cranpose_ui/modifier/

slices.rs

use std::{fmt, mem::size_of, rc::Rc};

use cranpose_foundation::{ModifierNodeChain, NodeCapabilities, PointerEvent, PointerEventKind};
use cranpose_ui_graphics::{
    ColorFilter, EdgeInsets, GraphicsLayer, LayerShape, PointerIcon, Rect, RenderEffect,
    RoundedCornerShape, Size,
};
use smallvec::SmallVec;

use super::{
    ModifierChainHandle, Point,
    coordinator_geometry::{CoordinatorGeometry, CoordinatorRect},
};
use crate::{
    draw::DrawCommand,
    modifier::{
        Modifier,
        scroll::{MotionContextAnimatedNode, TranslatedContentContextNode},
    },
    modifier_nodes::{
        BackgroundNode, ClipToBoundsNode, CornerShapeNode, DrawCommandNode, GraphicsLayerNode,
        PaddingNode, PointerIconNode, SelectableTextNode, WindowRectReporterNode,
    },
    text::{TextLayoutOptions, TextStyle},
    text_field_modifier_node::{TextFieldLayoutHandle, TextFieldModifierNode, TextPanResolver},
    text_modifier_node::{TextModifierNode, TextPreparedLayoutHandle},
};

/// Snapshot of modifier node slices that impact draw and pointer subsystems.
#[derive(Default)]
pub struct ModifierNodeSlices {
    draw_commands: Vec<DrawCommand>,
    layer_draw_boundary: Option<usize>,
    pointer_inputs: Vec<Rc<dyn Fn(PointerEvent)>>,
    pointer_input_sizes: Vec<Rc<std::cell::Cell<cranpose_ui_graphics::Size>>>,
    pointer_icon: Option<PointerIcon>,
    clip_to_bounds: bool,
    /// Where the outermost clip or graphics layer of the chain sits: its
    /// coordinator, which a later offset moves the content inside of.
    layer_coordinator: Option<CoordinatorRect>,
    motion_context_animated: bool,
    translated_content_context: bool,
    translated_content_context_identity: Option<usize>,
    translated_content_offset_reader: Option<Rc<dyn Fn() -> Point>>,
    text: Option<SliceText>,
    text_coordinator: Option<CoordinatorRect>,
    text_window_transform: Option<Rc<std::cell::Cell<cranpose_ui_graphics::ProjectiveTransform>>>,
    viewport_window_rect: Option<Rc<dyn crate::modifier_nodes::WindowRectSink>>,
    /// Boxed: few nodes carry a layer, and inline it took 240 bytes of
    /// every node's slices.
    graphics_layer: Option<Box<GraphicsLayer>>,
    graphics_layer_resolver: Option<Rc<dyn Fn() -> GraphicsLayer>>,
    corner_shape: Option<RoundedCornerShape>,
    chain_guard: Option<Rc<ChainGuard>>,
}

struct ChainGuard {
    _handle: ModifierChainHandle,
}

/// The text a node shows. A `Text`'s string, style and options are its
/// prepared layout's, read from there rather than copied into every
/// rebuild of the slices; a text field's are held here.
#[derive(Clone)]
enum SliceText {
    Text(TextPreparedLayoutHandle),
    Field(Rc<FieldText>),
}

struct FieldText {
    content: Rc<crate::text::AnnotatedString>,
    style: TextStyle,
    layout: TextFieldLayoutHandle,
    pan: Option<TextPanResolver>,
}

#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct ModifierNodeSlicesDebugStats {
    pub draw_command_count: usize,
    pub draw_command_capacity: usize,
    pub pointer_input_count: usize,
    pub pointer_input_capacity: usize,
    pub has_text_content: bool,
    pub has_text_style: bool,
    pub has_text_layout_options: bool,
    pub has_prepared_text_layout: bool,
    pub has_graphics_layer: bool,
    pub has_graphics_layer_resolver: bool,
    pub heap_bytes: usize,
}

impl Clone for ModifierNodeSlices {
    fn clone(&self) -> Self {
        Self {
            draw_commands: self.draw_commands.clone(),
            layer_draw_boundary: self.layer_draw_boundary,
            pointer_inputs: self.pointer_inputs.clone(),
            pointer_input_sizes: self.pointer_input_sizes.clone(),
            pointer_icon: self.pointer_icon.clone(),
            clip_to_bounds: self.clip_to_bounds,
            layer_coordinator: self.layer_coordinator.clone(),
            motion_context_animated: self.motion_context_animated,
            translated_content_context: self.translated_content_context,
            translated_content_context_identity: self.translated_content_context_identity,
            translated_content_offset_reader: self.translated_content_offset_reader.clone(),
            text: self.text.clone(),
            text_coordinator: self.text_coordinator.clone(),
            text_window_transform: self.text_window_transform.clone(),
            viewport_window_rect: self.viewport_window_rect.clone(),
            graphics_layer: self.graphics_layer.clone(),
            graphics_layer_resolver: self.graphics_layer_resolver.clone(),
            corner_shape: self.corner_shape,
            chain_guard: self.chain_guard.clone(),
        }
    }
}

fn merge_graphics_layers(base: GraphicsLayer, overlay: GraphicsLayer) -> GraphicsLayer {
    GraphicsLayer {
        alpha: (base.alpha * overlay.alpha).clamp(0.0, 1.0),
        scale: base.scale * overlay.scale,
        scale_x: base.scale_x * overlay.scale_x,
        scale_y: base.scale_y * overlay.scale_y,
        rotation_x: base.rotation_x + overlay.rotation_x,
        rotation_y: base.rotation_y + overlay.rotation_y,
        rotation_z: base.rotation_z + overlay.rotation_z,
        camera_distance: transform_frame(&base, &overlay).camera_distance,
        transform_origin: transform_frame(&base, &overlay).transform_origin,
        translation_x: base.translation_x + overlay.translation_x,
        translation_y: base.translation_y + overlay.translation_y,
        shadow_elevation: overlay.shadow_elevation,
        ambient_shadow_color: overlay.ambient_shadow_color,
        spot_shadow_color: overlay.spot_shadow_color,
        shape: merged_layer_shape(&base, &overlay),
        clip: base.clip || overlay.clip,
        compositing_strategy: overlay.compositing_strategy,
        blend_mode: overlay.blend_mode,
        color_filter: compose_color_filters(base.color_filter, overlay.color_filter),
        render_effect: compose_render_effects(base.render_effect, overlay.render_effect),
        backdrop_effect: overlay.backdrop_effect.or(base.backdrop_effect),
    }
}

/// Which of two stacked layers merged into one frames the merged transform:
/// the later layer when it rotates or scales itself, else the earlier one. A
/// layer that neither rotates nor scales, such as a glass surface under a
/// tilt, leaves its camera and pivot at their defaults, and those would
/// replace the tilt's own.
fn transform_frame<'a>(base: &'a GraphicsLayer, overlay: &'a GraphicsLayer) -> &'a GraphicsLayer {
    let transforms = |layer: &GraphicsLayer| {
        layer.rotation_x != 0.0
            || layer.rotation_y != 0.0
            || layer.rotation_z != 0.0
            || layer.scale != 1.0
            || layer.scale_x != 1.0
            || layer.scale_y != 1.0
    };
    if transforms(overlay) || !transforms(base) {
        overlay
    } else {
        base
    }
}

/// The shape of two stacked layers merged into one: the layer that clips
/// owns it, else the layer that carries the backdrop (its effect covers that
/// shape), else the later layer, whose default resets it like every other
/// parent-local field.
fn merged_layer_shape(base: &GraphicsLayer, overlay: &GraphicsLayer) -> LayerShape {
    if overlay.clip || (!base.clip && overlay.backdrop_effect.is_some()) {
        overlay.shape
    } else if base.clip || base.backdrop_effect.is_some() {
        base.shape
    } else {
        overlay.shape
    }
}

fn compose_render_effects(
    outer: Option<RenderEffect>,
    inner: Option<RenderEffect>,
) -> Option<RenderEffect> {
    match (outer, inner) {
        (None, None) => None,
        (Some(effect), None) | (None, Some(effect)) => Some(effect),
        (Some(outer_effect), Some(inner_effect)) => Some(inner_effect.then(outer_effect)),
    }
}

fn compose_color_filters(
    base: Option<ColorFilter>,
    overlay: Option<ColorFilter>,
) -> Option<ColorFilter> {
    match (base, overlay) {
        (None, None) => None,
        (Some(filter), None) | (None, Some(filter)) => Some(filter),
        (Some(filter), Some(next)) => Some(filter.compose(next)),
    }
}

impl ModifierNodeSlices {
    pub fn draw_commands(&self) -> &[DrawCommand] {
        &self.draw_commands
    }

    /// How many leading [`draw_commands`](Self::draw_commands) come from
    /// modifiers chained before the node's graphics layer or clip. Those draw
    /// around the layer in the parent's space, outside its clip, alpha and
    /// transform, exactly as an outer `drawBehind` wraps a `graphicsLayer`.
    pub fn outer_draw_command_count(&self) -> usize {
        self.layer_draw_boundary.unwrap_or(0)
    }

    fn insert_background_draw(
        &mut self,
        insert_index: usize,
        precedes_layer: bool,
        command: DrawCommand,
    ) {
        let insert_index = insert_index.min(self.draw_commands.len());
        self.draw_commands.insert(insert_index, command);
        if let Some(boundary) = self.layer_draw_boundary.as_mut()
            && precedes_layer
        {
            *boundary += 1;
        }
    }

    fn mark_layer_draw_boundary(&mut self) {
        if self.layer_draw_boundary.is_none() {
            self.layer_draw_boundary = Some(self.draw_commands.len());
        }
    }

    pub fn pointer_inputs(&self) -> &[Rc<dyn Fn(PointerEvent)>] {
        &self.pointer_inputs
    }

    /// Dispatches an event whose position is already local to this layout node.
    /// Consumed events stop propagation except for release and cancellation,
    /// which reach every handler so each can finish its active interaction.
    pub fn dispatch_pointer_event(&self, event: PointerEvent) {
        let terminal = matches!(event.kind, PointerEventKind::Up | PointerEventKind::Cancel);
        for handler in &self.pointer_inputs {
            if event.is_consumed() && !terminal {
                break;
            }
            handler(event.clone());
        }
    }

    /// The write targets for this node's resolved size, one per pointer-input
    /// node that exposes a size to its handler. See
    /// [`ModifierNodeSlices::publish_pointer_input_size`].
    pub fn pointer_input_size_sinks(&self) -> &[Rc<std::cell::Cell<cranpose_ui_graphics::Size>>] {
        &self.pointer_input_sizes
    }

    /// Publishes this layout node's resolved size to every pointer-input
    /// handler attached to it, so `PointerInputScope::size()` reports the
    /// node's real dimensions.
    ///
    /// Called by every pass that resolves a node's geometry (the layout `place`
    /// passes and the per-frame scene build), so the size is current before any
    /// pointer event for that frame is dispatched and tracks resizes.
    ///
    /// `size` is the node's layout box — the same box the dispatched
    /// [`PointerEvent`] positions are made local to — so handlers can compare
    /// event coordinates against it directly.
    pub fn publish_pointer_input_size(&self, size: cranpose_ui_graphics::Size) {
        for sink in &self.pointer_input_sizes {
            sink.set(size);
        }
    }

    /// The pointer's appearance over this node, when a `pointer_icon`
    /// modifier names one. The innermost declaration in the chain wins.
    pub fn pointer_icon(&self) -> Option<&PointerIcon> {
        self.pointer_icon.as_ref()
    }

    pub fn clip_to_bounds(&self) -> bool {
        self.clip_to_bounds
    }

    /// The bounds of the node's layer in a node of `node_size`, where its
    /// clip cuts and its transforms pivot: as in Compose, the rect of the
    /// coordinator its outermost clip or graphics layer wraps, so an offset
    /// declared after it moves the content inside the layer rather than the
    /// layer. The node's own rect when it has neither.
    pub fn layer_bounds(&self, node_size: Size) -> Rect {
        self.layer_coordinator
            .as_ref()
            .map_or_else(|| Rect::from_size(node_size), |layer| layer.rect(node_size))
    }

    /// Publishes geometry in window coordinates through the same transform as
    /// rendering. Pointer-input sizes stay in the node's local coordinates.
    #[doc(hidden)]
    pub fn publish_window_geometry(
        &self,
        origin: Point,
        transform: cranpose_ui_graphics::ProjectiveTransform,
        size: Size,
    ) {
        if self.text_window_transform.is_some() || self.viewport_window_rect.is_some() {
            let local_to_window =
                cranpose_ui_graphics::ProjectiveTransform::translation(origin.x, origin.y)
                    .then(transform);
            if let Some(sink) = self.text_window_transform() {
                sink.set(local_to_window);
            }
            if let Some(sink) = self.viewport_window_rect() {
                sink.set(cranpose_ui_graphics::WindowCoordinates {
                    size,
                    local_to_window,
                });
            }
        }
        self.publish_pointer_input_size(size);
    }

    pub fn motion_context_animated(&self) -> bool {
        self.motion_context_animated
    }

    pub fn translated_content_context(&self) -> bool {
        self.translated_content_context
    }

    pub fn translated_content_context_identity(&self) -> Option<usize> {
        self.translated_content_context_identity
    }

    pub fn translated_content_offset(&self) -> Option<Point> {
        self.translated_content_offset_reader
            .as_ref()
            .map(|reader| reader())
    }

    pub fn text_content(&self) -> Option<&str> {
        self.annotated_text().map(|text| text.text.as_str())
    }

    pub fn annotated_text(&self) -> Option<&Rc<crate::text::AnnotatedString>> {
        match self.text.as_ref()? {
            SliceText::Text(layout) => Some(layout.annotated_text()),
            SliceText::Field(field) => Some(&field.content),
        }
    }

    /// Where the text draws in a node of `node_size`: the rect its layout put
    /// the text in, after every layout modifier before it.
    pub fn text_content_rect(&self, node_size: Size) -> Rect {
        self.text_coordinator.as_ref().map_or(
            Rect {
                x: 0.0,
                y: 0.0,
                width: node_size.width,
                height: node_size.height,
            },
            |coordinator| coordinator.rect(node_size),
        )
    }

    pub fn text_style(&self) -> Option<&TextStyle> {
        match self.text.as_ref()? {
            SliceText::Text(layout) => Some(layout.style()),
            SliceText::Field(field) => Some(&field.style),
        }
    }

    pub fn text_layout_options(&self) -> Option<TextLayoutOptions> {
        match self.text.as_ref()? {
            SliceText::Text(layout) => Some(layout.options()),
            SliceText::Field(_) => Some(TextLayoutOptions::default()),
        }
    }

    /// Returns the horizontal pan resolver for single-line text fields.
    ///
    /// The resolver takes the content viewport width (px) and returns the
    /// horizontal scroll offset that keeps the cursor visible. Renderers
    /// subtract this offset from the text origin so the glyphs pan together
    /// with the cursor and selection.
    pub fn text_pan_resolver(&self) -> Option<TextPanResolver> {
        match self.text.as_ref()? {
            SliceText::Field(field) => field.pan.clone(),
            SliceText::Text(_) => None,
        }
    }

    /// The write target for a text node's local-to-window transform, used by
    /// editable and selectable text for caret, selection and pointer geometry.
    pub fn text_window_transform(
        &self,
    ) -> Option<&Rc<std::cell::Cell<cranpose_ui_graphics::ProjectiveTransform>>> {
        self.text_window_transform.as_ref()
    }

    /// The write target for a scroll container's composited window rect, if this
    /// node carries a `report_window_rect` modifier. The layout pass writes the
    /// node's true on-screen viewport rect here so a `BringIntoViewResponder`
    /// can scroll a focused field's caret above the soft keyboard.
    pub fn viewport_window_rect(&self) -> Option<&Rc<dyn crate::modifier_nodes::WindowRectSink>> {
        self.viewport_window_rect.as_ref()
    }

    /// Returns the text layout this node's `Text` or text field produced when
    /// layout last measured it, laid out at the same width.
    ///
    /// A text field's layout is its current text wrapped at the width its
    /// caret and selection are placed on. `None` when the node carries no text,
    /// or carries a `Text` that has not been measured yet.
    pub fn measured_text_layout(&self) -> Option<Rc<crate::text::PreparedTextLayout>> {
        match self.text.as_ref()? {
            SliceText::Text(layout) => layout.measured_layout(),
            SliceText::Field(field) => Some(field.layout.measured_layout(&field.style)),
        }
    }

    pub fn graphics_layer(&self) -> Option<GraphicsLayer> {
        if let Some(resolve) = &self.graphics_layer_resolver {
            Some(resolve())
        } else {
            self.graphics_layer.as_deref().cloned()
        }
    }

    pub fn corner_shape(&self) -> Option<RoundedCornerShape> {
        self.corner_shape
    }

    /// Records the layer's coordinator when this is the chain's outermost
    /// clip or graphics layer.
    fn enter_layer(&mut self, site: &DrawSite) {
        if self.layer_coordinator.is_none() {
            self.layer_coordinator = Some(site.coordinator.clone());
        }
    }

    fn push_graphics_layer(
        &mut self,
        layer: GraphicsLayer,
        resolver: Option<Rc<dyn Fn() -> GraphicsLayer>>,
    ) {
        let existing_snapshot = self.graphics_layer.as_deref().cloned();
        let next_snapshot = existing_snapshot.as_ref().map_or_else(
            || layer.clone(),
            |current| merge_graphics_layers(current.clone(), layer.clone()),
        );
        let existing_resolver = self.graphics_layer_resolver.clone();

        match &mut self.graphics_layer {
            Some(held) => **held = next_snapshot,
            None => self.graphics_layer = Some(Box::new(next_snapshot)),
        }
        self.graphics_layer_resolver = match (existing_resolver, resolver) {
            (None, None) => None,
            (Some(current_resolver), None) => Some(Rc::new(move || {
                merge_graphics_layers(current_resolver(), layer.clone())
            })),
            (None, Some(next_resolver)) => {
                let base = existing_snapshot.unwrap_or_default();
                Some(Rc::new(move || {
                    merge_graphics_layers(base.clone(), next_resolver())
                }))
            }
            (Some(current_resolver), Some(next_resolver)) => Some(Rc::new(move || {
                merge_graphics_layers(current_resolver(), next_resolver())
            })),
        };
    }

    pub fn with_chain_guard(mut self, handle: ModifierChainHandle) -> Self {
        self.chain_guard = Some(Rc::new(ChainGuard { _handle: handle }));
        self
    }

    pub fn debug_stats(&self) -> ModifierNodeSlicesDebugStats {
        let draw_command_bytes = self.draw_commands.capacity() * size_of::<DrawCommand>();
        let pointer_input_bytes =
            self.pointer_inputs.capacity() * size_of::<Rc<dyn Fn(PointerEvent)>>();
        ModifierNodeSlicesDebugStats {
            draw_command_count: self.draw_commands.len(),
            draw_command_capacity: self.draw_commands.capacity(),
            pointer_input_count: self.pointer_inputs.len(),
            pointer_input_capacity: self.pointer_inputs.capacity(),
            has_text_content: self.text.is_some(),
            has_text_style: self.text.is_some(),
            has_text_layout_options: self.text.is_some(),
            has_prepared_text_layout: self.text.is_some(),
            has_graphics_layer: self.graphics_layer.is_some(),
            has_graphics_layer_resolver: self.graphics_layer_resolver.is_some(),
            heap_bytes: draw_command_bytes + pointer_input_bytes,
        }
    }

    /// Resets the slice collection for reuse, retaining vector capacity.
    pub fn clear(&mut self) {
        self.draw_commands.clear();
        self.layer_draw_boundary = None;
        self.pointer_inputs.clear();
        self.pointer_input_sizes.clear();
        self.pointer_icon = None;
        self.clip_to_bounds = false;
        self.layer_coordinator = None;
        self.motion_context_animated = false;
        self.translated_content_context = false;
        self.translated_content_context_identity = None;
        self.translated_content_offset_reader = None;
        self.text = None;
        self.text_coordinator = None;
        self.text_window_transform = None;
        self.viewport_window_rect = None;
        self.graphics_layer = None;
        self.graphics_layer_resolver = None;
        self.corner_shape = None;
        self.chain_guard = None;
    }
}

impl fmt::Debug for ModifierNodeSlices {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ModifierNodeSlices")
            .field("draw_commands", &self.draw_commands.len())
            .field("pointer_inputs", &self.pointer_inputs.len())
            .field("pointer_icon", &self.pointer_icon)
            .field("clip_to_bounds", &self.clip_to_bounds)
            .field("motion_context_animated", &self.motion_context_animated)
            .field(
                "translated_content_context",
                &self.translated_content_context,
            )
            .field(
                "translated_content_context_identity",
                &self.translated_content_context_identity,
            )
            .field(
                "translated_content_offset",
                &self.translated_content_offset(),
            )
            .field("text_content", &self.annotated_text())
            .field("text_style", &self.text_style())
            .field("text_layout_options", &self.text_layout_options())
            .field("graphics_layer", &self.graphics_layer)
            .field(
                "graphics_layer_resolver",
                &self.graphics_layer_resolver.is_some(),
            )
            .field("corner_shape", &self.corner_shape)
            .finish()
    }
}

/// Records `node`'s pointer icon when it declares one, leaving the icon already
/// collected in place when it does not.
///
/// The chain is walked head to tail, so the innermost declaration is the last
/// one written and the one that survives.
fn collect_pointer_icon(node: &dyn std::any::Any, slices: &mut ModifierNodeSlices) {
    if let Some(icon_node) = node.downcast_ref::<PointerIconNode>() {
        slices.pointer_icon = Some(icon_node.icon().clone());
    }
}

/// Collects modifier node slices directly from a reconciled [`ModifierNodeChain`]
/// that no layout places: its draws sit inside the padding before them.
pub fn collect_modifier_slices(chain: &ModifierNodeChain) -> ModifierNodeSlices {
    let mut slices = ModifierNodeSlices::default();
    collect_modifier_slices_into(chain, &mut slices, &Rc::default(), 1.0);
    slices
}

/// Collects modifier node slices into an existing buffer to reuse allocations.
///
/// Single-pass: iterates the chain once instead of 4 separate capability-filtered
/// traversals, reducing per-node `RefCell::borrow()` overhead.
/// Collects `chain`'s slices into the node's shared snapshot. The snapshot's
/// storage is reused when nothing else holds it; one the render graph still
/// shares is left to the graph and replaced, not cloned only to be cleared.
/// Its draws and text read their place from `geometry`, which the node's
/// layout writes, and until then sit inside the padding before them on the
/// device pixel grid of `density`.
pub(crate) fn collect_modifier_slices_into_shared(
    chain: &ModifierNodeChain,
    slices: &mut Rc<ModifierNodeSlices>,
    geometry: &Rc<CoordinatorGeometry>,
    density: f32,
) {
    if Rc::get_mut(slices).is_none() {
        *slices = Rc::default();
    }
    if let Some(slices) = Rc::get_mut(slices) {
        collect_modifier_slices_into(chain, slices, geometry, density);
    }
}

fn collect_modifier_slices_into(
    chain: &ModifierNodeChain,
    slices: &mut ModifierNodeSlices,
    geometry: &Rc<CoordinatorGeometry>,
    density: f32,
) {
    slices.clear();

    let caps = chain.capabilities();
    let has_pointer = caps.intersects(NodeCapabilities::POINTER_INPUT);
    let has_draw = caps.intersects(NodeCapabilities::DRAW);
    let has_layout = caps.intersects(NodeCapabilities::LAYOUT);

    if !has_pointer && !has_draw && !has_layout {
        return;
    }

    let mut backgrounds = Backgrounds::default();
    let mut padding = EdgeInsets::default();
    // The layout nodes walked so far: the index of the coordinator the next
    // draw belongs to, which is the next layout node's (or a layout node's
    // own) or, past the last one, the node's content.
    let mut layout_ordinal = 0_usize;

    for (modifier_index, node_ref) in chain.head_to_tail().enumerate() {
        let node_caps = node_ref.kind_set();

        node_ref.with_node(|node| {
            let any = node.as_any();

            if has_pointer
                && node_caps.intersects(NodeCapabilities::POINTER_INPUT)
                && let Some(pointer_node) = node.as_pointer_input_node()
            {
                if let Some(handler) = pointer_node.pointer_input_handler() {
                    slices.pointer_inputs.push(handler);
                }
                if let Some(sink) = pointer_node.layout_size_sink() {
                    slices.pointer_input_sizes.push(sink);
                }
                collect_pointer_icon(any, slices);
            }

            if has_draw && node_caps.intersects(NodeCapabilities::DRAW) {
                let draw = DrawSite {
                    coordinator: CoordinatorRect::new(geometry, layout_ordinal, padding),
                    displaceable: has_layout,
                    modifier_index,
                };
                collect_draw_node(node, &draw, slices, &mut backgrounds);
            }

            if has_layout && node_caps.intersects(NodeCapabilities::LAYOUT) {
                if let Some(padding_node) = any.downcast_ref::<PaddingNode>() {
                    padding +=
                        crate::modifier_nodes::device_padding(padding_node.padding(), density);
                }

                if let Some(motion_context_node) = any.downcast_ref::<MotionContextAnimatedNode>() {
                    slices.motion_context_animated = motion_context_node.is_active();
                }

                collect_window_geometry_sink(
                    any,
                    // The text a selectable wraps is the layout node after it.
                    CoordinatorRect::new(geometry, layout_ordinal + 1, padding),
                    slices,
                );

                if let Some(translated_content_node) =
                    any.downcast_ref::<TranslatedContentContextNode>()
                {
                    slices.translated_content_context = translated_content_node.is_active();
                    slices.translated_content_context_identity =
                        Some(translated_content_node.identity());
                    slices.translated_content_offset_reader =
                        translated_content_node.content_offset_reader();
                }

                if let Some(text_node) = any.downcast_ref::<TextModifierNode>() {
                    slices.text_coordinator =
                        Some(CoordinatorRect::new(geometry, layout_ordinal, padding));
                    slices.text = Some(SliceText::Text(text_node.prepared_layout_handle()));
                }

                if let Some(text_field_node) = any.downcast_ref::<TextFieldModifierNode>() {
                    slices.text = Some(SliceText::Field(Rc::new(FieldText {
                        content: Rc::new(crate::text::AnnotatedString::from(
                            text_field_node.text(),
                        )),
                        style: text_field_node.style().clone(),
                        layout: text_field_node.layout_handle(),
                        pan: text_field_node.text_pan_resolver(),
                    })));
                    slices.text_window_transform = Some(text_field_node.window_transform_sink());

                    let coordinator = CoordinatorRect::new(geometry, layout_ordinal, padding);
                    text_field_node.set_content_origin(coordinator.clone());
                    slices.text_coordinator = Some(coordinator);
                }
                layout_ordinal += 1;
            }
        });
    }

    backgrounds.insert_into(slices);
}

/// Where a draw modifier draws: in its coordinator's rect, which only moves
/// off the node's own rect when the chain has layout modifiers.
struct DrawSite {
    coordinator: CoordinatorRect,
    displaceable: bool,
    modifier_index: usize,
}

/// The chain's backgrounds as the walk finds them. Each draws at its place in
/// the draw order, in its coordinator's rect, as Compose draws every
/// background of a chain; a corner shape shapes the nearest background
/// before it, or the next one when none comes before.
#[derive(Default)]
struct Backgrounds {
    slots: SmallVec<[BackgroundSlot; 2]>,
    pending_shape: Option<RoundedCornerShape>,
    last_shape: Option<RoundedCornerShape>,
}

struct BackgroundSlot {
    color: crate::modifier::Color,
    coordinator: CoordinatorRect,
    insert_index: usize,
    precedes_layer: bool,
    corner_shape: Option<RoundedCornerShape>,
}

impl Backgrounds {
    fn push(
        &mut self,
        color: crate::modifier::Color,
        shape: Option<RoundedCornerShape>,
        site: &DrawSite,
        slices: &ModifierNodeSlices,
    ) {
        let corner_shape = shape.or_else(|| self.pending_shape.take());
        if shape.is_some() {
            self.last_shape = shape;
        }
        self.slots.push(BackgroundSlot {
            color,
            coordinator: site.coordinator.clone(),
            insert_index: slices.draw_commands.len(),
            precedes_layer: slices.layer_draw_boundary.is_none(),
            corner_shape,
        });
    }

    fn shape(&mut self, shape: RoundedCornerShape) {
        self.last_shape = Some(shape);
        match self.slots.last_mut() {
            Some(slot) => slot.corner_shape = Some(shape),
            None => self.pending_shape = Some(shape),
        }
    }

    /// Inserts every background at its place, the last first so the places
    /// of those before it still hold.
    fn insert_into(self, slices: &mut ModifierNodeSlices) {
        slices.corner_shape = self.last_shape;
        for slot in self.slots.into_iter().rev() {
            let BackgroundSlot {
                color,
                coordinator,
                insert_index,
                precedes_layer,
                corner_shape,
            } = slot;
            let draw_cmd = Rc::new(move |scope: &mut cranpose_ui_graphics::DrawScopeDefault| {
                use cranpose_ui_graphics::{CornerRadii, DrawScope as _};

                use crate::modifier::Brush;

                let rect = coordinator.rect(scope.size());
                let brush = Brush::solid(color);
                if let Some(shape) = corner_shape {
                    let radii: CornerRadii = shape.resolve(rect.width, rect.height);
                    scope.draw_round_rect_at(rect, brush, radii);
                } else {
                    scope.draw_rect_at(rect, brush);
                }
            });
            slices.insert_background_draw(
                insert_index,
                precedes_layer,
                DrawCommand::Behind(draw_cmd),
            );
        }
    }
}

/// Where layout reports a node's window geometry: a scroll viewport's rect,
/// or the origin of a selectable text, whose content is `text`'s rect.
fn collect_window_geometry_sink(
    any: &dyn std::any::Any,
    text: CoordinatorRect,
    slices: &mut ModifierNodeSlices,
) {
    if let Some(reporter) = any.downcast_ref::<WindowRectReporterNode>() {
        slices.viewport_window_rect = Some(reporter.window_rect_sink());
    }
    if let Some(selectable) = any.downcast_ref::<SelectableTextNode>() {
        slices.text_window_transform = Some(selectable.geometry().window_transform_sink());
        selectable.geometry().set_content_origin(text);
    }
}

/// Collects what a draw-capable node contributes, drawn at `site`.
fn collect_draw_node(
    node: &dyn cranpose_foundation::ModifierNode,
    site: &DrawSite,
    slices: &mut ModifierNodeSlices,
    backgrounds: &mut Backgrounds,
) {
    let any = node.as_any();
    if let Some(bg_node) = any.downcast_ref::<BackgroundNode>() {
        backgrounds.push(bg_node.color(), bg_node.shape(), site, slices);
    }

    if let Some(shape_node) = any.downcast_ref::<CornerShapeNode>() {
        backgrounds.shape(shape_node.shape());
    }

    if let Some(commands) = any.downcast_ref::<DrawCommandNode>() {
        slices.draw_commands.extend(
            commands
                .observed_commands(site.modifier_index)
                .map(|command| placed_draw_command(command, site)),
        );
    }

    if let Some(draw_node) = node.as_draw_node() {
        collect_draw_closures(draw_node, site, slices);
    }

    if let Some(layer_node) = any.downcast_ref::<GraphicsLayerNode>() {
        slices.mark_layer_draw_boundary();
        slices.push_graphics_layer(
            layer_node.layer_snapshot(),
            layer_node.layer_resolver(site.modifier_index),
        );
        slices.enter_layer(site);
    }

    if any.is::<ClipToBoundsNode>() {
        slices.mark_layer_draw_boundary();
        slices.clip_to_bounds = true;
        slices.enter_layer(site);
    }
}

/// A draw node's behind and overlay closures, or what its `draw` records
/// when it has no overlay closure.
fn collect_draw_closures(
    draw_node: &dyn cranpose_foundation::DrawModifierNode,
    site: &DrawSite,
    slices: &mut ModifierNodeSlices,
) {
    if let Some(closure) = draw_node.create_behind_draw_closure() {
        slices
            .draw_commands
            .push(placed_draw_command(DrawCommand::Behind(closure), site));
    }
    if let Some(closure) = draw_node.create_draw_closure() {
        slices
            .draw_commands
            .push(placed_draw_command(DrawCommand::Overlay(closure), site));
    }
}

/// `command` drawn where Compose draws a draw modifier: in its coordinator's
/// rect, sized to it, with what it records moved to that rect.
fn placed_draw_command(command: DrawCommand, site: &DrawSite) -> DrawCommand {
    if !site.displaceable {
        return command;
    }
    let place = |draw: crate::draw::DrawCommandFn| -> crate::draw::DrawCommandFn {
        let coordinator = site.coordinator.clone();
        Rc::new(move |scope: &mut cranpose_ui_graphics::DrawScopeDefault| {
            use cranpose_ui_graphics::DrawScope as _;

            let insets = coordinator.insets(scope.size());
            if insets.is_zero() {
                draw(scope);
            } else {
                scope.inset(insets, |inner| draw(inner));
            }
        })
    };
    match command {
        DrawCommand::Behind(draw) => DrawCommand::Behind(place(draw)),
        DrawCommand::WithContent(draw) => DrawCommand::WithContent(place(draw)),
        DrawCommand::Overlay(draw) => DrawCommand::Overlay(place(draw)),
    }
}

/// Collects modifier node slices by instantiating a temporary node chain from a [`Modifier`].
pub fn collect_slices_from_modifier(modifier: &Modifier) -> ModifierNodeSlices {
    let mut handle = ModifierChainHandle::new();
    let _ = handle.update(modifier);
    collect_modifier_slices(handle.chain()).with_chain_guard(handle)
}

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