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//! The display list pass.
//!
//! The pass flattens the tree into one list of drawing items in paint order,
//! so a renderer replays one buffer instead of walking the tree. Every node
//! stores its operations in its own paint coordinates, so the structure the
//! list needs is the clip that a clipping node puts around its children and
//! the offset scope that a node with a scroll offset puts around the children
//! its scroll moves: a renderer replays the items translated by the open
//! offsets, which lands the operations back in the window without the tree
//! repainting anything that only scrolled.
use crate::geometry::Point;
use crate::paint::DisplayList;
use crate::style::Position;
use super::{WidgetId, WidgetTree};
impl WidgetTree {
/// Rebuilds `list` from the tree in paint order.
///
/// A node contributes its cached operations, then its children in order.
/// The children of a node that clips are wrapped in a
/// [`PushClip`](crate::paint::DisplayItem::PushClip) carrying the area
/// inside the node's border, in the node's paint coordinates, and a
/// matching [`PopClip`](crate::paint::DisplayItem::PopClip), so a child
/// that overflows its parent is cut off by the renderer before it reaches
/// the border. The children a node's scroll offset moves are wrapped in a
/// [`PushOffset`](crate::paint::DisplayItem::PushOffset) scope of minus
/// the offset, so a renderer draws them moved by the scroll without the
/// operations being repainted. A hidden node and its subtree contribute
/// nothing.
///
/// The list is cleared first, which keeps its allocated capacity, and
/// [`DisplayList::finish`] runs at the end to check that the clips and
/// offsets balance.
pub(crate) fn build_display_list(&self, list: &mut DisplayList) {
list.clear();
if let Some(root) = self.root {
self.emit(root, list);
}
list.finish();
}
/// Appends `id` and its subtree to `list` in paint order.
fn emit(&self, id: WidgetId, list: &mut DisplayList) {
let Some(style) = self.style(id) else {
return;
};
if style.layout.hidden {
return;
}
let clips = style.layout.overflow.clips();
let node = self.nodes.get(id).expect("the id is a live node");
list.extend_ops(&node.ops);
if clips {
list.push_clip(self.inside_border(id));
}
// The runs of children the scroll offset moves share one offset
// scope, and an absolutely positioned child between them is emitted
// outside it, because the scroll does not move it.
let scroll = node.scroll_offset;
let offset = Point {
x: -scroll.x,
y: -scroll.y,
};
let mut offset_open = false;
for &child in &node.children {
let Some(child_style) = self.style(child) else {
continue;
};
if child_style.layout.hidden {
continue;
}
let flowing = child_style.layout.position == Position::Relative;
if offset != Point::default() {
if flowing && !offset_open {
list.push_offset(offset);
offset_open = true;
} else if !flowing && offset_open {
list.pop_offset();
offset_open = false;
}
}
self.emit(child, list);
}
if offset_open {
list.pop_offset();
}
if clips {
list.pop_clip();
}
}
}