inset_rendering/

paragraph.rs

//! Flutter counterpart: `rendering/paragraph.dart` (`RenderParagraph`).
//!
//! Selection (`SelectionRegistrar` / `_SelectableFragment`), the fade overflow shader and
//! semantics wait; see `PORTING.md`.

use std::any::{Any, TypeId};
use std::fmt;
use std::rc::Rc;

use inset_embedder::{
    BoxHeightStyle, BoxWidthStyle, ClipOp, FontCollection, Matrix4, Offset, PlaceholderAlignment,
    Rect, Size, TextAlign, TextBaseline, TextBox, TextDirection, TextHeightBehavior, TextPosition,
    TextRange, TextSelection,
};
use inset_foundation::{App, Handle};
use inset_painting::{
    ClipContext, InlineSpanRef, PaintingBinding, PlaceholderDimensions, PlaceholderSpan,
    RenderComparison, TextOverflow, TextPainter, TextScaler, TextWidthBasis,
};

use crate::box_::{AnyRenderBox, BoxConstraints, BoxHitTestResult, RenderBox, RenderBoxData};
use crate::layout_helper::{ChildBaselineGetter, ChildLayoutHelper, ChildLayouter};
use crate::object::{
    AnyRenderObject, Constraints, ContainerParentData, ContainerParentDataMixin,
    ContainerRenderObjectData, ContainerRenderObjectMixin, ParentData, RenderHandle, RenderObject,
    RenderObjectData,
};
use crate::painting_context::PaintingContext;
use crate::pipeline_owner::PipelineOwner;
use crate::{RelayoutWhenSystemFontsChangeData, RelayoutWhenSystemFontsChangeMixin};

const K_ELLIPSIS: &str = "\u{2026}";

/// Parent data used by [`RenderParagraph`] and `RenderEditable` to annotate inline contents
/// (such as `WidgetSpan`s) with.
#[derive(Debug, Default)]
pub struct TextParentData {
    container_parent_data: ContainerParentData<AnyRenderBox>,
    offset: Option<Offset>,

    /// The [`PlaceholderSpan`] associated with this render child.
    ///
    /// This field is usually set by a `ParentDataWidget`, and is typically not `None` when
    /// `perform_layout` is called.
    pub span: Option<Rc<dyn PlaceholderSpan>>,
}

impl TextParentData {
    /// The offset at which to paint the child in the parent's coordinate system.
    ///
    /// A `None` value indicates this inline widget is not laid out. For instance, when the
    /// inline widget has never been laid out, or the inline widget is ellipsized away.
    pub fn offset(&self) -> Option<Offset> {
        self.offset
    }
}

impl ParentData for TextParentData {
    fn detach(&mut self) {
        self.span = None;
        self.offset = None;
        ContainerParentDataMixin::detach(self);
    }

    fn provide(&self, id: TypeId) -> Option<&dyn Any> {
        (id == TypeId::of::<TextParentData>()).then_some(self)
    }

    fn provide_mut(&mut self, id: TypeId) -> Option<&mut dyn Any> {
        (id == TypeId::of::<TextParentData>()).then_some(self)
    }
}

impl ContainerParentDataMixin for TextParentData {
    type ChildType = AnyRenderBox;

    fn container_parent_data(&self) -> &ContainerParentData<AnyRenderBox> {
        &self.container_parent_data
    }

    fn container_parent_data_mut(&mut self) -> &mut ContainerParentData<AnyRenderBox> {
        &mut self.container_parent_data
    }
}

impl fmt::Display for TextParentData {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "widget: {:?}, ", self.span)?;
        match self.offset {
            None => f.write_str("not laid out"),
            Some(offset) => write!(f, "offset: {offset:?}"),
        }
    }
}

/// A mixin that provides useful default behaviors for text [`RenderBox`]es
/// ([`RenderParagraph`] and `RenderEditable` for example) with inline content children
/// managed by the [`ContainerRenderObjectMixin`] mixin.
///
/// This mixin assumes every child managed by the [`ContainerRenderObjectMixin`] mixin
/// corresponds to a [`PlaceholderSpan`], and they are organized in logical order of the text
/// (the order each [`PlaceholderSpan`] is encountered when the user reads the text).
///
/// To use this mixin in a [`RenderBox`] class:
///
///  * Call [`layout_inline_children`](Self::layout_inline_children) in the `perform_layout`
///    and `compute_dry_layout` implementation, and during intrinsic size calculations, to get
///    the size information of the inline widgets as a `Vec` of [`PlaceholderDimensions`].
///    Determine the positioning of the inline widgets (which is usually done by a
///    [`TextPainter`] using its line break algorithm).
///
///  * Call [`position_inline_children`](Self::position_inline_children) with the positioning
///    information of the inline widgets.
///
///  * Implement [`RenderBox::apply_paint_transform`], optionally with
///    [`default_apply_paint_transform`](Self::default_apply_paint_transform).
///
///  * Call [`paint_inline_children`](Self::paint_inline_children) in [`RenderObject::paint`]
///    to paint the inline widgets.
///
///  * Call [`hit_test_inline_children`](Self::hit_test_inline_children) in
///    [`RenderBox::hit_test_children`] to hit test the inline widgets.
///
/// See also:
///
///  * `WidgetSpan::extract_from_inline_span`, a helper function for extracting `WidgetSpan`s
///    from an `InlineSpan` tree.
pub trait RenderInlineChildrenContainerDefaults:
    RenderBox + ContainerRenderObjectMixin<ChildType = AnyRenderBox, ParentDataType = TextParentData>
{
    /// Installs [`TextParentData`] on `child`, the mixin's `setupParentData`.
    fn setup_parent_data(self: RenderHandle<Self>, app: &mut App, child: AnyRenderObject) {
        if !child.parent_data_is::<TextParentData>(app) {
            child.set_parent_data(app, TextParentData::default());
        }
    }

    /// Computes the layout for every inline child using the `max_width` constraint.
    ///
    /// Returns a list of [`PlaceholderDimensions`], representing the layout results for each
    /// child managed by the [`ContainerRenderObjectMixin`] mixin.
    ///
    /// The `get_child_baseline` parameter and the `layout_child` parameter must be
    /// consistent: if `layout_child` computes the size of the child without modifying the
    /// actual layout of that child, then `get_child_baseline` must also be "dry", and vice
    /// versa.
    ///
    /// Since this method does not impose a maximum height constraint on the inline children,
    /// some children may become taller than this [`RenderBox`].
    ///
    /// See also:
    ///
    ///  * [`TextPainter::set_placeholder_dimensions`], the method that usually takes the
    ///    layout results from this method as the input.
    fn layout_inline_children(
        self: RenderHandle<Self>,
        app: &mut App,
        max_width: f64,
        layout_child: ChildLayouter,
        get_child_baseline: ChildBaselineGetter,
    ) -> Vec<PlaceholderDimensions> {
        let constraints = BoxConstraints::new().max_width(max_width);
        let mut dimensions = Vec::new();
        let mut child = self.first_child(app);
        while let Some(current) = child {
            dimensions.push(layout_child_inline(
                app,
                current,
                constraints,
                layout_child,
                get_child_baseline,
            ));
            child = self.child_after(app, current);
        }
        dimensions
    }

    /// Positions each inline child according to the coordinates provided in the `boxes`
    /// list.
    ///
    /// The `boxes` list must be in logical order, which is the order each child is
    /// encountered when the user reads the text. Usually the length of the list equals
    /// [`child_count`](ContainerRenderObjectMixin::child_count), but it can be less than
    /// that, when some children are omitted due to ellipsing. It never exceeds
    /// [`child_count`](ContainerRenderObjectMixin::child_count).
    ///
    /// See also:
    ///
    ///  * [`TextPainter::inline_placeholder_boxes`], the method that can be used to get the
    ///    input `boxes`.
    fn position_inline_children(self: RenderHandle<Self>, app: &mut App, boxes: &[TextBox]) {
        let mut child = self.first_child(app);
        for text_box in boxes {
            let Some(current) = child else {
                debug_assert!(
                    false,
                    "Invalid number of boxes provided to positionInlineChildren. The number of \
                     boxes ({}) exceeds the number of child render objects ({}).",
                    boxes.len(),
                    self.child_count(app)
                );
                return;
            };
            text_parent_data_mut(app, current).offset =
                Some(Offset::new(text_box.left, text_box.top));
            child = self.child_after(app, current);
        }
        while let Some(current) = child {
            text_parent_data_mut(app, current).offset = None;
            child = self.child_after(app, current);
        }
    }

    /// Applies the transform that would be applied when painting the given child to the given
    /// matrix.
    ///
    /// Render children whose [`TextParentData::offset`] is `None` zeros out the `transform` to
    /// indicate they're invisible thus should not be painted.
    fn default_apply_paint_transform(
        self: RenderHandle<Self>,
        app: &App,
        child: AnyRenderObject,
        transform: &mut Matrix4,
    ) {
        match child.parent_data_of::<TextParentData>(app).offset {
            None => *transform = crate::object::zero_matrix(),
            Some(offset) => crate::object::translate(transform, offset.dx(), offset.dy()),
        }
    }

    /// Paints each inline child.
    ///
    /// Render children whose [`TextParentData::offset`] is `None` will be skipped by this
    /// method.
    fn paint_inline_children(
        self: RenderHandle<Self>,
        app: &mut App,
        context: &mut PaintingContext,
        offset: Offset,
    ) {
        let mut child = self.first_child(app);
        while let Some(current) = child {
            let Some(child_offset) = text_parent_data(app, current).offset else {
                return;
            };
            context.paint_child(app, current.as_object(), child_offset + offset);
            child = self.child_after(app, current);
        }
    }

    /// Performs a hit test on each inline child.
    ///
    /// Render children whose [`TextParentData::offset`] is `None` will be skipped by this
    /// method.
    fn hit_test_inline_children(
        self: RenderHandle<Self>,
        app: &mut App,
        result: &mut BoxHitTestResult<'_>,
        position: Offset,
    ) -> bool {
        let mut child = self.first_child(app);
        while let Some(current) = child {
            let Some(child_offset) = text_parent_data(app, current).offset else {
                return false;
            };
            let is_hit = result.add_with_paint_offset(
                Some(child_offset),
                position,
                |result, transformed| current.hit_test(app, result, transformed),
            );
            if is_hit {
                return true;
            }
            child = self.child_after(app, current);
        }
        false
    }
}

/// The mixin's `_layoutChild`: lays `child` out with `layout_child` and describes it as a
/// placeholder for its span.
fn layout_child_inline(
    app: &mut App,
    child: AnyRenderBox,
    child_constraints: BoxConstraints,
    layout_child: ChildLayouter,
    get_baseline: ChildBaselineGetter,
) -> PlaceholderDimensions {
    let span = text_parent_data(app, child).span.clone();
    debug_assert!(span.is_some());
    let Some(span) = span else {
        return PlaceholderDimensions::EMPTY;
    };
    let size = layout_child(app, child, child_constraints);
    let baseline_offset = match span.alignment() {
        PlaceholderAlignment::AboveBaseline
        | PlaceholderAlignment::BelowBaseline
        | PlaceholderAlignment::Bottom
        | PlaceholderAlignment::Middle
        | PlaceholderAlignment::Top => None,
        PlaceholderAlignment::Baseline => get_baseline(
            app,
            child,
            child_constraints,
            span.baseline()
                .expect("a baseline-aligned placeholder names its baseline"),
        ),
    };
    PlaceholderDimensions {
        size,
        alignment: span.alignment(),
        baseline: span.baseline(),
        baseline_offset,
    }
}

fn text_parent_data(app: &App, child: AnyRenderBox) -> &TextParentData {
    child.as_object().parent_data_of::<TextParentData>(app)
}

fn text_parent_data_mut(app: &mut App, child: AnyRenderBox) -> &mut TextParentData {
    child.as_object().parent_data_of_mut::<TextParentData>(app)
}

/// A render object that displays a paragraph of text.
///
/// Dart's constructor arguments beyond `text` and `textDirection` are the setters; add the
/// inline children with [`add`](ContainerRenderObjectMixin::add) or
/// [`add_all`](ContainerRenderObjectMixin::add_all). The paragraph shapes against `fonts` when
/// given, else the app-wide collection of `PaintingBinding`.
pub struct RenderParagraph {
    render_object: RenderObjectData,
    render_box: RenderBoxData,
    container: ContainerRenderObjectData<AnyRenderBox>,
    text_painter: TextPainter,
    text_intrinsics: Option<TextPainter>,
    fonts_override: Option<Handle<FontCollection>>,
    soft_wrap: bool,
    overflow: TextOverflow,
    device_pixel_ratio: f64,
    needs_clipping: bool,
    // Placeholder dimensions representing the sizes of child inline widgets.
    //
    // These need to be cached because the text painter's placeholder dimensions
    // will be overwritten during intrinsic width/height calculations and must be
    // restored to the original values before final layout and painting.
    placeholder_dimensions: Option<Vec<PlaceholderDimensions>>,
    system_fonts: RelayoutWhenSystemFontsChangeData,
}

impl RenderParagraph {
    /// Creates a paragraph render object.
    pub fn new(
        app: &mut App,
        text: InlineSpanRef,
        text_direction: TextDirection,
        fonts: Option<Handle<FontCollection>>,
    ) -> RenderHandle<Self> {
        debug_assert!(text.debug_assert_is_valid());
        let mut text_painter = TextPainter::new();
        text_painter.set_text(Some(text));
        text_painter.set_text_direction(Some(text_direction));
        RenderHandle::new_box(
            app,
            RenderParagraph {
                render_object: RenderObjectData::new(),
                render_box: RenderBoxData::new(),
                container: ContainerRenderObjectData::new(),
                text_painter,
                text_intrinsics: None,
                fonts_override: fonts,
                soft_wrap: true,
                overflow: TextOverflow::Clip,
                device_pixel_ratio: 1.0,
                needs_clipping: false,
                placeholder_dimensions: None,
                system_fonts: RelayoutWhenSystemFontsChangeData::new(),
            },
        )
    }

    fn painter(self: RenderHandle<Self>, app: &App) -> &TextPainter {
        &self.get(app).text_painter
    }

    fn painter_mut(self: RenderHandle<Self>, app: &mut App) -> &mut TextPainter {
        &mut self.get_mut(app).text_painter
    }

    /// The collection this paragraph shapes against: the override, else the app-wide one.
    pub fn fonts(self: RenderHandle<Self>, app: &mut App) -> Handle<FontCollection> {
        match self.get(app).fonts_override {
            Some(fonts) => fonts,
            None => PaintingBinding::instance(app).fonts(app),
        }
    }

    /// The painter together with the collection it shapes against.
    fn painter_with_fonts(
        self: RenderHandle<Self>,
        app: &mut App,
    ) -> (&mut TextPainter, &mut FontCollection) {
        let fonts = self.fonts(app);
        let (this, fonts) = app.get_disjoint_mut(self.handle(), fonts);
        (&mut this.text_painter, fonts)
    }

    /// The text to display.
    pub fn text(self: RenderHandle<Self>, app: &App) -> InlineSpanRef {
        self.painter(app)
            .text()
            .cloned()
            .expect("a paragraph always has text")
    }

    /// Sets [`text`](Self::text).
    pub fn set_text(self: RenderHandle<Self>, app: &mut App, value: InlineSpanRef) {
        match self.text(app).compare_to(&*value) {
            RenderComparison::Identical => {}
            RenderComparison::Metadata => {
                self.painter_mut(app).set_text(Some(value));
            }
            RenderComparison::Paint => {
                self.painter_mut(app).set_text(Some(value));
                self.mark_needs_paint(app);
            }
            RenderComparison::Layout => {
                self.painter_mut(app).set_text(Some(value));
                self.mark_needs_layout(app);
            }
        }
    }

    /// How the text should be aligned horizontally.
    pub fn text_align(self: RenderHandle<Self>, app: &App) -> TextAlign {
        self.painter(app).text_align()
    }

    /// Sets [`text_align`](Self::text_align).
    pub fn set_text_align(self: RenderHandle<Self>, app: &mut App, value: TextAlign) {
        if self.painter(app).text_align() == value {
            return;
        }
        self.painter_mut(app).set_text_align(value);
        self.mark_needs_paint(app);
    }

    /// The directionality of the text.
    ///
    /// This decides how the [`TextAlign::Start`], [`TextAlign::End`], and
    /// [`TextAlign::Justify`] values of [`text_align`](Self::text_align) are interpreted.
    ///
    /// This is also used to disambiguate how to render bidirectional text. For example, if
    /// the [`text`](Self::text) is an English phrase followed by a Hebrew phrase, in a
    /// [`TextDirection::Ltr`] context the English phrase will be on the left and the Hebrew
    /// phrase to its right, while in a [`TextDirection::Rtl`] context, the English phrase
    /// will be on the right and the Hebrew phrase on its left.
    pub fn text_direction(self: RenderHandle<Self>, app: &App) -> TextDirection {
        self.painter(app)
            .text_direction()
            .expect("a paragraph always has a direction")
    }

    /// Sets [`text_direction`](Self::text_direction).
    pub fn set_text_direction(self: RenderHandle<Self>, app: &mut App, value: TextDirection) {
        if self.painter(app).text_direction() == Some(value) {
            return;
        }
        self.painter_mut(app).set_text_direction(Some(value));
        self.mark_needs_layout(app);
    }

    /// Whether the text should break at soft line breaks.
    ///
    /// If false, the glyphs in the text will be positioned as if there was unlimited
    /// horizontal space.
    ///
    /// If the text exceeds the given number of lines, it will be truncated according to
    /// [`overflow`](Self::overflow) and [`soft_wrap`](Self::soft_wrap).
    pub fn soft_wrap(self: RenderHandle<Self>, app: &App) -> bool {
        self.get(app).soft_wrap
    }

    /// Sets [`soft_wrap`](Self::soft_wrap).
    pub fn set_soft_wrap(self: RenderHandle<Self>, app: &mut App, value: bool) {
        if self.get(app).soft_wrap == value {
            return;
        }
        self.get_mut(app).soft_wrap = value;
        self.mark_needs_layout(app);
    }

    /// How visual overflow should be handled.
    pub fn overflow(self: RenderHandle<Self>, app: &App) -> TextOverflow {
        self.get(app).overflow
    }

    /// Sets [`overflow`](Self::overflow).
    pub fn set_overflow(self: RenderHandle<Self>, app: &mut App, value: TextOverflow) {
        if self.get(app).overflow == value {
            return;
        }
        self.get_mut(app).overflow = value;
        let ellipsis = (value == TextOverflow::Ellipsis).then(|| K_ELLIPSIS.to_owned());
        self.painter_mut(app).set_ellipsis(ellipsis);
        self.mark_needs_layout(app);
    }

    /// The font scaling strategy to use when laying out and rendering the text.
    pub fn text_scaler(self: RenderHandle<Self>, app: &App) -> TextScaler {
        self.painter(app).text_scaler().clone()
    }

    /// Sets [`text_scaler`](Self::text_scaler).
    pub fn set_text_scaler(self: RenderHandle<Self>, app: &mut App, value: TextScaler) {
        if *self.painter(app).text_scaler() == value {
            return;
        }
        self.painter_mut(app).set_text_scaler(value);
        self.mark_needs_layout(app);
    }

    /// The pixel ratio of the device this paragraph is being rendered on.
    ///
    /// Flutter repaints on change only on the web; here it is bookkeeping.
    pub fn device_pixel_ratio(self: RenderHandle<Self>, app: &App) -> f64 {
        self.get(app).device_pixel_ratio
    }

    /// Sets [`device_pixel_ratio`](Self::device_pixel_ratio).
    pub fn set_device_pixel_ratio(self: RenderHandle<Self>, app: &mut App, value: f64) {
        if self.get(app).device_pixel_ratio == value {
            return;
        }
        self.get_mut(app).device_pixel_ratio = value;
    }

    /// An optional maximum number of lines for the text to span, wrapping if necessary.
    ///
    /// If the text exceeds the given number of lines, it will be truncated according to
    /// [`overflow`](Self::overflow) and [`soft_wrap`](Self::soft_wrap).
    pub fn max_lines(self: RenderHandle<Self>, app: &App) -> Option<i32> {
        self.painter(app).max_lines()
    }

    /// Sets [`max_lines`](Self::max_lines). The value may be `None`. If it is not `None`, then
    /// it must be greater than zero.
    pub fn set_max_lines(self: RenderHandle<Self>, app: &mut App, value: Option<i32>) {
        debug_assert!(value.is_none_or(|value| value > 0));
        if self.painter(app).max_lines() == value {
            return;
        }
        self.painter_mut(app).set_max_lines(value);
        self.mark_needs_layout(app);
    }

    /// Defines how to measure the width of the rendered text.
    pub fn text_width_basis(self: RenderHandle<Self>, app: &App) -> TextWidthBasis {
        self.painter(app).text_width_basis()
    }

    /// Sets [`text_width_basis`](Self::text_width_basis).
    pub fn set_text_width_basis(self: RenderHandle<Self>, app: &mut App, value: TextWidthBasis) {
        if self.painter(app).text_width_basis() == value {
            return;
        }
        self.painter_mut(app).set_text_width_basis(value);
        self.mark_needs_layout(app);
    }

    /// Defines how to apply `TextStyle.height` over and under text.
    pub fn text_height_behavior(self: RenderHandle<Self>, app: &App) -> Option<TextHeightBehavior> {
        self.painter(app).text_height_behavior()
    }

    /// Sets [`text_height_behavior`](Self::text_height_behavior).
    pub fn set_text_height_behavior(
        self: RenderHandle<Self>,
        app: &mut App,
        value: Option<TextHeightBehavior>,
    ) {
        if self.painter(app).text_height_behavior() == value {
            return;
        }
        self.painter_mut(app).set_text_height_behavior(value);
        self.mark_needs_layout(app);
    }

    /// The height of a space in [`text`](Self::text) in logical pixels.
    pub fn preferred_line_height(self: RenderHandle<Self>, app: &mut App) -> f64 {
        let (painter, fonts) = self.painter_with_fonts(app);
        painter.preferred_line_height(fonts)
    }

    /// Flutter's `_textIntrinsics`: a second painter kept in sync with this paragraph's own,
    /// together with the collection it shapes against.
    ///
    /// Laying the paragraph's painter out for intrinsics would destroy the state its own layout
    /// left behind.
    fn text_intrinsics_with_fonts(
        self: RenderHandle<Self>,
        app: &mut App,
    ) -> (&mut TextPainter, &mut FontCollection) {
        let fonts = self.fonts(app);
        let (this, fonts) = app.get_disjoint_mut(self.handle(), fonts);
        let painter = &this.text_painter;
        let intrinsics = this.text_intrinsics.get_or_insert_with(TextPainter::new);
        intrinsics.set_text(painter.text().cloned());
        intrinsics.set_text_align(painter.text_align());
        intrinsics.set_text_direction(painter.text_direction());
        intrinsics.set_text_scaler(painter.text_scaler().clone());
        intrinsics.set_max_lines(painter.max_lines());
        intrinsics.set_ellipsis(painter.ellipsis().map(str::to_owned));
        intrinsics.set_text_width_basis(painter.text_width_basis());
        intrinsics.set_text_height_behavior(painter.text_height_behavior());
        (intrinsics, fonts)
    }

    /// Flutter's `_computeIntrinsicHeight`: the height the text takes at `width`.
    fn compute_intrinsic_height(self: RenderHandle<Self>, app: &mut App, width: f64) -> f64 {
        let placeholder_dimensions = self.layout_inline_children(
            app,
            width,
            ChildLayoutHelper::dry_layout_child,
            ChildLayoutHelper::get_dry_baseline,
        );
        let max_width = self.adjust_max_width(app, width);
        let (intrinsics, fonts) = self.text_intrinsics_with_fonts(app);
        intrinsics.set_placeholder_dimensions(Some(placeholder_dimensions));
        intrinsics.layout(fonts, width, max_width);
        intrinsics.height()
    }

    fn adjust_max_width(self: RenderHandle<Self>, app: &App, max_width: f64) -> f64 {
        if self.get(app).soft_wrap || self.get(app).overflow == TextOverflow::Ellipsis {
            max_width
        } else {
            f64::INFINITY
        }
    }

    fn layout_text_with_constraints(
        self: RenderHandle<Self>,
        app: &mut App,
        constraints: BoxConstraints,
    ) {
        let max_width = self.adjust_max_width(app, constraints.max_width);
        let placeholder_dimensions = self.get(app).placeholder_dimensions.clone();
        let (painter, fonts) = self.painter_with_fonts(app);
        painter.set_placeholder_dimensions(placeholder_dimensions);
        painter.layout(fonts, constraints.min_width, max_width);
    }

    /// Returns the offset at which to paint the caret.
    ///
    /// Valid only after layout.
    pub fn get_offset_for_caret(
        self: RenderHandle<Self>,
        app: &mut App,
        position: TextPosition,
        caret_prototype: Rect,
    ) -> Offset {
        debug_assert!(!self.debug_needs_layout(app));
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);
        let (painter, fonts) = self.painter_with_fonts(app);
        painter.get_offset_for_caret(fonts, position, caret_prototype)
    }

    /// Returns the full height of the caret at the given position.
    ///
    /// Valid only after layout.
    pub fn get_full_height_for_caret(
        self: RenderHandle<Self>,
        app: &mut App,
        position: TextPosition,
    ) -> f64 {
        debug_assert!(!self.debug_needs_layout(app));
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);
        let (painter, fonts) = self.painter_with_fonts(app);
        painter.get_full_height_for_caret(fonts, position, Rect::ZERO)
    }

    /// Returns a list of rects that bound the given selection.
    ///
    /// The `box_height_style` and `box_width_style` arguments may be used to select the
    /// shape of the [`TextBox`]es. These properties default to [`BoxHeightStyle::Tight`] and
    /// [`BoxWidthStyle::Tight`] respectively.
    ///
    /// A given selection might have more than one rect if this text painter contains
    /// bidirectional text because logically contiguous text might not be visually contiguous.
    ///
    /// Valid only after layout.
    pub fn get_boxes_for_selection(
        self: RenderHandle<Self>,
        app: &mut App,
        selection: TextSelection,
        box_height_style: BoxHeightStyle,
        box_width_style: BoxWidthStyle,
    ) -> Vec<TextBox> {
        debug_assert!(!self.debug_needs_layout(app));
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);
        self.painter(app)
            .get_boxes_for_selection(selection, box_height_style, box_width_style)
    }

    /// Returns the position within the text for the given pixel offset.
    ///
    /// Valid only after layout.
    pub fn get_position_for_offset(
        self: RenderHandle<Self>,
        app: &mut App,
        offset: Offset,
    ) -> TextPosition {
        debug_assert!(!self.debug_needs_layout(app));
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);
        self.painter(app).get_position_for_offset(offset)
    }

    /// Returns the text range of the word at the given offset. Characters not part of a word,
    /// such as spaces, symbols, and punctuation, have word breaks on both sides. In such
    /// cases, this method will return a text range that contains the given text position.
    ///
    /// Word boundaries are defined more precisely in Unicode Standard Annex #29
    /// <http://www.unicode.org/reports/tr29/#Word_Boundaries>.
    ///
    /// Valid only after layout.
    pub fn get_word_boundary(
        self: RenderHandle<Self>,
        app: &mut App,
        position: TextPosition,
    ) -> TextRange {
        debug_assert!(!self.debug_needs_layout(app));
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);
        self.painter(app).get_word_boundary(position)
    }

    /// Returns the size of the text as laid out.
    ///
    /// This can differ from [`size`](RenderBox::size) if the text overflowed or if the
    /// [`constraints`](RenderBox::constraints) provided by the parent
    /// [`RenderObject`](crate::RenderObject) forced the layout to be bigger than necessary
    /// for the given [`text`](Self::text).
    ///
    /// This returns the [`TextPainter::size`] of the underlying [`TextPainter`].
    ///
    /// Valid only after layout.
    pub fn text_size(self: RenderHandle<Self>, app: &App) -> Size {
        debug_assert!(!self.debug_needs_layout(app));
        self.painter(app).size()
    }

    /// Whether the text was truncated or ellipsized as laid out.
    ///
    /// This returns the [`TextPainter::did_exceed_max_lines`] of the underlying
    /// [`TextPainter`].
    ///
    /// Valid only after layout.
    pub fn did_exceed_max_lines(self: RenderHandle<Self>, app: &App) -> bool {
        debug_assert!(!self.debug_needs_layout(app));
        self.painter(app).did_exceed_max_lines()
    }
}

impl RelayoutWhenSystemFontsChangeMixin for RenderParagraph {
    fn relayout_when_system_fonts_change_data(
        self: RenderHandle<Self>,
        app: &App,
    ) -> &RelayoutWhenSystemFontsChangeData {
        &self.get(app).system_fonts
    }

    fn relayout_when_system_fonts_change_data_mut(
        self: RenderHandle<Self>,
        app: &mut App,
    ) -> &mut RelayoutWhenSystemFontsChangeData {
        &mut self.get_mut(app).system_fonts
    }

    fn system_fonts_did_change(self: RenderHandle<Self>, app: &mut App) {
        self.as_render_object(app).mark_needs_layout(app);
        self.painter_mut(app).mark_needs_layout();
    }
}

impl ContainerRenderObjectMixin for RenderParagraph {
    type ChildType = AnyRenderBox;
    type ParentDataType = TextParentData;

    fn container_data(
        self: RenderHandle<Self>,
        app: &App,
    ) -> &ContainerRenderObjectData<AnyRenderBox> {
        &self.get(app).container
    }

    fn container_data_mut(
        self: RenderHandle<Self>,
        app: &mut App,
    ) -> &mut ContainerRenderObjectData<AnyRenderBox> {
        &mut self.get_mut(app).container
    }
}

impl RenderInlineChildrenContainerDefaults for RenderParagraph {}

impl RenderObject for RenderParagraph {
    crate::render_object_accessors!();

    fn visit_children(
        self: RenderHandle<Self>,
        app: &App,
        visitor: &mut dyn FnMut(AnyRenderObject),
    ) {
        ContainerRenderObjectMixin::visit_children(self, app, visitor)
    }

    fn did_attach(self: RenderHandle<Self>, app: &mut App, owner: Handle<PipelineOwner>) {
        ContainerRenderObjectMixin::did_attach(self, app, owner);
        RelayoutWhenSystemFontsChangeMixin::attach_system_fonts(self, app);
    }

    fn did_detach(self: RenderHandle<Self>, app: &mut App) {
        RelayoutWhenSystemFontsChangeMixin::detach_system_fonts(self, app);
        ContainerRenderObjectMixin::did_detach(self, app);
    }

    fn redepth_children(self: RenderHandle<Self>, app: &mut App) {
        ContainerRenderObjectMixin::redepth_children(self, app)
    }

    fn perform_layout(self: RenderHandle<Self>, app: &mut App) {
        let constraints = self.constraints(app);
        let placeholder_dimensions = self.layout_inline_children(
            app,
            constraints.max_width,
            ChildLayoutHelper::layout_child,
            ChildLayoutHelper::get_baseline,
        );
        self.get_mut(app).placeholder_dimensions = Some(placeholder_dimensions);
        self.layout_text_with_constraints(app, constraints);
        let boxes = self
            .painter_mut(app)
            .inline_placeholder_boxes()
            .expect("laid out just now");
        self.position_inline_children(app, &boxes);
        let text_size = self.painter(app).size();
        let size = constraints.constrain(text_size);
        self.set_size(app, size);

        let did_overflow_height =
            size.height() < text_size.height() || self.painter(app).did_exceed_max_lines();
        let did_overflow_width = size.width() < text_size.width();
        // TODO(abarth): We're only measuring the sizes of the line boxes here. If
        // the glyphs draw outside the line boxes, we might think that there isn't
        // visual overflow when there actually is visual overflow. This can become
        // a problem if we start having horizontal overflow and introduce a clip
        // that affects the actual (but undetected) vertical overflow.
        let has_visual_overflow = did_overflow_width || did_overflow_height;
        let needs_clipping = has_visual_overflow
            && match self.get(app).overflow {
                TextOverflow::Visible => false,
                // The fade shader waits on gradients; a fade clips meanwhile.
                TextOverflow::Clip | TextOverflow::Ellipsis | TextOverflow::Fade => true,
            };
        self.get_mut(app).needs_clipping = needs_clipping;
    }

    fn paint(
        self: RenderHandle<Self>,
        app: &mut App,
        context: &mut PaintingContext,
        offset: Offset,
    ) {
        // Ideally we could compute the min/max intrinsic width/height with a
        // non-destructive operation. However, currently, computing these values
        // will destroy state inside the painter. If that happens, we need to get back
        // the correct state by calling _layout again.
        //
        // TODO(abarth): Make computing the min/max intrinsic width/height a
        //  non-destructive operation.
        //
        // If you remove this call, make sure that changing the textAlign still
        // works properly.
        let constraints = self.constraints(app);
        self.layout_text_with_constraints(app, constraints);

        let needs_clipping = self.get(app).needs_clipping;
        if needs_clipping {
            let bounds = offset & self.size(app);
            context.canvas().save();
            context.canvas().clip_rect(bounds, ClipOp::Intersect);
        }
        let (painter, fonts) = self.painter_with_fonts(app);
        painter.paint(fonts, context.canvas(), offset);

        self.paint_inline_children(app, context, offset);

        if needs_clipping {
            context.canvas().restore();
        }
    }
}

impl RenderBox for RenderParagraph {
    crate::render_box_accessors!();

    fn setup_parent_data(self: RenderHandle<Self>, app: &mut App, child: AnyRenderObject) {
        RenderInlineChildrenContainerDefaults::setup_parent_data(self, app, child);
    }

    fn compute_min_intrinsic_width(self: RenderHandle<Self>, app: &mut App, _height: f64) -> f64 {
        let placeholder_dimensions = self.layout_inline_children(
            app,
            f64::INFINITY,
            |app, child, _| Size::new(child.get_min_intrinsic_width(app, f64::INFINITY), 0.0),
            ChildLayoutHelper::get_dry_baseline,
        );
        let (intrinsics, fonts) = self.text_intrinsics_with_fonts(app);
        intrinsics.set_placeholder_dimensions(Some(placeholder_dimensions));
        intrinsics.layout(fonts, 0.0, f64::INFINITY);
        intrinsics.min_intrinsic_width()
    }

    fn compute_max_intrinsic_width(self: RenderHandle<Self>, app: &mut App, _height: f64) -> f64 {
        let placeholder_dimensions = self.layout_inline_children(
            app,
            f64::INFINITY,
            // Height and baseline is irrelevant as all text will be laid
            // out in a single line. Therefore, using 0.0 as a dummy for the height.
            |app, child, _| Size::new(child.get_max_intrinsic_width(app, f64::INFINITY), 0.0),
            ChildLayoutHelper::get_dry_baseline,
        );
        let (intrinsics, fonts) = self.text_intrinsics_with_fonts(app);
        intrinsics.set_placeholder_dimensions(Some(placeholder_dimensions));
        intrinsics.layout(fonts, 0.0, f64::INFINITY);
        intrinsics.max_intrinsic_width()
    }

    fn compute_min_intrinsic_height(self: RenderHandle<Self>, app: &mut App, width: f64) -> f64 {
        self.compute_intrinsic_height(app, width)
    }

    fn compute_max_intrinsic_height(self: RenderHandle<Self>, app: &mut App, width: f64) -> f64 {
        self.compute_intrinsic_height(app, width)
    }

    fn compute_dry_layout(
        self: RenderHandle<Self>,
        app: &mut App,
        constraints: BoxConstraints,
    ) -> Size {
        let placeholder_dimensions = self.layout_inline_children(
            app,
            constraints.max_width,
            ChildLayoutHelper::dry_layout_child,
            ChildLayoutHelper::get_dry_baseline,
        );
        let max_width = self.adjust_max_width(app, constraints.max_width);
        let (intrinsics, fonts) = self.text_intrinsics_with_fonts(app);
        intrinsics.set_placeholder_dimensions(Some(placeholder_dimensions));
        intrinsics.layout(fonts, constraints.min_width, max_width);
        let size = intrinsics.size();
        constraints.constrain(size)
    }

    fn compute_distance_to_actual_baseline(
        self: RenderHandle<Self>,
        app: &mut App,
        _baseline: TextBaseline,
    ) -> Option<f64> {
        debug_assert!(!self.as_object().debug_needs_layout(app));
        let constraints = self.constraints(app);
        debug_assert!(constraints.debug_assert_is_valid(false));
        self.layout_text_with_constraints(app, constraints);
        // Since the metric for the ideographic baseline is inaccurate and the non-alphabetic
        // baselines are based off of the alphabetic baseline, the alphabetic one is used for
        // now to produce correct layouts.
        Some(
            self.painter(app)
                .compute_distance_to_actual_baseline(TextBaseline::Alphabetic),
        )
    }

    fn compute_dry_baseline(
        self: RenderHandle<Self>,
        app: &mut App,
        constraints: BoxConstraints,
        _baseline: TextBaseline,
    ) -> Option<f64> {
        debug_assert!(constraints.debug_assert_is_valid(false));
        let placeholder_dimensions = self.layout_inline_children(
            app,
            constraints.max_width,
            ChildLayoutHelper::dry_layout_child,
            ChildLayoutHelper::get_dry_baseline,
        );
        let max_width = self.adjust_max_width(app, constraints.max_width);
        let (intrinsics, fonts) = self.text_intrinsics_with_fonts(app);
        intrinsics.set_placeholder_dimensions(Some(placeholder_dimensions));
        intrinsics.layout(fonts, constraints.min_width, max_width);
        Some(intrinsics.compute_distance_to_actual_baseline(TextBaseline::Alphabetic))
    }

    fn apply_paint_transform(
        self: RenderHandle<Self>,
        app: &App,
        child: AnyRenderObject,
        transform: &mut Matrix4,
    ) {
        self.default_apply_paint_transform(app, child, transform);
    }

    fn hit_test_self(self: RenderHandle<Self>, _app: &App, _position: Offset) -> bool {
        true
    }

    fn hit_test_children(
        self: RenderHandle<Self>,
        app: &mut App,
        result: &mut BoxHitTestResult<'_>,
        position: Offset,
    ) -> bool {
        let painter = self.painter(app);
        let glyph = painter.get_closest_glyph_for_offset(position);
        // The hit-test can't fall through the horizontal gaps between visually
        // adjacent characters on the same line, even with a large letter-spacing or
        // text justification, as grapheme_cluster_layout_bounds.width is the advance
        // width to the next character, so there's no gap between their
        // grapheme_cluster_layout_bounds rects.
        let span_hit = glyph
            .filter(|glyph| glyph.grapheme_cluster_layout_bounds.contains(position))
            .and_then(|glyph| {
                painter
                    .text()
                    .expect("a paragraph always has text")
                    .clone()
                    .get_span_for_position(TextPosition::new(
                        glyph.grapheme_cluster_code_unit_range.start,
                    ))
                    .and_then(|span| span.to_hit_test_target())
            });
        match span_hit {
            Some(span) => {
                result.add(span.into());
                true
            }
            None => self.hit_test_inline_children(app, result, position),
        }
    }
}

#[cfg(test)]
mod tests {
    use inset_embedder::Color;
    use inset_foundation::AppCell;
    use inset_gestures::HitTestResult;
    use inset_painting::{TextSpan, TextStyle};

    use super::*;
    use crate::layer::{ContainerLayer, ErasedLayer, OffsetLayer};
    use crate::pipeline_owner::PipelineOwner;
    use crate::proxy_box::RenderRepaintBoundary;

    /// What the shell does at start-up: the app-wide fonts, here the OS fonts.
    fn install_fonts(app: &mut App) {
        let binding = PaintingBinding::instance(app);
        if !binding.has_fonts(app) {
            binding.install_fonts(app, |fonts| {
                fonts.add_source(valo_system_fonts::SystemFonts::load());
            });
        }
    }

    fn span(text: &str) -> InlineSpanRef {
        TextSpan::new()
            .text(text)
            .style(TextStyle::new().font_size(20.0))
            .into_span()
    }

    fn laid_out(
        app: &mut App,
        text: &str,
        constraints: BoxConstraints,
    ) -> (
        RenderHandle<RenderParagraph>,
        RenderHandle<RenderRepaintBoundary>,
    ) {
        laid_out_span(app, span(text), constraints)
    }

    fn laid_out_span(
        app: &mut App,
        text: InlineSpanRef,
        constraints: BoxConstraints,
    ) -> (
        RenderHandle<RenderParagraph>,
        RenderHandle<RenderRepaintBoundary>,
    ) {
        install_fonts(app);
        let paragraph = RenderParagraph::new(app, text, TextDirection::Ltr, None);
        let root = RenderRepaintBoundary::new(app, Some(paragraph.as_box()));
        let owner = PipelineOwner::new(app, None, None);
        owner.set_root_node(app, Some(root.as_object()));
        root.schedule_initial_layout(app);
        root.layout(app, constraints, false);
        let paint_root = OffsetLayer::new(app, Offset::ZERO);
        paint_root.as_layer().attach(app, root.as_object().id());
        root.as_object()
            .schedule_initial_paint(app, paint_root.as_container_layer());
        (paragraph, root)
    }

    /// `paragraph_test.dart`: the minimum intrinsic width is the widest word, the maximum is the
    /// whole text on one line, and the intrinsic height at a width is what the text takes there.
    #[test]
    fn paragraph_intrinsics_come_from_the_text() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        install_fonts(&mut app);
        let paragraph = RenderParagraph::new(
            &mut app,
            span("one two three four"),
            TextDirection::Ltr,
            None,
        );
        let box_ = paragraph.as_box();
        let min_width = box_.get_min_intrinsic_width(&mut app, f64::INFINITY);
        let max_width = box_.get_max_intrinsic_width(&mut app, f64::INFINITY);
        assert!(min_width > 0.0);
        assert!(
            min_width < max_width,
            "the widest word is narrower than the whole text"
        );

        let one_line = box_.get_min_intrinsic_height(&mut app, max_width);
        let wrapped = box_.get_min_intrinsic_height(&mut app, max_width / 2.0);
        assert!(wrapped > one_line, "half the width takes more lines");
        assert_eq!(box_.get_max_intrinsic_height(&mut app, max_width), one_line);
    }

    /// The dry layout of a paragraph is the size it lays out to, and its dry baseline is the
    /// baseline that layout reports.
    #[test]
    fn paragraph_dry_layout_and_baseline_match_its_layout() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let constraints = BoxConstraints::new().max_width(200.0).max_height(1000.0);
        install_fonts(&mut app);
        let paragraph =
            RenderParagraph::new(&mut app, span("one two three"), TextDirection::Ltr, None);
        let dry = paragraph.as_box().get_dry_layout(&mut app, constraints);
        let dry_baseline =
            paragraph
                .as_box()
                .get_dry_baseline(&mut app, constraints, TextBaseline::Alphabetic);

        let (paragraph, _root) = laid_out(&mut app, "one two three", constraints);
        assert_eq!(dry, paragraph.size(&app));
        let baseline =
            paragraph
                .as_box()
                .get_distance_to_baseline(&mut app, TextBaseline::Alphabetic, true);
        assert_eq!(dry_baseline, baseline);
        assert!(baseline.is_some_and(|baseline| baseline > 0.0));
    }

    #[test]
    fn a_paragraph_sizes_itself_to_its_text_within_loose_constraints() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (paragraph, _root) = laid_out(&mut app, "Hello", loose);
        let size = paragraph.size(&app);
        assert!(size.width() > 0.0 && size.width() < 1000.0);
        assert_eq!(size, paragraph.text_size(&app));
        assert!(!paragraph.did_exceed_max_lines(&app));
    }

    #[test]
    fn narrow_constraints_wrap_the_text() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (wide, _root) = laid_out(&mut app, "one two three four", loose);
        let natural = wide.size(&app);
        let narrow = BoxConstraints::new()
            .max_width(natural.width() / 2.0)
            .max_height(1000.0);
        let (wrapped, _root) = laid_out(&mut app, "one two three four", narrow);
        assert!(wrapped.size(&app).height() > natural.height());
        assert!(wrapped.size(&app).width() <= natural.width() / 2.0);
    }

    #[test]
    fn soft_wrap_off_lets_the_text_overflow_and_clips_it() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (wide, _root) = laid_out(&mut app, "one two three four", loose);
        let natural = wide.size(&app);
        install_fonts(&mut app);
        let paragraph = RenderParagraph::new(
            &mut app,
            span("one two three four"),
            TextDirection::Ltr,
            None,
        );
        paragraph.set_soft_wrap(&mut app, false);
        let root = RenderRepaintBoundary::new(&mut app, Some(paragraph.as_box()));
        let owner = PipelineOwner::new(&mut app, None, None);
        owner.set_root_node(&mut app, Some(root.as_object()));
        root.schedule_initial_layout(&mut app);
        let narrow = BoxConstraints::new()
            .max_width(natural.width() / 2.0)
            .max_height(1000.0);
        root.layout(&mut app, narrow, false);
        assert_eq!(paragraph.size(&app).width(), natural.width() / 2.0);
        assert_eq!(
            paragraph.text_size(&app).width(),
            natural.width(),
            "laid out unwrapped"
        );
        assert!(paragraph.get(&app).needs_clipping);

        paragraph.set_overflow(&mut app, TextOverflow::Visible);
        owner.flush_layout(&mut app);
        assert!(!paragraph.get(&app).needs_clipping);
    }

    #[test]
    fn ellipsis_overflow_reports_exceeded_lines() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (wide, _root) = laid_out(&mut app, "one two three four", loose);
        let natural = wide.size(&app);
        install_fonts(&mut app);
        let paragraph = RenderParagraph::new(
            &mut app,
            span("one two three four"),
            TextDirection::Ltr,
            None,
        );
        paragraph.set_max_lines(&mut app, Some(1));
        paragraph.set_overflow(&mut app, TextOverflow::Ellipsis);
        let root = RenderRepaintBoundary::new(&mut app, Some(paragraph.as_box()));
        let owner = PipelineOwner::new(&mut app, None, None);
        owner.set_root_node(&mut app, Some(root.as_object()));
        root.schedule_initial_layout(&mut app);
        root.layout(
            &mut app,
            BoxConstraints::new()
                .max_width(natural.width() / 2.0)
                .max_height(1000.0),
            false,
        );
        assert!(paragraph.did_exceed_max_lines(&app));
        assert_eq!(paragraph.size(&app).height(), natural.height());
    }

    #[test]
    fn setting_text_marks_layout_or_paint_by_the_comparison() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (paragraph, root) = laid_out(&mut app, "Hello", loose);
        let owner = root.owner(&app).unwrap();
        owner.flush_compositing_bits(&mut app);
        owner.flush_paint(&mut app);
        assert!(!paragraph.debug_needs_layout(&app));

        let recolored = TextSpan::new()
            .text("Hello")
            .style(
                TextStyle::new()
                    .font_size(20.0)
                    .color(Color::from_argb(255, 255, 0, 0)),
            )
            .into_span();
        paragraph.set_text(&mut app, recolored);
        assert!(
            !paragraph.debug_needs_layout(&app),
            "a color change is paint only"
        );
        assert!(paragraph.as_object().debug_needs_paint(&app));

        paragraph.set_text(&mut app, span("Hello there"));
        assert!(paragraph.debug_needs_layout(&app));
    }

    #[test]
    fn painting_records_glyphs_and_hit_testing_lands_on_the_span_and_the_paragraph() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let loose = BoxConstraints::new().max_width(1000.0).max_height(1000.0);
        let (paragraph, root) = laid_out(&mut app, "Hello", loose);
        let owner = root.owner(&app).unwrap();
        owner.flush_compositing_bits(&mut app);
        owner.flush_paint(&mut app);
        assert!(!paragraph.as_object().debug_needs_paint(&app));

        let mut result = HitTestResult::new();
        let hit = paragraph.hit_test(
            &mut app,
            &mut BoxHitTestResult::wrap(&mut result),
            Offset::new(2.0, 2.0),
        );
        assert!(hit);
        assert_eq!(result.path().len(), 2, "the span, then the paragraph");
        let position = paragraph.get_position_for_offset(&mut app, Offset::new(0.0, 1.0));
        assert_eq!(position.offset, 0);
        assert_eq!(
            paragraph.get_word_boundary(&mut app, TextPosition::new(1)),
            TextRange::new(0, 5)
        );
    }

    // ---- spans as hit-test targets and mouse tracker annotations ----

    /// The boxes of the text between UTF-16 offsets `start` and `end`, one per line.
    fn boxes_of(
        app: &mut App,
        paragraph: RenderHandle<RenderParagraph>,
        start: i32,
        end: i32,
    ) -> Vec<TextBox> {
        paragraph.get_boxes_for_selection(
            app,
            TextSelection::new(start, end),
            BoxHeightStyle::Tight,
            BoxWidthStyle::Tight,
        )
    }

    fn center_of(text_box: &TextBox) -> Offset {
        Offset::new(
            (text_box.left + text_box.right) / 2.0,
            (text_box.top + text_box.bottom) / 2.0,
        )
    }

    /// The text of each span a hit test at `position` added, front to back.
    fn spans_hit(
        app: &mut App,
        paragraph: RenderHandle<RenderParagraph>,
        position: Offset,
    ) -> (bool, Vec<String>) {
        let mut result = HitTestResult::new();
        let hit = paragraph.hit_test(app, &mut BoxHitTestResult::wrap(&mut result), position);
        let spans = result
            .path()
            .iter()
            .filter_map(|entry| {
                let target: &dyn Any = entry.target();
                target.downcast_ref::<TextSpan>()?.text.clone()
            })
            .collect();
        (hit, spans)
    }

    /// `paragraph_test.dart`, "Basic TextSpan Hit testing": a hit on a glyph adds the span the
    /// glyph is in, and a hit off the glyphs adds no span.
    #[test]
    fn a_hit_on_a_glyph_adds_its_span_and_a_hit_off_the_glyphs_adds_none() {
        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let text = TextSpan::new()
            .style(TextStyle::new().font_size(10.0))
            .children(vec![
                TextSpan::new().text("A".repeat(10)).into_span(),
                TextSpan::new()
                    .text("BC")
                    .style(TextStyle::new().letter_spacing(26.0))
                    .into_span(),
            ])
            .into_span();
        let (paragraph, _root) =
            laid_out_span(&mut app, text, BoxConstraints::tight_for(Some(300.0), None));

        let first_a = center_of(&boxes_of(&mut app, paragraph, 0, 1)[0]);
        let last_a = center_of(&boxes_of(&mut app, paragraph, 9, 10)[0]);
        let b = boxes_of(&mut app, paragraph, 10, 11)[0];
        let c = center_of(&boxes_of(&mut app, paragraph, 11, 12)[0]);
        for position in [first_a, last_a] {
            assert_eq!(
                spans_hit(&mut app, paragraph, position),
                (true, vec!["A".repeat(10)])
            );
        }
        // Past B's glyph, in its letter spacing: still B, as the advance reaches C.
        let between_b_and_c = Offset::new(b.right - 1.0, center_of(&b).dy());
        for position in [center_of(&b), between_b_and_c, c] {
            assert_eq!(
                spans_hit(&mut app, paragraph, position),
                (true, vec!["BC".to_owned()])
            );
        }

        let after_c = Offset::new(299.0, c.dy());
        assert_eq!(
            spans_hit(&mut app, paragraph, after_c),
            (true, Vec::new()),
            "the paragraph hits itself, and no span is under the pointer"
        );
        assert_eq!(
            spans_hit(&mut app, paragraph, Offset::new(9999.0, 9999.0)),
            (false, Vec::new())
        );
    }

    /// Plays `RendererBinding`'s part: hit-tests the paragraph's root before the gesture binding
    /// adds itself to the path.
    struct RootHitTester {
        root: RenderHandle<RenderRepaintBoundary>,
    }

    impl inset_gestures::GestureBindingOverridesObject for RootHitTester {
        fn hit_test_in_view(
            self: Handle<Self>,
            app: &mut App,
            result: &mut HitTestResult,
            position: Offset,
            _view_id: inset_embedder::ViewId,
        ) {
            let root = app.get(self).root;
            root.hit_test(app, &mut BoxHitTestResult::wrap(result), position);
        }

        fn will_dispatch_event(
            self: Handle<Self>,
            _app: &mut App,
            _event: &inset_gestures::PointerEvent,
            _hit_test_result: Option<&HitTestResult>,
        ) {
        }
    }

    fn tap_at(app: &mut App, position: Offset) {
        use inset_gestures::{GestureBinding, PointerDownEvent, PointerEvent, PointerUpEvent};
        let binding = GestureBinding::instance(app);
        binding.handle_pointer_event(
            app,
            PointerEvent::Down(PointerDownEvent {
                position,
                ..PointerDownEvent::default()
            }),
        );
        app.drain_microtasks();
        binding.handle_pointer_event(
            app,
            PointerEvent::Up(PointerUpEvent {
                position,
                ..PointerUpEvent::default()
            }),
        );
        app.drain_microtasks();
    }

    /// A press on a span reaches its `TapGestureRecognizer`; a tap on the text beside it does
    /// not.
    #[test]
    fn a_tap_on_a_span_runs_its_recognizer_and_a_tap_beside_it_does_not() {
        use inset_gestures::{GestureBinding, GestureRecognizerLeaf, TapGestureRecognizer};

        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let taps = Rc::new(std::cell::Cell::new(0));
        let recognizer = TapGestureRecognizer::new(&mut app);
        let counter = Rc::clone(&taps);
        recognizer.set_on_tap(
            &mut app,
            Some(inset_foundation::Listener::new(move |_app| {
                counter.set(counter.get() + 1)
            })),
        );
        let text = TextSpan::new()
            .style(TextStyle::new().font_size(20.0))
            .children(vec![
                TextSpan::new().text("Tap ").into_span(),
                TextSpan::new()
                    .text("here")
                    .recognizer(recognizer.as_recognizer())
                    .into_span(),
                TextSpan::new().text(" please").into_span(),
            ])
            .into_span();
        let (paragraph, root) = laid_out_span(
            &mut app,
            text,
            BoxConstraints::new().max_width(1000.0).max_height(1000.0),
        );
        let hit_tester = app.create(RootHitTester { root });
        GestureBinding::instance(&mut app).set_overrides(&mut app, Rc::new(hit_tester));

        let here = center_of(&boxes_of(&mut app, paragraph, 4, 8)[0]);
        tap_at(&mut app, here);
        assert_eq!(taps.get(), 1);

        let beside = center_of(&boxes_of(&mut app, paragraph, 0, 3)[0]);
        tap_at(&mut app, beside);
        assert_eq!(taps.get(), 1, "the plain span has no recognizer");
        recognizer.as_recognizer().dispose(&mut app);
    }

    /// A span's cursor shows while the mouse is over it: the click cursor by default for a span
    /// with a recognizer, the one it names otherwise, and the one behind when it defers.
    #[test]
    fn the_mouse_shows_the_cursor_of_the_span_under_it() {
        use inset_gestures::{
            GestureRecognizerLeaf, PointerAddedEvent, PointerEvent, PointerHoverEvent,
            TapGestureRecognizer,
        };
        use inset_services::{MouseCursorRef, SystemMouseCursors};

        use crate::mouse_tracker::MouseTracker;

        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let recognizer = TapGestureRecognizer::new(&mut app);
        let text = TextSpan::new()
            .style(TextStyle::new().font_size(20.0))
            .children(vec![
                TextSpan::new().text("plain ").into_span(),
                TextSpan::new()
                    .text("link")
                    .recognizer(recognizer.as_recognizer())
                    .into_span(),
                TextSpan::new()
                    .text(" forbidden")
                    .mouse_cursor(SystemMouseCursors::FORBIDDEN)
                    .into_span(),
            ])
            .into_span();
        let (paragraph, root) = laid_out_span(
            &mut app,
            text,
            BoxConstraints::new().max_width(1000.0).max_height(1000.0),
        );
        let tracker = MouseTracker::new(
            &mut app,
            Rc::new(move |app: &mut App, position: Offset, _view| {
                let mut result = HitTestResult::new();
                root.hit_test(app, &mut BoxHitTestResult::wrap(&mut result), position);
                result
            }),
        );
        let plain = center_of(&boxes_of(&mut app, paragraph, 0, 5)[0]);
        let link = center_of(&boxes_of(&mut app, paragraph, 6, 10)[0]);
        let forbidden = center_of(&boxes_of(&mut app, paragraph, 11, 20)[0]);
        tracker.update_with_event(
            &mut app,
            &PointerEvent::Added(PointerAddedEvent {
                kind: inset_embedder::PointerDeviceKind::Mouse,
                position: plain,
                ..PointerAddedEvent::default()
            }),
            None,
        );
        let cursor_at = |app: &mut App, position: Offset| -> MouseCursorRef {
            tracker.update_with_event(
                app,
                &PointerEvent::Hover(PointerHoverEvent {
                    kind: inset_embedder::PointerDeviceKind::Mouse,
                    position,
                    ..PointerHoverEvent::default()
                }),
                None,
            );
            tracker.debug_device_active_cursor(app, 0).unwrap()
        };
        let basic: MouseCursorRef = SystemMouseCursors::BASIC.into();
        let click: MouseCursorRef = SystemMouseCursors::CLICK.into();
        let forbidden_cursor: MouseCursorRef = SystemMouseCursors::FORBIDDEN.into();
        assert!(*cursor_at(&mut app, link) == *click);
        assert!(*cursor_at(&mut app, plain) == *basic, "a plain span defers");
        assert!(*cursor_at(&mut app, forbidden) == *forbidden_cursor);
        recognizer.as_recognizer().dispose(&mut app);
    }

    /// A span that wraps over two lines is one annotation: the mouse enters it once on the
    /// first line, stays in it on the second, and exits it once.
    #[test]
    fn a_wrapped_span_is_entered_once_across_its_lines() {
        use std::cell::RefCell;

        use inset_gestures::{
            PointerAddedEvent, PointerEnterEvent, PointerEvent, PointerExitEvent, PointerHoverEvent,
        };

        use crate::mouse_tracker::MouseTracker;

        let cell = AppCell::new();
        let mut app = cell.borrow_mut();
        let log: Rc<RefCell<Vec<&str>>> = Rc::default();
        let enter_log = Rc::clone(&log);
        let exit_log = Rc::clone(&log);
        let words = "one two three four five six seven eight";
        let text = TextSpan::new()
            .text(words)
            .style(TextStyle::new().font_size(20.0))
            .on_enter(Rc::new(move |_app: &mut App, _event: PointerEnterEvent| {
                enter_log.borrow_mut().push("enter")
            }))
            .on_exit(Rc::new(move |_app: &mut App, _event: PointerExitEvent| {
                exit_log.borrow_mut().push("exit")
            }))
            .into_span();
        let (paragraph, root) = laid_out_span(
            &mut app,
            text,
            BoxConstraints::new().max_width(120.0).max_height(1000.0),
        );
        let boxes = boxes_of(&mut app, paragraph, 0, 3);
        let first_line = center_of(&boxes[0]);
        let last = words.len() as i32;
        let last_box = *boxes_of(&mut app, paragraph, last - 1, last)
            .last()
            .unwrap();
        assert!(last_box.top > boxes[0].bottom - 1.0, "the span wraps");
        let last_line = center_of(&last_box);
        let tracker = MouseTracker::new(
            &mut app,
            Rc::new(move |app: &mut App, position: Offset, _view| {
                let mut result = HitTestResult::new();
                root.hit_test(app, &mut BoxHitTestResult::wrap(&mut result), position);
                result
            }),
        );
        let mouse = |position: Offset| {
            PointerEvent::Hover(PointerHoverEvent {
                kind: inset_embedder::PointerDeviceKind::Mouse,
                position,
                ..PointerHoverEvent::default()
            })
        };
        tracker.update_with_event(
            &mut app,
            &PointerEvent::Added(PointerAddedEvent {
                kind: inset_embedder::PointerDeviceKind::Mouse,
                position: Offset::new(5000.0, 5000.0),
                ..PointerAddedEvent::default()
            }),
            None,
        );
        tracker.update_with_event(&mut app, &mouse(first_line), None);
        tracker.update_with_event(&mut app, &mouse(last_line), None);
        assert_eq!(*log.borrow(), ["enter"]);
        tracker.update_with_event(&mut app, &mouse(Offset::new(5000.0, 5000.0)), None);
        assert_eq!(*log.borrow(), ["enter", "exit"]);
    }
}