sx/library/modules/ui/scroll_view.sx

#import "modules/std.sx";
#import "modules/math";
#import "modules/ui/types.sx";
#import "modules/ui/render.sx";
#import "modules/ui/events.sx";
#import "modules/ui/view.sx";

ScrollAxes :: enum { vertical; horizontal; both; }

// Persistent scroll state — lives outside the frame arena
ScrollState :: struct {
    offset: Point;
    content_size: Size;
    viewport_size: Size;
    dragging: bool;
    drag_pending: bool;
    drag_start: Point;
    drag_offset: Point;
}

ScrollView :: struct {
    child: ViewChild;
    state: *ScrollState;
    axes: ScrollAxes;

    SCROLL_SPEED  :f32: 20.0;
    DRAG_THRESHOLD :f32: 4.0;

    clamp_offset :: (self: *ScrollView) {
        s := self.state;
        max_x := max(0.0, s.content_size.width - s.viewport_size.width);
        max_y := max(0.0, s.content_size.height - s.viewport_size.height);

        if self.axes == .vertical or self.axes == .both {
            s.offset.y = clamp(s.offset.y, 0.0, max_y);
        } else {
            s.offset.y = 0.0;
        }

        if self.axes == .horizontal or self.axes == .both {
            s.offset.x = clamp(s.offset.x, 0.0, max_x);
        } else {
            s.offset.x = 0.0;
        }
    }
}

impl View for ScrollView {
    size_that_fits :: (self: *ScrollView, proposal: ProposedSize) -> Size {
        // ScrollView takes all proposed space (default 200 if unspecified)
        Size.{
            width = proposal.width ?? 200.0,
            height = proposal.height ?? 200.0
        }
    }

    layout :: (self: *ScrollView, bounds: Frame) {
        s := self.state;
        s.viewport_size = bounds.size;

        // Measure child with infinite space on scroll axes
        child_proposal := ProposedSize.{
            width = if self.axes == .horizontal or self.axes == .both then null else bounds.size.width,
            height = if self.axes == .vertical or self.axes == .both then null else bounds.size.height
        };
        s.content_size = self.child.view.size_that_fits(child_proposal);
        self.clamp_offset();

        // Layout child offset by scroll position
        self.child.computed_frame = Frame.make(
            bounds.origin.x - s.offset.x,
            bounds.origin.y - s.offset.y,
            s.content_size.width,
            s.content_size.height
        );
        self.child.view.layout(self.child.computed_frame);
    }

    render :: (self: *ScrollView, ctx: *RenderContext, frame: Frame) {
        ctx.push_clip(frame);
        self.child.view.render(ctx, self.child.computed_frame);
        ctx.pop_clip();
    }

    handle_event :: (self: *ScrollView, event: *Event, frame: Frame) -> bool {
        s := self.state;

        if pos := event_position(event) {
            if !frame.contains(pos) { return false; }
        }

        if event.* == {
            case .mouse_wheel: (d) {
                if self.axes == .vertical or self.axes == .both {
                    s.offset.y -= d.delta.y * ScrollView.SCROLL_SPEED;
                }
                if self.axes == .horizontal or self.axes == .both {
                    s.offset.x -= d.delta.x * ScrollView.SCROLL_SPEED;
                }
                self.clamp_offset();
                return true;
            }
            case .mouse_down: (d) {
                s.drag_pending = true;
                s.drag_start = d.position;
                s.drag_offset = s.offset;
                self.child.view.handle_event(event, self.child.computed_frame);
                return true;
            }
            case .mouse_moved: (d) {
                if s.drag_pending and !s.dragging {
                    dx := d.position.x - s.drag_start.x;
                    dy := d.position.y - s.drag_start.y;
                    dist := sqrt(dx * dx + dy * dy);
                    if dist >= ScrollView.DRAG_THRESHOLD {
                        s.dragging = true;
                        s.drag_pending = false;
                        cancel :Event = .mouse_up(.{
                            position = .{ x = 0.0 - 10000.0, y = 0.0 - 10000.0 },
                            button = .none
                        });
                        self.child.view.handle_event(@cancel, self.child.computed_frame);
                    }
                }
                if s.dragging {
                    if self.axes == .vertical or self.axes == .both {
                        s.offset.y = s.drag_offset.y - (d.position.y - s.drag_start.y);
                    }
                    if self.axes == .horizontal or self.axes == .both {
                        s.offset.x = s.drag_offset.x - (d.position.x - s.drag_start.x);
                    }
                    self.clamp_offset();
                    return true;
                }
                return self.child.view.handle_event(event, self.child.computed_frame);
            }
            case .mouse_up: {
                was_dragging := s.dragging;
                s.dragging = false;
                s.drag_pending = false;
                if was_dragging {
                    return true;
                }
                return self.child.view.handle_event(event, self.child.computed_frame);
            }
        }
        self.child.view.handle_event(event, self.child.computed_frame)
    }
}