Bevy_save

A framework for saving and loading game state in Bevy.

preview-0.6.0.mp4

Features

Serialization and Deserialization

While Bevy's DynamicScene only allows you to save entities and components, bevy_save enables you to save everything, including resources.

• World::serialize<S>() and World::deserialize<D>() allow you to manually serialize and deserialize game state with your own serializer / deserializer.

Save file management

bevy_save automatically uses your app's workspace name to create a unique, permanent save location in the correct place for whatever platform it is running on.

• World::save() and World::load() uses your app's save location to save and load your game state, handling all serialization and deserialization for you.
• The AppSaver and AppLoader resources determine what save format is used.
  • By default, this is set up to use rmp_serde for serialization and deserialization.
  • However, is extremely easy to switch to a custom save file format, see "examples/json.rs" for how you can do this.
• The AppBackend resource determines how and where to store save files.
  • The default FileIO backend saves each named snapshot to an individual file on the disk.
  • Many games have different requirements like saving to multiple directories, to a database, or to WebStorage.
  • You can override the backend by modifying the AppBackend resource with your own Backend implementation.

Save directory location

WORKSPACE is the name of your project's workspace (parent folder) name.

Windows	Linux/*BSD	MacOS
C:\Users\%USERNAME%\AppData\Local\WORKSPACE\saves	~/.local/share/WORKSPACE/saves	~/Library/Application Support/WORKSPACE/saves

Snapshots and Rollback

bevy_save is not just about save files, it is about total control over game state.

This crate introduces a few snapshot types which may be used directly:

• Snapshot is a serializable snapshot of all saveable resources, entities, and components.
• Rollback is a serializable snapshot of all saveable resources, entities, and components that are included in rollbacks.

Or via the World extension methods:

• World::snapshot() captures a snapshot of the current game state, including resources. (equivalent to Snapshot::from_world())
• World::checkpoint() captures a snapshot for later rollback / rollforward.
• World::rollback() rolls the game state backwards or forwards through any checkpoints you have created.

The Rollbacks resource also gives you fine-tuned control of the currently stored rollbacks.

Type registration

bevy_save adds methods to Bevy's App for registering types that should be saved. As long as the type implements Reflect, it can be registered and used with bevy_save. Types that are not explicitly registered in the SaveableRegistry are not included in save/load.

• App.register_saveable::<T>() registers a type as saveable, allowing it to be included in saves and rollbacks.
• App.ignore_rollback::<T>() excludes a type from rollback.
• App.allow_rollback::<T>() allows you to re-include a type in rollback after it has already been set to ignore rollback.

Type filtering

While types that are not registered with SaveableRegistry are automatically filtered out for you, bevy_save also allows you to explicitly filter types when creating a snapshot.

• Snapshot::from_world_with_filter()
• Rollback::from_world_with_filter()

Entity mapping

As Entity ids are not intended to be used as unique identifiers, bevy_save supports mapping Entity ids.

First, you'll need to get a SnapshotApplier:

• World::deserialize_applier() while manually deserializing a snapshot.
• World::load_applier() while loading from a named save.
• World::rollback_applier() while rolling back / forward.

Or directly on the snapshot types:

• Snapshot::applier()
• Rollback::applier()

The SnapshotApplier will then allow you to configure the EntityMap (and other settings) before applying:

let snapshot = Snapshot::from_world(world);

snapshot
    .applier(world)

    // Your entity map - in many cases this can be omitted
    .map(EntityMap::default())

    // Despawn all entities matching (With<A>, Without<B>)
    .despawn(DespawnMode::all_with::<(With<A>, Without<B>)>())

    // Do not overwrite existing entities
    .mapping(MappingMode::Strict)

    .apply();

By default, bevy_save snapshots do not behave like Bevy's DynamicScene when applying.

If you use the methods that do not return an Applier (deserialize, load, rollback, apply), the default settings are used:

• DespawnMode::Missing - Any entities not present in the snapshot are despawned.
• MappingMode::Simple - Existing entities may be overridden with snapshot data.

You can change the default behavior using the AppDespawnMode and AppMappingMode resources.

It is also possible to match DynamicScene behavior by using DespawnMode::None and MappingMode::Strict.

MapEntities

bevy_save also supports MapEntities via reflection to allow you to update entity ids within components and resources.

See Bevy's Parent Component for a simple example.

Entity hooks

You are also able to add hooks when applying snapshots, similar to bevy-scene-hook.

This can be used for many things, like spawning the snapshot as a child of an entity:

let snapshot = Snapshot::from_world(world);

snapshot
    .applier(world)

    // This will be run for every Entity in the snapshot
    // It runs after the Entity's Components are loaded
    .hook(move |entity, cmds| {
        // You can use the hook to add, get, or remove Components
        if !entity.contains::<Parent>() {
            cmds.set_parent(parent);
        }
    })

    .apply();

Hooks may also despawn entities:

let snapshot = Snapshot::from_world(world);

snapshot
    .applier(world)
    
    .hook(|entity, cmds| {
        if entity.contains::<A>() {
            cmds.despawn();
        }
    })

Partial Snapshots

While bevy_save aims to make it as easy as possible to save your entire world, some games also need to be able to save only parts of the world.

Builder allows you to manually create snapshots like DynamicSceneBuilder:

fn build_snapshot(world: &World, target: Entity, children: Query<&Children>) -> Snapshot {
    Snapshot::builder(world)
        // Extract all saveable resources
        .extract_all_resources()

        // Extract all descendants of `target`
        // This will include all saveable components
        .extract_entities(children.iter_descendants(target))

        // Entities without any saveable components will also be extracted
        // You can use `clear_empty` to remove them
        // NOTE: If applied with the default `MappingMode` this may cause your `Window` entity to be despawned
        // .clear_empty()

        // Build the `Snapshot`
        .build()
}

You are also able to extract resources by type name:

Snapshot::builder(world)
    // Extract the resource by the type name
    // In this case, we extract the resource from the `manual` example
    .extract_resource("manual::FancyMap")

    // Build the `Snapshot`
    // It will only contain the one resource we extracted
    .build()

Additionally, explicit type filtering like Applier is available when building snapshots:

Snapshot::builder(world)
    // Exclude `Transform` from this `Snapshot`
    .filter(|reg| reg.type_name() != "bevy_transform::components::transform::Transform")

    // Extract all matching entities and resources
    .extract_all()
    
    // Clear all extracted entities without any components
    .clear_empty()
    
    // Build the `Snapshot`
    .build()

License

bevy_save is dual-licensed under MIT and Apache-2.0.

Compatibility

Bevy

NOTE: We do not track Bevy main.

Bevy Version	Crate Version
0.11	0.9
0.10	0.4, 0.5, 0.6, 0.7, 0.8
0.9	0.1, 0.2, 0.3

Platforms

Platform	Support
Windows	✔️
MacOS	✔️
Linux	✔️
WASM	🛠️†
Android	❓
iOS	❓

✔️ = First Class Support — 🆗 = Best Effort Support — ⚡ = Untested, but should work — ❓ = Untested, probably won't work — 🛠️ = In progress

† Everything but World::save and World::load should work, full support is possible now via a custom backend

Third-party Crates

bevy_save should work with most third-party crates, but you must register their types as saveable to be included in saves.

Registering as saveable requires the type implements Reflect. Components will also need to implement ReflectComponent and Resources will need to implement ReflectResource.

If a type stores Entity values, it must also have a MapEntities implementation and ReflectMapEntities registration to handle entity remapping properly.

Automatic registration for certain crates may be available via a feature flag. Only some types from those crates will be registered.

Registering a type again after it has already been registered will have no effect.

Name	Support	Feature Flag	Example	Notes
bevy	✔️	✅	✅
bevy_ecs_tilemap	🆗	✅	✅	No MapEntities support
bevy_rapier	⚡	🛠️	🛠️
bevy_tweening	❓	🛠️	🛠️
leafwing-input-manager	⚡	🛠️	🛠️

✔️ = First Class Support — 🆗 = Best Effort Support — ⚡ = Untested, but should work — ❓ = Untested, probably won't work — 🛠️ = In progress