cranpose-ui 0.1.84

//! SwipeToDismiss composable
//!
//! Mirrors Jetpack Compose's `SwipeToDismissBox` (Material) in spirit: wraps
//! content that can be dragged horizontally; releasing past a threshold
//! animates the content off-screen and fires `on_dismiss`, otherwise the
//! content springs back into place.
//!
//! # Gesture disambiguation
//!
//! The drag capture mirrors the axis-locking rules of the scroll modifier so
//! a `SwipeToDismiss` row inside a vertical `LazyColumn` coexists with the
//! list scroll (children receive pointer events before ancestors):
//!
//! - the swipe captures the gesture only once the *horizontal* travel exceeds
//!   the drag slop while dominating the vertical travel; from then on events
//!   are consumed so the parent scroll abandons the gesture;
//! - a decisively *vertical* start locks the swipe out for the rest of the
//!   gesture, leaving every event unconsumed for the parent to scroll;
//! - taps (no travel beyond the slop) consume nothing, so clickable rows
//!   keep working.

#![allow(non_snake_case)]

use crate::composable;
use crate::modifier::{GraphicsLayer, Modifier, PointerEvent, PointerEventKind};
use crate::subcompose_layout::{Constraints, SubcomposeLayoutScope, SubcomposeMeasureScope};
use crate::widgets::box_widget::{Box, BoxSpec};
use crate::widgets::layout::{BoxWithConstraints, SubcomposeLayout};
use crate::widgets::scopes::BoxWithConstraintsScope;
use cranpose_animation::{spring, Animatable, AnimationType, Spring};
use cranpose_core::internal::FrameCallbackRegistration;
use cranpose_core::{
    with_current_composer, NodeId, Owned, OwnedMutableState, RuntimeHandle, SlotId,
};
use cranpose_foundation::DRAG_THRESHOLD;
use cranpose_ui_layout::Placement;
use std::cell::{Cell, RefCell};
use std::rc::Rc;
use std::sync::atomic::{AtomicU64, Ordering};

/// Offset (logical px) within which a dismiss animation counts as settled.
const DISMISS_SETTLE_EPSILON: f32 = 0.5;

/// Height-scale (fraction of the natural height) at or below which the
/// post-dismiss collapse is considered complete.
const COLLAPSE_SETTLE_EPSILON: f32 = 0.01;

/// Spring used both for the dismissal fling and the spring-back.
fn swipe_spring() -> AnimationType {
    spring(Spring::DampingRatioNoBouncy, Spring::StiffnessMediumLow)
}

/// Which edge the dismiss background is revealed on, i.e. the side the row is
/// being swiped away from. Passed to the [`SwipeToDismissSpec::with_background`]
/// closure so its label/icon can follow the swipe direction instead of being
/// pinned to one side.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum SwipeDismissSide {
    /// The row is moving right (offset > 0); the background shows on the leading
    /// (start / left-in-LTR) edge.
    Start,
    /// The row is moving left (offset < 0); the background shows on the trailing
    /// (end / right-in-LTR) edge.
    End,
}

/// Boxed background closure, invoked each frame with the current
/// [`SwipeDismissSide`] so the reveal can follow the swipe direction.
type BackgroundFn = Rc<RefCell<dyn FnMut(SwipeDismissSide)>>;

/// Configuration for [`SwipeToDismiss`].
#[derive(Clone)]
pub struct SwipeToDismissSpec {
    /// Fraction of the content width the offset must exceed on release for
    /// the row to dismiss (default `0.5`). Clamped to `(0, 1]`.
    pub threshold_fraction: f32,
    /// Optional content drawn behind the swiped row (e.g. a delete bin),
    /// revealed as the row slides away. Receives the [`SwipeDismissSide`] the
    /// row is being swiped toward so the reveal can follow the swipe direction.
    background: Option<BackgroundFn>,
    /// Identity of the wrapped content (e.g. the row's database id). When a
    /// composition slot is reused for a DIFFERENT item — unkeyed lazy-list
    /// rows all shift up after a removal — the remembered swipe state must
    /// not leak onto the new item: without a key, the next row inherits the
    /// dismissed row's displacement, revealed background and collapsed
    /// height ("items go shuffled, red boxes stick"). `None` keeps the
    /// keyless per-slot behaviour unchanged.
    pub key: Option<u64>,
}

impl SwipeToDismissSpec {
    pub fn new() -> Self {
        Self {
            threshold_fraction: 0.5,
            background: None,
            key: None,
        }
    }

    /// Declares the identity of the wrapped content. When the key changes,
    /// the swipe state resets to rest — the new item starts untouched.
    pub fn with_key(mut self, key: u64) -> Self {
        self.key = Some(key);
        self
    }

    /// Sets the dismiss threshold as a fraction of the content width.
    pub fn with_threshold_fraction(mut self, fraction: f32) -> Self {
        self.threshold_fraction = fraction;
        self
    }

    /// Sets the background content revealed behind the swiped row. The closure
    /// receives the [`SwipeDismissSide`] the row is currently being swiped
    /// toward, so it can align its label/icon to the revealed edge.
    pub fn with_background(mut self, background: impl FnMut(SwipeDismissSide) + 'static) -> Self {
        self.background = Some(Rc::new(RefCell::new(background)));
        self
    }
}

impl Default for SwipeToDismissSpec {
    fn default() -> Self {
        Self::new()
    }
}

/// Phase of the swipe gesture state machine.
#[derive(Clone, Copy, Debug, PartialEq)]
enum SwipePhase {
    /// No active pointer sequence.
    Idle,
    /// Pointer down, axis not decided yet.
    Tracking {
        down_x: f32,
        down_y: f32,
        start_offset: f32,
    },
    /// Horizontal axis won: the swipe owns the gesture and consumes events.
    Dragging { down_x: f32, start_offset: f32 },
    /// Vertical axis won decisively: never capture for this gesture.
    LockedOut,
}

/// Axis decision for the initial slop check. Mirrors the scroll modifier:
/// the main axis captures when it exceeds the slop *and* dominates; the
/// cross axis locks out when it exceeds the slop and strictly dominates.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub(crate) enum SwipeAxisDecision {
    Undecided,
    Horizontal,
    Vertical,
}

/// Decides the gesture axis from the total travel since pointer down.
pub(crate) fn decide_axis(total_dx: f32, total_dy: f32, slop: f32) -> SwipeAxisDecision {
    let horizontal = total_dx.abs();
    let vertical = total_dy.abs();
    if horizontal > slop && horizontal >= vertical {
        SwipeAxisDecision::Horizontal
    } else if vertical > slop && vertical > horizontal {
        SwipeAxisDecision::Vertical
    } else {
        SwipeAxisDecision::Undecided
    }
}

/// Where the released row should animate to: `Some(±width)` when the offset
/// crossed the dismiss threshold, `None` to spring back to rest.
pub(crate) fn dismissal_target(offset: f32, width: f32, threshold_fraction: f32) -> Option<f32> {
    if !width.is_finite() || width <= 0.0 {
        return None;
    }
    let threshold = width * threshold_fraction.clamp(f32::EPSILON, 1.0);
    (offset.abs() >= threshold).then(|| width * offset.signum())
}

/// Clamps the dragged offset so the content cannot travel further than one
/// full width in either direction.
pub(crate) fn clamp_offset(offset: f32, width: f32) -> f32 {
    if width.is_finite() && width > 0.0 {
        offset.clamp(-width, width)
    } else {
        offset
    }
}

static NEXT_SWIPE_ID: AtomicU64 = AtomicU64::new(0);

/// State shared between the composable (which reads the animated offset) and
/// the pointer-input handler (which drives it). Main-thread only.
struct SwipeToDismissController {
    /// Stable key for the pointer-input modifier.
    id: u64,
    runtime: RuntimeHandle,
    /// Animated horizontal offset of the content in logical px.
    offset: RefCell<Animatable<f32>>,
    /// Reactive flag mirroring `|offset| > 0`, read inside the subcompose so
    /// the row's background is *composed* only while the row is displaced.
    /// Conditional composition (adding/removing the background node) re-applies
    /// when the slot recomposes, unlike a layout-offset value change which is
    /// placement-cached — so this bool, not the raw offset, gates the reveal.
    /// A plain bool also recomposes only on rest/displaced transitions rather
    /// than on every drag frame. Without gating, the always-composed background
    /// flashes through content that fades in with alpha < 1 during an entrance.
    revealed: OwnedMutableState<bool>,
    /// Height scale of the row (fraction of its natural height). `1.0` at rest;
    /// after a successful dismiss it animates to `0.0` so the row collapses and
    /// leaves no gap — and no red background strip — even if the host removes
    /// the item a few frames later.
    collapse: RefCell<Animatable<f32>>,
    phase: Cell<SwipePhase>,
    /// Content width in logical px, refreshed from the incoming constraints.
    width_px: Cell<f32>,
    threshold_fraction: Cell<f32>,
    /// Refreshed every recomposition so the latest captured environment runs.
    on_dismiss: RefCell<Option<Rc<dyn Fn()>>>,
    /// `on_dismiss` fired for the current dismissal (fires at most once).
    dismissed: Cell<bool>,
    /// The row's layout node, so the collapse watcher can force a re-measure
    /// each frame as the height shrinks. Set on every recomposition.
    node_id: Cell<Option<NodeId>>,
    /// Frame callback watching the dismiss animation for completion.
    settle_watcher: RefCell<Option<FrameCallbackRegistration>>,
    /// Frame callback driving the post-dismiss height collapse.
    collapse_watcher: RefCell<Option<FrameCallbackRegistration>>,
    /// Identity of the content this controller's state belongs to (from
    /// [`SwipeToDismissSpec::with_key`]); state resets when it changes.
    identity: Cell<Option<u64>>,
}

impl SwipeToDismissController {
    fn new(runtime: RuntimeHandle) -> Rc<Self> {
        Rc::new(Self {
            id: NEXT_SWIPE_ID.fetch_add(1, Ordering::Relaxed),
            offset: RefCell::new(Animatable::new(0.0, runtime.clone())),
            revealed: OwnedMutableState::with_runtime(false, runtime.clone()),
            collapse: RefCell::new(Animatable::new(1.0, runtime.clone())),
            runtime,
            phase: Cell::new(SwipePhase::Idle),
            width_px: Cell::new(f32::NAN),
            threshold_fraction: Cell::new(0.5),
            on_dismiss: RefCell::new(None),
            dismissed: Cell::new(false),
            node_id: Cell::new(None),
            settle_watcher: RefCell::new(None),
            collapse_watcher: RefCell::new(None),
            identity: Cell::new(None),
        })
    }

    /// Instantly return to the untouched state. Used when the composition
    /// slot is reused for a different item (the spec key changed): the new
    /// item must not inherit the old item's displacement, reveal, collapse
    /// or a pending dismissal.
    fn reset_to_rest(&self) {
        self.settle_watcher.borrow_mut().take();
        self.collapse_watcher.borrow_mut().take();
        self.offset.borrow_mut().snapTo(0.0);
        self.collapse.borrow_mut().snapTo(1.0);
        self.phase.set(SwipePhase::Idle);
        self.dismissed.set(false);
        self.set_revealed(false);
    }

    fn current_offset(&self) -> f32 {
        self.offset.borrow().state().value()
    }

    /// The edge the background is revealed on, from the current displacement:
    /// a rightward offset reveals the start edge, a leftward offset the end.
    fn revealed_side(&self) -> SwipeDismissSide {
        if self.current_offset() >= 0.0 {
            SwipeDismissSide::Start
        } else {
            SwipeDismissSide::End
        }
    }

    /// Current row height scale (`1.0` at rest, `0.0` fully collapsed).
    fn collapse_fraction(&self) -> f32 {
        self.collapse.borrow().state().value()
    }

    /// Reactive read of the revealed flag; subscribes the reading scope (the
    /// subcompose slot) so it recomposes when the background reveal toggles.
    fn revealed(&self) -> bool {
        self.revealed.value()
    }

    /// Updates the revealed flag (only writes on change, to avoid needless
    /// recompositions each drag frame while already displaced).
    fn set_revealed(&self, revealed: bool) {
        if self.revealed.get_non_reactive() != revealed {
            self.revealed.set_value(revealed);
        }
    }

    fn snap_to(&self, offset: f32) {
        self.offset.borrow_mut().snapTo(offset);
        // A live drag frame: the background is revealed exactly while the row
        // is displaced. Rest (offset 0) hides it, even mid-entrance.
        self.set_revealed(offset != 0.0);
    }

    fn animate_to(&self, target: f32) {
        self.offset.borrow_mut().animateTo(target, swipe_spring());
        // A dismissal fling keeps the row displaced, so reveal immediately.
        // A spring-back keeps the reveal on until the settle watcher observes
        // the row return to rest (so the background does not vanish out from
        // under the still-sliding content).
        if target != 0.0 {
            self.set_revealed(true);
        }
    }
}

/// Appends the swipe pointer-input handler to `base`. Split out of the
/// composable so headless tests can drive the exact production modifier
/// through a manually built modifier chain.
fn swipe_gesture_modifier(base: Modifier, controller: Rc<SwipeToDismissController>) -> Modifier {
    let key = controller.id;
    base.pointer_input(key, move |scope| {
        let controller = Rc::clone(&controller);
        async move {
            scope
                .await_pointer_event_scope(|await_scope| async move {
                    loop {
                        let event = await_scope.await_pointer_event().await;
                        handle_swipe_event(&controller, &event);
                    }
                })
                .await;
        }
    })
}

/// Handles one pointer event for the swipe gesture. Returns nothing; event
/// consumption communicates ownership to sibling/ancestor handlers.
fn handle_swipe_event(controller: &Rc<SwipeToDismissController>, event: &PointerEvent) {
    // Secondary fingers never participate: the swipe is a one-finger gesture
    // and multi-touch handlers (zoom) key off the extra pointers themselves.
    if event.id != 0 && event.kind != PointerEventKind::Cancel {
        return;
    }

    match event.kind {
        PointerEventKind::Down => {
            if event.is_consumed() {
                return;
            }
            // Catch an in-flight spring: pressing pins the content where it
            // currently is, like Compose's swipeable.
            let current = controller.current_offset();
            controller.snap_to(current);
            controller.phase.set(SwipePhase::Tracking {
                down_x: event.global_position.x,
                down_y: event.global_position.y,
                start_offset: current,
            });
            // Down is never consumed: taps must keep reaching clickables.
        }
        PointerEventKind::Move => {
            if event.is_consumed() {
                // Another handler owns this sequence (parent scroll started,
                // or a sibling captured); abandon and rest.
                if matches!(controller.phase.get(), SwipePhase::Dragging { .. }) {
                    animate_spring_back(controller);
                }
                controller.phase.set(SwipePhase::Idle);
                return;
            }
            match controller.phase.get() {
                SwipePhase::Tracking {
                    down_x,
                    down_y,
                    start_offset,
                } => {
                    let total_dx = event.global_position.x - down_x;
                    let total_dy = event.global_position.y - down_y;
                    match decide_axis(total_dx, total_dy, DRAG_THRESHOLD) {
                        SwipeAxisDecision::Horizontal => {
                            controller.phase.set(SwipePhase::Dragging {
                                down_x,
                                start_offset,
                            });
                            let width = controller.width_px.get();
                            controller.snap_to(clamp_offset(start_offset + total_dx, width));
                            event.consume();
                        }
                        SwipeAxisDecision::Vertical => {
                            controller.phase.set(SwipePhase::LockedOut);
                        }
                        SwipeAxisDecision::Undecided => {}
                    }
                }
                SwipePhase::Dragging {
                    down_x,
                    start_offset,
                } => {
                    let total_dx = event.global_position.x - down_x;
                    let width = controller.width_px.get();
                    controller.snap_to(clamp_offset(start_offset + total_dx, width));
                    event.consume();
                }
                SwipePhase::Idle | SwipePhase::LockedOut => {}
            }
        }
        PointerEventKind::Up => {
            let phase = controller.phase.get();
            controller.phase.set(SwipePhase::Idle);
            if let SwipePhase::Dragging { .. } = phase {
                settle_release(controller);
                event.consume();
            }
        }
        PointerEventKind::Cancel => {
            if matches!(controller.phase.get(), SwipePhase::Dragging { .. }) {
                animate_spring_back(controller);
            }
            controller.phase.set(SwipePhase::Idle);
        }
        PointerEventKind::Scroll
        | PointerEventKind::Zoom
        | PointerEventKind::RotaryScrollPre
        | PointerEventKind::RotaryScroll
        | PointerEventKind::Enter
        | PointerEventKind::Exit => {}
    }
}

/// Applies the release decision: animate off-screen and watch for completion
/// (firing `on_dismiss`), or spring back to rest.
fn settle_release(controller: &Rc<SwipeToDismissController>) {
    let offset = controller.current_offset();
    let width = controller.width_px.get();
    match dismissal_target(offset, width, controller.threshold_fraction.get()) {
        Some(target) => animate_dismiss(controller, target),
        None => animate_spring_back(controller),
    }
}

/// Flings the row off-screen and watches for `on_dismiss` to fire once settled.
fn animate_dismiss(controller: &Rc<SwipeToDismissController>, target: f32) {
    controller.animate_to(target);
    watch_settle(controller, true);
}

/// Springs the row back to rest and watches so the reveal is hidden again once
/// the content actually returns to offset 0.
fn animate_spring_back(controller: &Rc<SwipeToDismissController>) {
    controller.animate_to(0.0);
    watch_settle(controller, false);
}

/// Watches the settle animation frame-by-frame. Once the offset reaches its
/// target it syncs the reveal flag to the resting displacement and, for a
/// dismissal, fires `on_dismiss` exactly once. Runs in frame callbacks
/// (outside composition), so the callback may freely mutate state.
fn watch_settle(controller: &Rc<SwipeToDismissController>, dismissing: bool) {
    let weak = Rc::downgrade(controller);
    let registration =
        controller
            .runtime
            .frame_clock()
            .with_frame_nanos(move |_frame_time_nanos| {
                let Some(controller) = weak.upgrade() else {
                    return;
                };
                controller.settle_watcher.borrow_mut().take();
                if dismissing && controller.dismissed.get() {
                    return;
                }
                // A new gesture retargeted the animation; a fresh snap already
                // owns the reveal flag, so stop watching.
                if matches!(controller.phase.get(), SwipePhase::Dragging { .. }) {
                    return;
                }
                let target = controller.offset.borrow().target();
                let value = controller.current_offset();
                if (value - target).abs() <= DISMISS_SETTLE_EPSILON {
                    // The row has settled. Stop drawing the background in every
                    // case: a spring-back rests at 0 (nothing revealed), and a
                    // dismissal must *not* leave the red strip behind while it
                    // waits for the host to drop the item.
                    controller.set_revealed(false);
                    if dismissing && !controller.dismissed.get() {
                        controller.dismissed.set(true);
                        // Collapse the row to zero height so no empty gap (or
                        // background strip) lingers if the host removes the item
                        // a frame or two later, then notify the host.
                        start_collapse(&controller);
                        let on_dismiss = controller.on_dismiss.borrow().clone();
                        if let Some(on_dismiss) = on_dismiss {
                            on_dismiss();
                        }
                    }
                } else {
                    watch_settle(&controller, dismissing);
                }
            });
    *controller.settle_watcher.borrow_mut() = Some(registration);
}

/// Animates the row's height scale to `0.0` and watches the animation so the
/// list re-measures the shrinking row each frame. Runs after a dismiss settles.
fn start_collapse(controller: &Rc<SwipeToDismissController>) {
    controller
        .collapse
        .borrow_mut()
        .animateTo(0.0, swipe_spring());
    watch_collapse(controller);
}

/// Frame-by-frame watcher for the post-dismiss collapse: the row height is a
/// layout output, so a plain animated value would not reach the parent list on
/// its own — each frame this forces a scoped re-measure of the row and a redraw
/// until the height scale reaches zero.
fn watch_collapse(controller: &Rc<SwipeToDismissController>) {
    let weak = Rc::downgrade(controller);
    let registration =
        controller
            .runtime
            .frame_clock()
            .with_frame_nanos(move |_frame_time_nanos| {
                let Some(controller) = weak.upgrade() else {
                    return;
                };
                controller.collapse_watcher.borrow_mut().take();
                if let Some(node_id) = controller.node_id.get() {
                    // Re-measure the row as it collapses. A plain layout repass
                    // only bubbles *layout* (placement) dirtiness up the tree,
                    // which does not set the row node's own `needs_measure` flag —
                    // and that flag is exactly what an enclosing `LazyColumn`
                    // consults (`children_need_relayout`) before reusing a cached
                    // item slot. `schedule_measure_repass` bubbles *measure*
                    // dirtiness so the list actually re-measures the shrinking
                    // row; otherwise the pre-dismiss height and the still-composed
                    // red background linger until the whole list is rebuilt.
                    crate::schedule_measure_repass(node_id);
                }
                crate::request_render_invalidation();
                if controller.collapse_fraction() > COLLAPSE_SETTLE_EPSILON {
                    watch_collapse(&controller);
                }
            });
    *controller.collapse_watcher.borrow_mut() = Some(registration);
}

/// Wraps `content` so it can be swiped away horizontally.
///
/// Dragging moves the content with the finger (clamped to one content width
/// in either direction). Releasing past `spec.threshold_fraction` of the
/// width animates the content off-screen with a spring and then fires
/// `on_dismiss` (exactly once); releasing earlier springs the content back.
/// An optional [`SwipeToDismissSpec::with_background`] content (delete bin,
/// archive icon, ...) is revealed behind the row while it is displaced.
///
/// Vertical drags are handed to ancestor scroll containers untouched, so
/// rows inside a `LazyColumn` still scroll the list (see the module docs for
/// the axis-locking rules).
#[composable(no_skip)]
pub fn SwipeToDismiss<D, F>(
    modifier: Modifier,
    spec: SwipeToDismissSpec,
    on_dismiss: D,
    content: F,
) -> cranpose_core::NodeId
where
    D: Fn() + 'static,
    F: FnMut() + 'static,
{
    let controller: Rc<SwipeToDismissController> = with_current_composer(|composer| {
        let runtime = composer.runtime_handle();
        let owned: Owned<Rc<SwipeToDismissController>> =
            composer.remember(|| SwipeToDismissController::new(runtime));
        owned.with(Rc::clone)
    });

    // Content identity: when this composition slot is reused for a different
    // item (unkeyed lazy rows shifting up after a removal), the remembered
    // controller must not leak the old item's swipe state onto the new one.
    if controller.identity.get() != spec.key {
        controller.reset_to_rest();
        controller.identity.set(spec.key);
    }

    // Refresh the per-recomposition inputs so the pointer handler (which
    // lives across recompositions) observes the latest values.
    controller
        .threshold_fraction
        .set(spec.threshold_fraction.clamp(f32::EPSILON, 1.0));
    *controller.on_dismiss.borrow_mut() = Some(Rc::new(on_dismiss));

    let background = spec.background.clone();
    let content = Rc::new(RefCell::new(content));

    // The gesture area is the whole (un-offset) row: like a scroll
    // container, the handler sits on the ancestor node so descendants
    // (clickables) see events first, and the hit region stays put while the
    // content slides away.
    let gesture_modifier = swipe_gesture_modifier(modifier, Rc::clone(&controller));

    // Outer collapsing layout: it measures the row and reports the row's height
    // scaled by the collapse fraction, so a dismissed row shrinks to nothing —
    // even when the row has a fixed height. It carries no user modifier: the
    // user modifier, the gesture handler and any fixed size stay on the *row*
    // (below), so the hit region and layout still belong to the row while only
    // the reported height is scaled here.
    let controller_for_layout = Rc::clone(&controller);
    let node = SubcomposeLayout(Modifier::empty(), move |scope, constraints| {
        // Height scale for the post-dismiss collapse (1.0 until dismissed), read
        // before measuring so the reported height tracks the animation.
        let collapse = controller_for_layout.collapse_fraction().clamp(0.0, 1.0);

        let gesture_modifier = gesture_modifier.clone();
        let background = background.clone();
        let content = Rc::clone(&content);
        let controller_for_row = Rc::clone(&controller_for_layout);
        let measurables = scope.subcompose(SlotId::new(0), move || {
            // The row itself: a `BoxWithConstraints` (so it can read its own
            // width for the dismiss threshold) carrying the user modifier + the
            // swipe pointer handler, overlaying the optional background and the
            // finger-translated content.
            let background = background.clone();
            let content = Rc::clone(&content);
            let controller_for_row = Rc::clone(&controller_for_row);
            BoxWithConstraints(gesture_modifier.clone(), move |row_scope| {
                controller_for_row
                    .width_px
                    .set(row_scope.constraints().max_width);

                // Only compose the background while the row is displaced (and
                // never once dismissed — the reveal flag is cleared on settle).
                // Reading it inside the subcompose subscribes this slot, so it
                // recomposes when the row leaves/returns to rest. At rest the
                // background is not composed at all, so it cannot flash through
                // content that fades in with alpha < 1 during an entrance.
                if controller_for_row.revealed() {
                    if let Some(background) = &background {
                        let background = Rc::clone(background);
                        let side = controller_for_row.revealed_side();
                        Box(Modifier::empty(), BoxSpec::new(), move || {
                            (background.borrow_mut())(side);
                        });
                    }
                }

                // Translate the content with the finger via a graphics-layer
                // resolver. The resolver re-reads the live offset on every draw
                // pass, and an offset write schedules a draw repass for this
                // node, so the content follows the finger 1:1 in real time and
                // settles with the spring on release — without re-measuring the
                // subcompose each frame.
                let content = Rc::clone(&content);
                let controller_for_layer = Rc::clone(&controller_for_row);
                Box(
                    Modifier::empty().graphics_layer(move || GraphicsLayer {
                        translation_x: controller_for_layer.current_offset(),
                        ..GraphicsLayer::default()
                    }),
                    BoxSpec::new(),
                    move || {
                        (content.borrow_mut())();
                    },
                );
            });
        });

        let child_constraints = Constraints {
            min_width: constraints.min_width,
            max_width: constraints.max_width,
            min_height: 0.0,
            max_height: constraints.max_height,
        };
        let mut width = 0.0_f32;
        let mut natural_height = 0.0_f32;
        let mut placements = Vec::with_capacity(measurables.len());
        for measurable in measurables {
            let placeable = scope.measure(measurable, child_constraints);
            width = width.max(placeable.width());
            natural_height = natural_height.max(placeable.height());
            placeable.place(0.0, 0.0);
            placements.push(Placement::new(placeable.node_id(), 0.0, 0.0, 0));
        }
        width = width.clamp(constraints.min_width, constraints.max_width);
        // Collapse the row height after a dismiss so the slot shrinks to
        // nothing: the content is already translated off-screen and the
        // background hidden, so scaling the reported height just closes the gap.
        let height = (natural_height * collapse).clamp(0.0, constraints.max_height);
        scope.layout(width, height, placements)
    });
    controller.node_id.set(Some(node));
    node
}

#[cfg(test)]
#[path = "../tests/swipe_to_dismiss_tests.rs"]
mod tests;