egui/emigui/src/containers/scroll_area.rs

use crate::*;

#[derive(Clone, Copy, Debug, Default)]
pub struct State {
    /// Positive offset means scrolling down/right
    pub offset: Vec2,
}

pub struct ScrollArea {
    max_height: f32,
}

impl Default for ScrollArea {
    fn default() -> Self {
        Self { max_height: 200.0 }
    }
}

impl ScrollArea {
    pub fn max_height(mut self, max_height: f32) -> Self {
        self.max_height = max_height;
        self
    }
}

impl ScrollArea {
    pub fn show(self, outer_region: &mut Region, add_contents: impl FnOnce(&mut Region)) {
        let ctx = outer_region.ctx().clone();

        let scroll_area_id = outer_region.id.with("scroll_area");
        let mut state = ctx
            .memory
            .lock()
            .scroll_areas
            .get(&scroll_area_id)
            .cloned()
            .unwrap_or_default();

        // content: size of contents (generally large)
        // outer: size of scroll area including scroll bar(s)
        // inner: excluding scroll bar(s). The area we clip the contents to.

        let scroll_bar_width = 16.0;

        let outer_size = vec2(outer_region.available_width(), self.max_height);
        let outer_rect = Rect::from_min_size(outer_region.cursor, outer_size);

        let inner_size = outer_size - vec2(scroll_bar_width, 0.0);
        let inner_rect = Rect::from_min_size(outer_region.cursor, inner_size);

        let mut content_region =
            outer_region.child_region(Rect::from_min_size(outer_region.cursor(), inner_size));
        content_region.cursor -= state.offset;
        content_region.desired_rect = content_region.desired_rect.translate(-state.offset);
        add_contents(&mut content_region);
        let content_size = content_region.bounding_size();

        let content_interact = ctx.interact(
            outer_region.layer,
            &inner_rect,
            Some(scroll_area_id.with("area")),
        );
        if content_interact.active {
            // Dragging scroll area to scroll:
            state.offset.y -= ctx.input.mouse_move.y;
        }

        // TODO: check that nothing else is being inteacted with
        if ctx.contains_mouse_pos(outer_region.layer, &outer_rect)
            && ctx.memory.lock().active_id.is_none()
        {
            state.offset.y -= ctx.input.scroll_delta.y;
        }

        let show_scroll = content_size.y > inner_size.y;
        if show_scroll {
            let left = inner_rect.right() + 2.0;
            let right = outer_rect.right();
            let corner_radius = (right - left) / 2.0;
            let top = inner_rect.top();
            let bottom = inner_rect.bottom();

            let outer_scroll_rect = Rect::from_min_max(
                pos2(left, inner_rect.top()),
                pos2(right, inner_rect.bottom()),
            );

            let from_content =
                |content_y| remap_clamp(content_y, 0.0..=content_size.y, top..=bottom);

            let handle_rect = Rect::from_min_max(
                pos2(left, from_content(state.offset.y)),
                pos2(right, from_content(state.offset.y + inner_rect.height())),
            );

            // intentionally use same id for inside and outside of handle
            let interact_id = Some(scroll_area_id.with("vertical"));
            let handle_interact = ctx.interact(outer_region.layer, &handle_rect, interact_id);

            if let Some(mouse_pos) = ctx.input.mouse_pos {
                if handle_interact.active {
                    if inner_rect.top() <= mouse_pos.y && mouse_pos.y <= inner_rect.bottom() {
                        state.offset.y +=
                            ctx.input.mouse_move.y * content_size.y / inner_rect.height();
                    }
                } else {
                    // Check for mouse down outside handle:
                    let scroll_bg_interact =
                        ctx.interact(outer_region.layer, &outer_scroll_rect, interact_id);

                    if scroll_bg_interact.active {
                        // Center scroll at mouse pos:
                        let mpos_top = mouse_pos.y - handle_rect.height() / 2.0;
                        state.offset.y = remap(mpos_top, top..=bottom, 0.0..=content_size.y);
                    }
                }
            }

            state.offset.y = state.offset.y.max(0.0);
            state.offset.y = state.offset.y.min(content_size.y - inner_rect.height());

            // Avoid frame-delay by calculating a new handle rect:
            let handle_rect = Rect::from_min_max(
                pos2(left, from_content(state.offset.y)),
                pos2(right, from_content(state.offset.y + inner_rect.height())),
            );

            let style = outer_region.style();
            let handle_fill_color = style.interact_fill_color(&handle_interact);
            let handle_outline = style.interact_outline(&handle_interact);

            outer_region.add_paint_cmd(PaintCmd::Rect {
                rect: outer_scroll_rect,
                corner_radius,
                fill_color: Some(color::BLACK),
                outline: None,
            });

            outer_region.add_paint_cmd(PaintCmd::Rect {
                rect: handle_rect.expand(-2.0),
                corner_radius,
                fill_color: handle_fill_color,
                outline: handle_outline,
            });
        }

        let size = content_size.min(content_region.clip_rect.size());
        outer_region.reserve_space_without_padding(size);

        state.offset.y = state.offset.y.max(0.0);
        state.offset.y = state.offset.y.min(content_size.y - inner_rect.height());

        outer_region
            .ctx()
            .memory
            .lock()
            .scroll_areas
            .insert(scroll_area_id, state);
    }
}