Bevy_save

A framework for saving and loading game 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 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.
• 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 games 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 game 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:

• World::snapshot() captures a snapshot of the current game state, including resources.
• 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.

• 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 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 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 Applier 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.

License

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

Feature Flags

Compatibility

Bevy

NOTE: We do not track Bevy main.

Platforms

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