Bevy Notify

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

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

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"),
    Monitoring(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(())
         },
     ),
));

Documentation

Docs aren't great atm, will improve in time but for the time being just make an issue whenever you run into something.

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>.