🌴 Moonshine Object

An extension to Bevy which provides an ergonomic interface for managing complex Entity hierarchies.

Entities are nice. Objects are better! 😎

Overview

This crate is designed to provide a wrapper for some commonly used operations when working with entities in Bevy.

It is often required for various systems to be able to traverse complex entity hierarchies. This is especially true for initialization code when various components need to reference various entities within a hierarchy of entities.

For example, consider a system which reacts to a flying bird by flapping its wings:

use bevy::prelude::*;

#[derive(Component)]
struct Bird;

#[derive(Component)]
struct Flying;

fn setup_bird(
    query: Query<Entity, (With<Bird>, Added<Flying>)>,
    children_query: Query<&Children>,
    name: Query<&Name>,
    mut commands: Commands
) {
    for entity in query.iter() {
        if let Ok(children) = children_query.get(entity) {
            for child in children.iter() {
                if let Ok(name) = name.get(*child) {
                    if name.as_str() == "Wings" {
                        if let Ok(wings) = children_query.get(*child) {
                            for wing in wings.iter() {
                                // TODO: Flap! Flap!
                            }
                        }
                    }
                }
            }
        }
    }
}

This code is intentionally verbose to show the hierarchy complexity.

This crate tries to make these situations more ergonomic by introducing Object<T>.

It behaves like an Entity or Instance<T> with some extra features:

use bevy::prelude::*;
use moonshine_object::prelude::*;

#[derive(Component)]
struct Bird;

#[derive(Component)]
struct Flying;

fn setup_bird(birds: Objects<Bird, Added<Flying>>, mut commands: Commands) {
    for bird in birds.iter() {
        if let Some(wings) = bird.find_by_path("./Wings") {
            for wing in wings.children() {
                // TODO: Flap! Flap!
            }
        }
    }
}

Features

Less boilerplate when dealing with complex entity hierarchies
Full type safety enforced through Kind semantics
No macros! No registration!

Usage

`Objects<T>`

Use Objects<T> as a system parameter to access all Object<T> instances.

This SystemParam is designed to be used like a Query:

use bevy::prelude::*;
use moonshine_object::prelude::*;

#[derive(Component)]
struct Bird;

fn update_birds(birds: Objects<Bird>) {
    for bird in birds.iter() {
        // ...
    }
}

Like a Query, you may also use a QueryFilter:

use bevy::prelude::*;
use moonshine_object::prelude::*;

#[derive(Component)]
struct Bird;

#[derive(Component)]
struct Flying;

fn update_flying_birds(birds: Objects<Bird, With<Flying>>) {
    for bird in birds.iter() {
        // ...
    }
}

Internally, Objects<T> is just a thin wrapper around some common queries:

Query<Instance<T>>
Query<&Parent> / Query<&Children>
Query<&Name>

`Object<T>`

Each Object<T> is a reference to an Entity with type, name, and hierarchy information. This provides a convenient way to pass this data between functions:

use bevy::prelude::*;
use moonshine_object::prelude::*;

#[derive(Component)]
struct Bird;

#[derive(Component)]
struct Flying;

fn update_flying_birds(birds: Objects<Bird, With<Flying>>) {
    for bird in birds.iter() {
        flap_wings(bird);
    }
}

fn flap_wings(bird: Object<Bird>) {
    if let Some(wings) = bird.find_by_path("./Wings") {
        for wing in wings.children() {
            // TODO: Flap! Flap!
        }
    }
}

⚠️ Unlike an Entity or Instance<T>, Object<T> has a non-static lifetime and may not be used as a Query term.

Casting

Like Instance<T>, any Object<T> may be be cast into an Object<U> if T implements CastInto<U>.

You may implement this trait for your own kinds using the safe_cast macro:

use bevy::prelude::*;
use moonshine_kind::prelude::*;
use moonshine_object::prelude::*;

#[derive(Component)]
struct Bird;

struct Creature;

// Every Bird is a Creature by definition:
impl Kind for Creature {
    type Filter = (With<Bird>, /* ... */);
}

// Therefore, all birds may safely be cast into creatures:
safe_cast!(Bird => Creature);

// Birds can chirp.
fn chirp(bird: Object<Bird>) {
    // TODO: Chirp!
}

// Creatures can find food.
fn find_food(creature: Object<Creature>) {
    // TODO: Find food!
}

// Birds chirp when they get hungry.
fn handle_hunger(bird: Object<Bird>) {
    chirp(bird);
    find_food(bird.cast_into()); // Safe! :)
}

Any Object<T> is safely convertible to Object<Any>.

Support

Please post an issue for any bugs, questions, or suggestions.

You may also contact me on the official Bevy Discord server as @Zeenobit.

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