egui retained widget trees

retained.rs

use bevy::prelude::*;
use bevy_egui::{egui, EguiContexts};
use bevy_retained_egui::{
    event::{Clicked, Draw, DrawRoot},
    widget::{CentralPanel, Label},
    RetainedEguiPlugin,
};

fn main() {
    let mut app = App::new();

    app.add_plugins(DefaultPlugins)
        .add_plugins(RetainedEguiPlugin)
        .add_systems(Startup, spawn)
        .add_systems(Update, draw)
        .observe(CounterButton::on_draw)
        .observe(CounterButton::on_click);

    app.run();
}

pub fn spawn(mut commands: Commands) {
    let root = commands
        .spawn(CentralPanel::default())
        .with_children(|parent| {
            parent.spawn(Label::heading(
                "This ui was built using a retained egui widget tree!",
            ));
            parent.spawn(CounterButton { count: 0 });
        })
        .id();
    commands.insert_resource(Root(root));
}

#[derive(Resource)]
pub struct Root(pub Entity);

pub fn draw(mut commands: Commands, root: Res<Root>, mut ctxs: EguiContexts) {
    let Some(ctx) = ctxs.try_ctx_mut() else {
        return;
    };

    commands.trigger_targets(DrawRoot { ctx: ctx.clone() }, root.0);
}

#[derive(Component)]
pub struct CounterButton {
    pub count: i32,
}

impl CounterButton {
    pub fn on_draw(
        mut trigger: Trigger<Draw>,
        mut commands: Commands,
        buttons: Query<&CounterButton>,
    ) {
        let Ok(button) = buttons.get(trigger.entity()) else {
            return;
        };

        let resp = trigger
            .event_mut()
            .ui
            .add(egui::Button::new(format!("Count: {}", button.count)));

        if resp.clicked() {
            commands.trigger_targets(Clicked, trigger.entity());
        }
    }

    pub fn on_click(trigger: Trigger<Clicked>, mut buttons: Query<&mut CounterButton>) {
        let Ok(mut button) = buttons.get_mut(trigger.entity()) else {
            return;
        };

        button.count += 1;
    }
}