oak-gpui/crates/gpui/src/elements/uniform_list.rs

//! A scrollable list of elements with uniform height, optimized for large lists.
//! Rather than use the full taffy layout system, uniform_list simply measures
//! the first element and then lays out all remaining elements in a line based on that
//! measurement. This is much faster than the full layout system, but only works for
//! elements with uniform height.

use crate::{
    point, size, AnyElement, AvailableSpace, Bounds, ContentMask, Element, ElementId,
    GlobalElementId, Hitbox, InteractiveElement, Interactivity, IntoElement, IsZero, LayoutId,
    ListSizingBehavior, Pixels, Render, ScrollHandle, Size, StyleRefinement, Styled, View,
    ViewContext, WindowContext,
};
use smallvec::SmallVec;
use std::{cell::RefCell, cmp, ops::Range, rc::Rc};
use taffy::style::Overflow;

use super::ListHorizontalSizingBehavior;

/// uniform_list provides lazy rendering for a set of items that are of uniform height.
/// When rendered into a container with overflow-y: hidden and a fixed (or max) height,
/// uniform_list will only render the visible subset of items.
#[track_caller]
pub fn uniform_list<I, R, V>(
    view: View<V>,
    id: I,
    item_count: usize,
    f: impl 'static + Fn(&mut V, Range<usize>, &mut ViewContext<V>) -> Vec<R>,
) -> UniformList
where
    I: Into<ElementId>,
    R: IntoElement,
    V: Render,
{
    let id = id.into();
    let mut base_style = StyleRefinement::default();
    base_style.overflow.y = Some(Overflow::Scroll);

    let render_range = move |range, cx: &mut WindowContext| {
        view.update(cx, |this, cx| {
            f(this, range, cx)
                .into_iter()
                .map(|component| component.into_any_element())
                .collect()
        })
    };

    UniformList {
        item_count,
        item_to_measure_index: 0,
        render_items: Box::new(render_range),
        decorations: Vec::new(),
        interactivity: Interactivity {
            element_id: Some(id),
            base_style: Box::new(base_style),

            #[cfg(debug_assertions)]
            location: Some(*core::panic::Location::caller()),

            ..Default::default()
        },
        scroll_handle: None,
        sizing_behavior: ListSizingBehavior::default(),
        horizontal_sizing_behavior: ListHorizontalSizingBehavior::default(),
    }
}

/// A list element for efficiently laying out and displaying a list of uniform-height elements.
pub struct UniformList {
    item_count: usize,
    item_to_measure_index: usize,
    render_items:
        Box<dyn for<'a> Fn(Range<usize>, &'a mut WindowContext) -> SmallVec<[AnyElement; 64]>>,
    decorations: Vec<Box<dyn UniformListDecoration>>,
    interactivity: Interactivity,
    scroll_handle: Option<UniformListScrollHandle>,
    sizing_behavior: ListSizingBehavior,
    horizontal_sizing_behavior: ListHorizontalSizingBehavior,
}

/// Frame state used by the [UniformList].
pub struct UniformListFrameState {
    items: SmallVec<[AnyElement; 32]>,
    decorations: SmallVec<[AnyElement; 1]>,
}

/// A handle for controlling the scroll position of a uniform list.
/// This should be stored in your view and passed to the uniform_list on each frame.
#[derive(Clone, Debug, Default)]
pub struct UniformListScrollHandle(pub Rc<RefCell<UniformListScrollState>>);

#[derive(Clone, Debug, Default)]
#[allow(missing_docs)]
pub struct UniformListScrollState {
    pub base_handle: ScrollHandle,
    pub deferred_scroll_to_item: Option<usize>,
    /// Size of the item, captured during last layout.
    pub last_item_size: Option<ItemSize>,
}

#[derive(Copy, Clone, Debug, Default)]
/// The size of the item and its contents.
pub struct ItemSize {
    /// The size of the item.
    pub item: Size<Pixels>,
    /// The size of the item's contents, which may be larger than the item itself,
    /// if the item was bounded by a parent element.
    pub contents: Size<Pixels>,
}

impl UniformListScrollHandle {
    /// Create a new scroll handle to bind to a uniform list.
    pub fn new() -> Self {
        Self(Rc::new(RefCell::new(UniformListScrollState {
            base_handle: ScrollHandle::new(),
            deferred_scroll_to_item: None,
            last_item_size: None,
        })))
    }

    /// Scroll the list to the given item index.
    pub fn scroll_to_item(&self, ix: usize) {
        self.0.borrow_mut().deferred_scroll_to_item = Some(ix);
    }

    /// Get the index of the topmost visible child.
    pub fn logical_scroll_top_index(&self) -> usize {
        let this = self.0.borrow();
        this.deferred_scroll_to_item
            .unwrap_or_else(|| this.base_handle.logical_scroll_top().0)
    }
}

impl Styled for UniformList {
    fn style(&mut self) -> &mut StyleRefinement {
        &mut self.interactivity.base_style
    }
}

impl Element for UniformList {
    type RequestLayoutState = UniformListFrameState;
    type PrepaintState = Option<Hitbox>;

    fn id(&self) -> Option<ElementId> {
        self.interactivity.element_id.clone()
    }

    fn request_layout(
        &mut self,
        global_id: Option<&GlobalElementId>,
        cx: &mut WindowContext,
    ) -> (LayoutId, Self::RequestLayoutState) {
        let max_items = self.item_count;
        let item_size = self.measure_item(None, cx);
        let layout_id = self
            .interactivity
            .request_layout(global_id, cx, |style, cx| match self.sizing_behavior {
                ListSizingBehavior::Infer => {
                    cx.with_text_style(style.text_style().cloned(), |cx| {
                        cx.request_measured_layout(
                            style,
                            move |known_dimensions, available_space, _cx| {
                                let desired_height = item_size.height * max_items;
                                let width = known_dimensions.width.unwrap_or(match available_space
                                    .width
                                {
                                    AvailableSpace::Definite(x) => x,
                                    AvailableSpace::MinContent | AvailableSpace::MaxContent => {
                                        item_size.width
                                    }
                                });
                                let height = match available_space.height {
                                    AvailableSpace::Definite(height) => desired_height.min(height),
                                    AvailableSpace::MinContent | AvailableSpace::MaxContent => {
                                        desired_height
                                    }
                                };
                                size(width, height)
                            },
                        )
                    })
                }
                ListSizingBehavior::Auto => cx.with_text_style(style.text_style().cloned(), |cx| {
                    cx.request_layout(style, None)
                }),
            });

        (
            layout_id,
            UniformListFrameState {
                items: SmallVec::new(),
                decorations: SmallVec::new(),
            },
        )
    }

    fn prepaint(
        &mut self,
        global_id: Option<&GlobalElementId>,
        bounds: Bounds<Pixels>,
        frame_state: &mut Self::RequestLayoutState,
        cx: &mut WindowContext,
    ) -> Option<Hitbox> {
        let style = self.interactivity.compute_style(global_id, None, cx);
        let border = style.border_widths.to_pixels(cx.rem_size());
        let padding = style.padding.to_pixels(bounds.size.into(), cx.rem_size());

        let padded_bounds = Bounds::from_corners(
            bounds.origin + point(border.left + padding.left, border.top + padding.top),
            bounds.lower_right()
                - point(border.right + padding.right, border.bottom + padding.bottom),
        );

        let can_scroll_horizontally = matches!(
            self.horizontal_sizing_behavior,
            ListHorizontalSizingBehavior::Unconstrained
        );

        let longest_item_size = self.measure_item(None, cx);
        let content_width = if can_scroll_horizontally {
            padded_bounds.size.width.max(longest_item_size.width)
        } else {
            padded_bounds.size.width
        };
        let content_size = Size {
            width: content_width,
            height: longest_item_size.height * self.item_count + padding.top + padding.bottom,
        };

        let shared_scroll_offset = self.interactivity.scroll_offset.clone().unwrap();
        let item_height = longest_item_size.height;
        let shared_scroll_to_item = self.scroll_handle.as_mut().and_then(|handle| {
            let mut handle = handle.0.borrow_mut();
            handle.last_item_size = Some(ItemSize {
                item: padded_bounds.size,
                contents: content_size,
            });
            handle.deferred_scroll_to_item.take()
        });

        self.interactivity.prepaint(
            global_id,
            bounds,
            content_size,
            cx,
            |style, mut scroll_offset, hitbox, cx| {
                let border = style.border_widths.to_pixels(cx.rem_size());
                let padding = style.padding.to_pixels(bounds.size.into(), cx.rem_size());

                let padded_bounds = Bounds::from_corners(
                    bounds.origin + point(border.left + padding.left, border.top),
                    bounds.lower_right() - point(border.right + padding.right, border.bottom),
                );

                if let Some(handle) = self.scroll_handle.as_mut() {
                    handle.0.borrow_mut().base_handle.set_bounds(bounds);
                }

                if self.item_count > 0 {
                    let content_height =
                        item_height * self.item_count + padding.top + padding.bottom;
                    let is_scrolled_vertically = !scroll_offset.y.is_zero();
                    let min_vertical_scroll_offset = padded_bounds.size.height - content_height;
                    if is_scrolled_vertically && scroll_offset.y < min_vertical_scroll_offset {
                        shared_scroll_offset.borrow_mut().y = min_vertical_scroll_offset;
                        scroll_offset.y = min_vertical_scroll_offset;
                    }

                    let content_width = content_size.width + padding.left + padding.right;
                    let is_scrolled_horizontally =
                        can_scroll_horizontally && !scroll_offset.x.is_zero();
                    if is_scrolled_horizontally && content_width <= padded_bounds.size.width {
                        shared_scroll_offset.borrow_mut().x = Pixels::ZERO;
                        scroll_offset.x = Pixels::ZERO;
                    }

                    if let Some(ix) = shared_scroll_to_item {
                        let list_height = padded_bounds.size.height;
                        let mut updated_scroll_offset = shared_scroll_offset.borrow_mut();
                        let item_top = item_height * ix + padding.top;
                        let item_bottom = item_top + item_height;
                        let scroll_top = -updated_scroll_offset.y;
                        if item_top < scroll_top + padding.top {
                            updated_scroll_offset.y = -(item_top) + padding.top;
                        } else if item_bottom > scroll_top + list_height - padding.bottom {
                            updated_scroll_offset.y = -(item_bottom - list_height) - padding.bottom;
                        }
                        scroll_offset = *updated_scroll_offset;
                    }

                    let first_visible_element_ix =
                        (-(scroll_offset.y + padding.top) / item_height).floor() as usize;
                    let last_visible_element_ix = ((-scroll_offset.y + padded_bounds.size.height)
                        / item_height)
                        .ceil() as usize;
                    let visible_range = first_visible_element_ix
                        ..cmp::min(last_visible_element_ix, self.item_count);

                    let mut items = (self.render_items)(visible_range.clone(), cx);

                    let content_mask = ContentMask { bounds };
                    cx.with_content_mask(Some(content_mask), |cx| {
                        for (mut item, ix) in items.into_iter().zip(visible_range.clone()) {
                            let item_origin = padded_bounds.origin
                                + point(
                                    if can_scroll_horizontally {
                                        scroll_offset.x + padding.left
                                    } else {
                                        scroll_offset.x
                                    },
                                    item_height * ix + scroll_offset.y + padding.top,
                                );
                            let available_width = if can_scroll_horizontally {
                                padded_bounds.size.width + scroll_offset.x.abs()
                            } else {
                                padded_bounds.size.width
                            };
                            let available_space = size(
                                AvailableSpace::Definite(available_width),
                                AvailableSpace::Definite(item_height),
                            );
                            item.layout_as_root(available_space, cx);
                            item.prepaint_at(item_origin, cx);
                            frame_state.items.push(item);
                        }

                        let bounds = Bounds::new(
                            padded_bounds.origin
                                + point(
                                    if can_scroll_horizontally {
                                        scroll_offset.x + padding.left
                                    } else {
                                        scroll_offset.x
                                    },
                                    scroll_offset.y + padding.top,
                                ),
                            padded_bounds.size,
                        );
                        for decoration in &self.decorations {
                            let mut decoration = decoration.as_ref().compute(
                                visible_range.clone(),
                                bounds,
                                item_height,
                                self.item_count,
                                cx,
                            );
                            let available_space = size(
                                AvailableSpace::Definite(bounds.size.width),
                                AvailableSpace::Definite(bounds.size.height),
                            );
                            decoration.layout_as_root(available_space, cx);
                            decoration.prepaint_at(bounds.origin, cx);
                            frame_state.decorations.push(decoration);
                        }
                    });
                }

                hitbox
            },
        )
    }

    fn paint(
        &mut self,
        global_id: Option<&GlobalElementId>,
        bounds: Bounds<crate::Pixels>,
        request_layout: &mut Self::RequestLayoutState,
        hitbox: &mut Option<Hitbox>,
        cx: &mut WindowContext,
    ) {
        self.interactivity
            .paint(global_id, bounds, hitbox.as_ref(), cx, |_, cx| {
                for item in &mut request_layout.items {
                    item.paint(cx);
                }
                for decoration in &mut request_layout.decorations {
                    decoration.paint(cx);
                }
            })
    }
}

impl IntoElement for UniformList {
    type Element = Self;

    fn into_element(self) -> Self::Element {
        self
    }
}

/// A decoration for a [`UniformList`]. This can be used for various things,
/// such as rendering indent guides, or other visual effects.
pub trait UniformListDecoration {
    /// Compute the decoration element, given the visible range of list items,
    /// the bounds of the list, and the height of each item.
    fn compute(
        &self,
        visible_range: Range<usize>,
        bounds: Bounds<Pixels>,
        item_height: Pixels,
        item_count: usize,
        cx: &mut WindowContext,
    ) -> AnyElement;
}

impl UniformList {
    /// Selects a specific list item for measurement.
    pub fn with_width_from_item(mut self, item_index: Option<usize>) -> Self {
        self.item_to_measure_index = item_index.unwrap_or(0);
        self
    }

    /// Sets the sizing behavior, similar to the `List` element.
    pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
        self.sizing_behavior = behavior;
        self
    }

    /// Sets the horizontal sizing behavior, controlling the way list items laid out horizontally.
    /// With [`ListHorizontalSizingBehavior::Unconstrained`] behavior, every item and the list itself will
    /// have the size of the widest item and lay out pushing the `end_slot` to the right end.
    pub fn with_horizontal_sizing_behavior(
        mut self,
        behavior: ListHorizontalSizingBehavior,
    ) -> Self {
        self.horizontal_sizing_behavior = behavior;
        match behavior {
            ListHorizontalSizingBehavior::FitList => {
                self.interactivity.base_style.overflow.x = None;
            }
            ListHorizontalSizingBehavior::Unconstrained => {
                self.interactivity.base_style.overflow.x = Some(Overflow::Scroll);
            }
        }
        self
    }

    /// Adds a decoration element to the list.
    pub fn with_decoration(mut self, decoration: impl UniformListDecoration + 'static) -> Self {
        self.decorations.push(Box::new(decoration));
        self
    }

    fn measure_item(&self, list_width: Option<Pixels>, cx: &mut WindowContext) -> Size<Pixels> {
        if self.item_count == 0 {
            return Size::default();
        }

        let item_ix = cmp::min(self.item_to_measure_index, self.item_count - 1);
        let mut items = (self.render_items)(item_ix..item_ix + 1, cx);
        let Some(mut item_to_measure) = items.pop() else {
            return Size::default();
        };
        let available_space = size(
            list_width.map_or(AvailableSpace::MinContent, |width| {
                AvailableSpace::Definite(width)
            }),
            AvailableSpace::MinContent,
        );
        item_to_measure.layout_as_root(available_space, cx)
    }

    /// Track and render scroll state of this list with reference to the given scroll handle.
    pub fn track_scroll(mut self, handle: UniformListScrollHandle) -> Self {
        self.interactivity.tracked_scroll_handle = Some(handle.0.borrow().base_handle.clone());
        self.scroll_handle = Some(handle);
        self
    }
}

impl InteractiveElement for UniformList {
    fn interactivity(&mut self) -> &mut crate::Interactivity {
        &mut self.interactivity
    }
}