Bevy_save

A framework for saving and loading application state in Bevy.

https://user-images.githubusercontent.com/29737477/234151375-4c561c53-a8f4-4bfe-a5e7-b69af883bf65.mp4

Features

Save file management

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

• World::save() and World::load() uses the managed save file location to save and load your application state, handling all serialization and deserialization for you.
• These methods accept a Pipeline, a strongly typed representation of how you are going to be saving and loading.
• The Pipeline trait uses the Backend trait as an interface between disk / database and bevy_save.
• The Backend trait uses the Format trait to determine what format should be used in the actual save files (MessagePack / RON / JSON / etc)
  • The default Pipeline uses the FileIO backend which saves each snapshot to an individual file on the disk by the given key.
    • Many applications have different requirements like saving to multiple directories, to a database, or to WebStorage.
    • You can use a different Backend by implementing your own Pipeline with a custom Backend.
  • The default Pipeline is set up to use rmp-serde as the file format.
    • You can use to a different Format by implementing your own Pipeline with a custom Format.

Save directory location

With the default FileIO backend, your save directory is managed for you.

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

On WASM, snapshots are saved to LocalStorage, with the key:

WORKSPACE.KEY

Snapshots and Rollback

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

This crate introduces a snapshot type which may be used directly:

• Snapshot is a serializable snapshot of all saveable resources, entities, and components.

Or via the World extension methods WorldSaveableExt and WorldRollbackExt:

• World::snapshot() captures a snapshot of the current application state, including resources.
• World::checkpoint() captures a snapshot for later rollback / rollforward.
• World::rollback() rolls the application 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.

• App.init_pipeline::<P>() initializes a Pipeline for use with save / load.
• App.allow_rollback::<T>() allows a type to roll back.
• App.deny_rollback::<T>() denies a type from rolling back.

Type filtering

bevy_save allows you to explicitly filter types when creating a snapshot.

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:

• Snapshot::applier()
• SnapshotApplier::new()

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

let snapshot = Snapshot::from_world(world);

snapshot
    .applier(world)

    // Your entity map
    .entity_map(HashMap::default())

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

    .apply();

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 applications also need to be able to save only parts of the world.

SnapshotBuilder allows you to manually create snapshots like DynamicSceneBuilder:

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

        // Extract all descendants of `target`
        // This will include all components not denied by the builder's filter
        .extract_entities(children.iter_descendants(target))

        // Entities without any components will also be extracted
        // You can use `clear_empty` to remove them
        .clear_empty()

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

You are also able to extract resources by type:

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

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

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

Snapshot::builder(world)
    // Exclude `Transform` from this `Snapshot`
    .deny::<Transform>()

    // Extract all matching entities and resources
    .extract_all()

    // Clear all extracted entities without any components
    .clear_empty()

    // Build the `Snapshot`
    .build()

Pipeline

Pipelines allow you to use multiple different configurations of Backend and Format in the same App.

Pipelines also let you re-use Snapshot appliers and extractors.

Stability warning

bevy_save does not yet provide any stability guarantees for save file format between crate versions.

bevy_save relies on serialization to create save files and as such is exposed to internal implementation details for types. Expect Bevy or other crate updates to break your save file format. It should be possible to mitigate this by overriding ReflectDeserialize for any offending types.

Changing what entities have what components or how you use your entities or resources in your logic can also result in broken saves. While bevy_save does not yet have explicit support for save file migration, you can use SnapshotApplier::hook to account for changes while applying a snapshot.

If your application has specific migration requirements, please open an issue.

Entity

For all intents and purposes, Entity should be treated as an opaque identifier. The internal bit representation is liable to change from release to release as are the behaviors or performance characteristics of any of its trait implementations (i.e. Ord, Hash, etc.). This means that changes in Entity’s representation, though made readable through various functions on the type, are not considered breaking changes under SemVer.

In particular, directly serializing with Serialize and Deserialize make zero guarantee of long term wire format compatibility. Changes in behavior will cause serialized Entity values persisted to long term storage (i.e. disk, databases, etc.) will fail to deserialize upon being updated.

— Bevy's Entity documentation

bevy_save serializes and deserializes entities directly. If you need to maintain compatibility across Bevy versions, consider adding a unique identifier Component to your tracked entities.

Stabilization

bevy_save will become a candidate for stabilization once save versioning and migration is finished.

Compatibility

Bevy

Save format changes (since 0.15)

1. bevy changed Entity's on-disk representation
2. bevy_save began using FromReflect when taking snapshots

Platforms

Feature Flags

License

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