertino);
        let mut w = widget(0.5);
        let mut rec = TrackRecorder::default();
        let mut ctx = PaintCtx::new(Point::ZERO, Size::new(200.0, HEIGHT)).with_theme(&theme);
        w.paint(&mut ctx, &mut rec);

        // track + fill + knob base fill (white, unlike the Material path's
        // tinted `THUMB_FILL`).
        assert_eq!(rec.rrects.len(), 3);
        assert_eq!(rec.rrects[2], CUPERTINO_THUMB_FILL);

        assert_eq!(rec.shadows.len(), 1, "one drop shadow under the knob");
        let expected = theme.glass.control.shadow;
        assert_eq!(
            rec.shadows[0].2,
            CUPERTINO_THUMB / 2.0,
            "circular, knob radius"
        );
        assert_eq!(rec.shadows[0].3, expected.blur_std_dev);

        assert_eq!(rec.brushes.len(), 1, "one specular gradient highlight");
        assert_eq!(
            rec.brushes[0].2,
            CUPERTINO_THUMB / 2.0,
            "circular, knob radius"
        );
        match &rec.brushes[0].3 {
            Brush::Gradient(g) => assert_eq!(g.stops.len(), 2, "peak + fade-to-transparent"),
            _ => panic!("expected a gradient brush for the specular highlight"),
        }
    }

    #[test]
    fn rebuild_adopts_new_value_without_self_mutation() {
        let mut counter = 0u64;
        let prev = slider::<Val, _>(0.2, |_s, _v| {});
        let mut w = View::<Val>::build(&prev, &mut BuildCtx::new(&mut counter));
        assert_eq!(w.value, 0.2);
        let next = slider::<Val, _>(0.8, |_s, _v| {});
        View::<Val>::rebuild(&next, &prev, &mut w, &mut BuildCtx::new(&mut counter));
        assert_eq!(w.value, 0.8);
    }
}