bevy/crates/bevy_ui/src/ui_update_system.rs

use super::Node;
use crate::Rect;
use bevy_core::transform::run_on_hierarchy_subworld_mut;
use bevy_transform::prelude::{Children, Parent};
use bevy_window::Windows;
use glam::Vec2;
use legion::{prelude::*, systems::SubWorld};

pub fn ui_update_system() -> Box<dyn Schedulable> {
    SystemBuilder::new("ui_update")
        .read_resource::<Windows>()
        .with_query(<Read<Node>>::query().filter(!component::<Parent>()))
        .write_component::<Node>()
        .write_component::<Rect>()
        .read_component::<Children>()
        .build(move |_, world, windows, node_query| {
            if let Some(window) = windows.get_primary() {
                let parent_size = glam::vec2(window.width as f32, window.height as f32);
                let parent_position = glam::vec2(0.0, 0.0);
                for entity in node_query
                    .iter_entities(world)
                    .map(|(e, _)| e)
                    .collect::<Vec<Entity>>()
                {
                    run_on_hierarchy_subworld_mut(
                        world,
                        entity,
                        (parent_size, parent_position, 0.9999),
                        &mut update_node_entity,
                    );
                }
            }
        })
}

fn update_node_entity(
    world: &mut SubWorld,
    entity: Entity,
    parent_properties: (Vec2, Vec2, f32),
) -> Option<(Vec2, Vec2, f32)> {
    let (parent_size, parent_position, z_index) = parent_properties;
    // TODO: Somehow remove this unsafe
    unsafe {
        if let Some(mut node) = world.get_component_mut_unchecked::<Node>(entity) {
            if let Some(mut rect) = world.get_component_mut_unchecked::<Rect>(entity) {
                node.update(&mut rect, parent_size, parent_position, z_index);
                return Some((rect.size, rect.position, z_index - 0.0001));
            }
        }
    }

    None
}