🍎 Moonshine Kind

Simple type safety solution for Bevy.

Overview

An Entity is a generic way to reference entities within Bevy:

use bevy::prelude::*;

#[derive(Component)]
struct FruitBasket {
    fruits: Vec<Entity>
}

A problem with using entities in this way is the lack of information about the "kind" of the entity. This results in code that is error prone, hard to debug, and read.

This crate attempts to solve this problem by introducing a new Instance<T> type which behaves like an Entity but also contains information about the "kind" of the entity:

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

#[derive(Component)]
struct Fruit;

#[derive(Component)]
struct FruitBasket {
    fruits: Vec<Instance<Fruit>>
}

Features

• Improved type safety and readability for Bevy code
• Ability to define custom entity kinds
• Ability to define commands for specific entity kinds
• Zero or minimal boilerplate

Usage

Kind and Instance

By definition, an Entity is of Kind T if it matches Query<(), <T as Kind>::Filter>.

Any Component automatically implements the Kind trait:

impl<T: Component> Kind for T {
    type Filter = With<T>;
}

This means you may use any Component as an argument to Instance<T>:

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

#[derive(Component)]
struct Apple;

fn count_apples(apples: Query<Instance<Apple>>) {
    println!("Apples: {}", apples.iter().count());
}

Alternatively, you may also define your own kind by implementing the Kind trait:

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

#[derive(Component)]
struct Apple;

#[derive(Component)]
struct Orange;

struct Fruit;

impl Kind for Fruit {
    type Filter = Or<(With<Apple>, With<Orange>)>;
}

fn count_fruits(fruits: Query<Instance<Fruit>>) {
    println!("Fruits: {}", fruits.iter().count());
}

InstanceRef and InstanceMut

If a Kind is also a Component, you may use InstanceRef<T> and InstanceMut<T> to access Instance<T> and the associated component data with a single query term:

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

#[derive(Component)]
struct Apple {
    freshness: f32
}

impl Apple {
    fn is_fresh(&self) -> bool {
        self.freshness >= 1.0
    }
}

fn fresh_apples(
    apples: Query<InstanceRef<Apple>>
) -> Vec<Instance<Apple>> {
    let mut fresh_apples = Vec::new();
    for apple in apples.iter() {
        if apple.is_fresh() {
            fresh_apples.push(apple.instance());
        }
    }
    fresh_apples
}

InstanceCommands

You may also extend InstanceCommands<T> to define Commands specific to a Kind.

InstanceCommands<T> behaves like EntityCommands, and is accessible via commands.instance(...) (see GetInstanceCommands<T> for details):

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

struct Fruit;

impl Kind for Fruit {
    type Filter = (/* ... */);
}

#[derive(Component)]
struct Human;

trait Eat {
    fn eat(&mut self, fruit: Instance<Fruit>);
}

// Humans can eat:
impl Eat for InstanceCommands<'_, Human> {
    fn eat(&mut self, fruit: Instance<Fruit>) {
        // ...
    }
}

fn eat(
    human: Query<Instance<Human>>,
    fruits: Query<Instance<Fruit>>, mut commands: Commands
) {
    let human = human.single();
    if let Some(fruit) = fruits.iter().next() {
        commands.instance(human).eat(fruit);
    }
}

Instance<Any>

When writing generic code, it may be desirable to have an instance that can be of Any kind:

use moonshine_kind::{prelude::*, Any};

struct Container<T: Kind = Any> {
    items: Vec<Instance<T>>
}

Instance<Any> is functionally equivalent to Entity.

Casting

An Instance<T> is safely convertible to an Instance<U> if CastInto<U> is implemented for T.

This is done by using the .cast_into() method:

You may use the kind macro to implement this trait for given kind pair:

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

#[derive(Component)]
struct Apple;

struct Fruit;

impl Kind for Fruit {
    type Filter = With<Apple>;
}

// An Apple is a Fruit because we said so:
kind!(Apple is Fruit);

fn init_apple(apple: Instance<Apple>, commands: &mut Commands) {
    init_fruit(apple.cast_into(), commands);
    // ...
}

fn init_fruit(fruit: Instance<Fruit>, commands: &mut Commands) {
    // ...
}

Examples

See examples/fruits.rs for a complete example.

Limitations

Instance Invalidation

This crate does not monitor instances for invalidation.

This means that if an entity is modified in such a way that it no longer matches some Kind T (such as removing Component T), any Instance<T> which references it would be invalid.

It is recommended to avoid using kind semantics for components that may be removed at runtime without despawning their associated entity.

However, if necessary, you may check instances for validity prior to usage:

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

struct Fruit;

impl Kind for Fruit {
    type Filter = (/* ... */);
}

fn prune_fruits(
    mut fruits: Vec<Instance<Fruit>>,
    query: &Query<(), <Fruit as Kind>::Filter>
) -> Vec<Instance<Fruit>> {
    fruits.retain(|fruit| {
        // Is the Fruit still a Fruit?
        query.get(fruit.entity()).is_ok()
    });
    fruits
}

Support

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

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