💾 Moonshine Save

A save/load framework for Bevy game engine.

Overview

In Bevy, it is possible to serialize and deserialize a [World] using a [DynamicScene] (see example for details). While this is useful for scene management and editing, it has some problems when using it as-is for saving or loading game state.

The main issue is that in most common applications, the saved game data is a very minimal subset of the actual scene. Visual and aesthetic elements such as transforms, scene hierarchy, camera, or UI components are typically added to the scene during game start or entity initialization.

This crate aims to solve this issue by providing a framework and a collection of systems for selectively saving and loading a world to disk.

Usage

Save

In order to save game state, start by marking components which must be saved using the Save marker. This is a component which can be added to bundles or inserted into entities like any other component: ``rust,ignore // Saved components must deriveReflect`

[derive(Component, Default, Reflect)]

[reflect(Component)]

struct Player;

[derive(Component, Default, Reflect)]

[reflect(Component)]

struct Level(u32);

[derive(Bundle)]

struct PlayerBundle { player: Player, level: Level, name: Name, save: Save, } ```

⚠️ Warning:
Components which are to be saved must derive Reflect. Otherwise, they are not saved.

Add SavePlugin and register your serialized components:

rust,ignore app.add_plugin(SavePlugin) .register_type::<Player>() .register_type::<Level>();

Finally, to invoke the save process, you must add a save pipeline. The default save pipeline is save_into_file:

rust,ignore app.add_system(save_into_file("saved.ron"));

When used on its own, save_into_file would save the world state on every application update. This is often undesirable because you typically want save to happen at specific times. To do this, you can combine save_into_file with run_if:

```rust,ignore app.addsystem(saveintofile("saved.ron").runif(should_save));

fn should_save( /* ... */ ) -> bool { todo!() } ```

Load

Before loading, mark your visual and aesthetic entities with Unload. Similar to Save, this is a marker which can be added to bundles or inserted into entities like a regular component. Any entity with Unload is despawned recursively prior to load.

```rust,ignore

[derive(Bundle)]

struct PlayerSpriteBundle { /* ... */ unload: Unload, } ```

You should try to design your game logic to keep saved data separate from game visuals. This can be done by using systems which spawn visuals for saved game data:

```rust,ignore

[derive(Component)] // <-- Does not derive Reflect, not saved!

struct PlayerSprite(Entity);

[derive(Bundle)]

struct PlayerSpriteBundle { sprite: SpriteBundle, unload: Unload, }

impl PlayerSpriteBundle { fn new() -> Self { todo!("create sprite bundle") } }

fn spawnplayervisuals(query: Query>, mut commands: Commands) { for entity in query.iter() { let sprite = PlayerSprite(commands.spawn(PlayerSpriteBundle::new()).id()); commands.entity(entity).insert(sprite); } } ```

Any saved components which reference entities must implement FromLoaded and be invoked during post load using component_from_loaded:

```rust,ignore

[derive(Component, Default, Reflect)]

[reflect(Component)]

struct PlayerWeapon(Option);

impl FromLoaded for PlayerWeapon { fn fromloaded(old: Self, loaded: &Loaded) -> Self { Self(Option::fromloaded(loaded)) } }

...

app.addsystem(componentfrom_loaded::()); ```

Make sure LoadPlugin is added and your types are registered:

rust,ignore app.add_plugin(LoadPlugin) .register_type::<Player>() .register_type::<Level>();

Finally, to invoke the load process, you must add a load pipeline. The default load pipeline is load_from_file:

rust,ignore app.add_system(load_from_file("saved.ron"));

Similar to save_into_file, you typically want to use load_from_file with run_if:

```rust,ignore app.addsystem(loadfromfile("saved.ron").runif(should_load));

fn should_load( /* ... */ ) -> bool { todo!() } ```

Example

See examples/army.rs for a minimal application which demonstrates how to save/load game state in detail.

Configuration

This crate is designed to be modular and fully configurable. The default save/load pipelines (save_into_file and load_from_file) are composed of sub-systems which can be used individually in any desirable configuration with other systems. You may refer to their implementation for details on how this can be done.