🥚 Moonshine Spawn

Collection of tools for spawning entities in Bevy.

Overview

In Bevy, complex hierarchies of entities are typically spawned using the ChildBuilder:

use bevy::prelude::*;

fn spawn_chicken(commands: &mut Commands) -> Entity {
    // Spawn logic is spread between this function and the bundle
    commands.spawn(ChickenBundle { /* Components */ }).with_children(|chicken| {
        // Children
    })
    .id()
}

#[derive(Bundle)]
struct ChickenBundle {
    // ...
}

While this pattern works for most cases, it tends to spread out the logic of entity spawning between the bundle and the function which builds the entity hierarchy. Arguably, this makes the code less modular and harder to read and maintain.

Additionally, there is no built-in functionality to reference a predefined entity hierarchy (i.e. a "prefab").

This crate aims to solve some of these issues by providing tools to make spawning more ergonomic:

use bevy::prelude::*;
use moonshine_spawn::prelude::*;

let mut app = App::new();
// Make sure `SpawnPlugin` is added to your `App`:
app.add_plugins((DefaultPlugins, SpawnPlugin));

// Register spawnables during initialization:
let chicken_key: SpawnKey = app.add_spawnable("chicken", Chicken);

// Spawn a spawnable with a key:
let chicken = app.world_mut().spawn_key(chicken_key); // .spawn_key("chicken") also works!

#[derive(Component)]
struct Chicken;

impl Spawn for Chicken {
    type Output = (Chicken, SpawnChildren);

    // Spawn logic is now unified:
    fn spawn(&self, world: &World, entity: Entity) -> Self::Output {
        // Components
        (Chicken,
        spawn_children(|chicken| {
            // Children
        }))
    }
}

Usage

Spawnables

A type is a "spawnable" if it implements either Spawn or SpawnOnce:

trait Spawn {
    type Output;

    fn spawn(&self, world: &World, entity: Entity) -> Self::Output;
}

trait SpawnOnce {
    type Output;

    fn spawn_once(self, world: &World, entity: Entity) -> Self::Output;
}

SpawnOnce is implemented by default for all Bevy bundles.

Spawn is implemented for all types which implement SpawnOnce + Clone. This means any Bundle + Clone implements Spawn.

The output of a spawn is always a bundle which is then inserted into the given entity at the end of spawn process.

You may use these traits to define functional spawnables:

use bevy::prelude::*;
use moonshine_spawn::prelude::*;

#[derive(Resource)]
struct DefaultChickenName(Name);

struct Egg;

impl SpawnOnce for Egg {
    type Output = ChickenBundle;

    fn spawn_once(self, world: &World, entity: Entity) -> Self::Output {
        let DefaultChickenName(name) = world.resource::<DefaultChickenName>();
        ChickenBundle::new(name.clone())
    }
}

#[derive(Bundle)]
struct ChickenBundle {
    chicken: Chicken,
    name: Name,
}

impl ChickenBundle {
    fn new(name: Name) -> Self {
        Self {
            chicken: Chicken,
            name,
        }
    }

}

#[derive(Component)]
struct Chicken;

fn open_egg(egg: Egg, commands: &mut Commands) -> Entity {
    commands.spawn_once_with(egg).id()
}

Bundles + Children

To spawn bundles with children, use the WithChildren trait:

use bevy::prelude::*;
use moonshine_spawn::prelude::*;

#[derive(Component)]
struct Chicken;

fn chicken() -> impl SpawnOnce {
    Chicken.with_children(|chicken| {
        // ...
    })
}

Or use the SpawnChildren component and the spawn_children function:

use bevy::prelude::*;
use moonshine_spawn::prelude::*;

#[derive(Bundle)]
struct ChickenBundle {
    chicken: Chicken,
    children: SpawnChildren,
}

#[derive(Component)]
struct Chicken;

fn chicken() -> impl SpawnOnce {
    ChickenBundle {
        chicken: Chicken,
        children: spawn_children(|chicken| {
            // ...
        })
    }
}

Spawn Keys

A SpawnKey is a reference to a registered spawnable.

Each key must be unique within the scope of a World and is registered using the AddSpawnable extension trait.

Use this to register your spawnables during app initialization:

app.add_spawnable("chicken", chicken());

You can then spawn a spawnable using a spawn key at runtime, either using Commands or a &mut World:

commands.spawn_with_key("chicken");

You may also use spawn keys when spawning children of a bundle:

fn chicken() -> impl SpawnOnce {
    ChickenBundle {
        chicken: Chicken,
        children: spawn_children(|chicken| {
            chicken.spawn_with_key("chicken_head");
        })
    }
}

force_spawn_children

This crate works by running a system which invokes any SpawnChildren Component during the First schedule.

Sometimes it may be necessary to spawn children manually before the First schedule runs due to system dependencies.

In such cases, you may use force_spawn_children to manually invoke these components:

use bevy::prelude::*;
use moonshine_spawn::{prelude::*, force_spawn_children};

let mut app = App::new();

// This system spawns a chicken during setup:
fn spawn_chicken(mut commands: Commands) {
    commands.spawn_once_with(chicken());
}

// This system depends on children of `Chicken`:
fn update_chicken_head(query: Query<(Entity, &Chicken, &Children)>, mut head: Query<&mut ChickenHead>) {
    for (entity, chicken, children) in query.iter() {
        if let Ok(mut head) = head.get_mut(children[0]) {
            // ...
        }
    }
}

#[derive(Component)]
struct Chicken;

fn chicken() -> impl SpawnOnce {
    Chicken.with_children(|chicken| {
        chicken.spawn(ChickenHead);
    })
}

#[derive(Component)]
struct ChickenHead;

let mut app = App::new();

app.add_plugins((DefaultPlugins, SpawnPlugin))
    .add_systems(Startup, spawn_chicken)
    // Without `force_spawn_children`, chicken head would only be updated on the second update cycle after spawn.
    // With `force_spawn_children`, chicken head would be updated in the same update cycle.
    .add_systems(Startup, force_spawn_children())
    .add_systems(Update, update_chicken_head);

Support

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

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