🍎 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
• No runtime overhead
• Zero boilerplate

This crate may be used separately, but is also included as part of 🍸 Moonshine Core.

Usage

[Kind] and [Instance<T>]

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

Any [Component] automatically implements the [Kind] trait:

```rust,ignore impl Kind for T { type Filter = With; }

An [`Instance<T>`] represents `Entity` of kind `T`. It is designed to behave exactly like an `Entity` with some added benefits.

This means you may use any component as an argument to `Instance`:

```rust
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<T>] and [InstanceMut<T>]

If a [Kind] is also a [Component], you may use [InstanceRef<T>] and [InstanceMut<T>] to access the [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
}

In other words, InstanceRef<T> is analogous to (Instance<T>, &T) and InstanceMut<T> is analogous to (Instance<T>, &mut T).

[InstanceCommands<T>]

You may also extend [InstanceCommands<T>] to define [Commands] specific to a [Kind]:

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().unwrap();
    if let Some(fruit) = fruits.iter().next() {
        commands.instance(human).eat(fruit);
    }
}

InstanceCommands<T> behaves like [EntityCommands], and is accessible via [commands.instance(...)][GetInstanceCommands<T>].

[Instance<Any>][Instance<T>]

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 and semantically identical to a regular [Entity].

[CastInto]

By definition, any [Instance<T>] is safely convertible to any [Instance<U>][Instance<T>] if [CastInto<U>][CastInto] is implemented for T.

This is done using the [CastInto] trait:

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:
impl CastInto<Fruit> for Apple {}

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

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

Required Components are a great way to enforce this type of "kind polymorphism" at runtime:

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

#[derive(Component, Default)]
struct Apple;

#[derive(Component)]
#[require(Apple)] // Require all GrannySmith instances to also have Apple
struct GrannySmith;

impl CastInto<Apple> for GrannySmith {} // GrannySmith is an Apple; Guaranteed!

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.