Bevy Notify

A reactive(ish) system for the bevy game engine using relationships.

Monitor are entities that watch for changes, additions, and removals of specified components with NotifyChanged<Component>, NotifyAdded<Component>, and NotifyRemoved<Component>. By default monitors watch for changes on all entities, but this can be restricted via, [Monitor], and [MonitorSelf].

use bevy_monitors::prelude::*;
use bevy::{prelude::*, ui_widgets::observe};

#[derive(Component)]
pub struct Health(pub u8);

let player = commands
    .spawn((
        Name::new("Player"),
        Health(100),
        MonitorSelf,
        observe(|mutation: On<Mutation<Health>>, health: Query<&Health>| -> Result<(), BevyError> {
            let current_health = health.get(mutation.entity)?;

            println!("My current health is {}", current_health);
        })
    ))
    .id();

commands.spawn((
    Name::new("Doctor"),
    Monitor(player),
    NotifyChanged::<Health>::default(),
    observe(
        |mutation: On<Mutation<Health>>,
        mut health: Query<&mut Health>|
        -> Result<(), BevyError> {
            let mut health = health.get_mut(mutation.mutated)?;

            if health.0 <= 20 {
                health.0 += 20;
            }

            Ok(())
         }
     ),
));

Future Work

Use many to many relationships once we have them so that multiple entities can be watched at once. For now the same effect can be achieved by using a custom relationship.

Also look into using ComponentId on the observer trigger to do fancy stuff like On<Mutation, Component>.