Bevy_save

A framework for saving and loading application state in Bevy.

preview-0.6.0.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.

By default, 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.

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.

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

On WASM, snapshots are saved to LocalStorage, with the key WORKSPACE.KEY.

Reflection-based Snapshots

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

With the "reflect" feature enabled, 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 method WorldPathwayExt and WorldCheckpointExt:

World::capture() captures a snapshot of the current application state, including resources.
World::apply() applies a snapshot to the World.

Versioning and Migrations

Applications can change over the history of development. Users expect that saves created in an older version will continue to work in newer versions.

bevy_save provides support for reflection-based migrations with the Migrate trait:

use bevy::prelude::*;
use bevy_save::prelude::*;

// `#[reflect(Migrate)]` registers `ReflectMigrate` with the `TypeRegistry`
// This allows `Snapshot`s to save the type version and apply migrations automatically
#[derive(Reflect, Component, Debug, PartialEq)]
#[type_path = "migrate"]
#[type_name = "Position"]
#[reflect(Component, Migrate)]
struct Position {
    xyz: (f32, f32, f32),
}

// The `Migrate` trait allows you to define a `Migrator`
// which will step the upgrade through each version
impl Migrate for Position {
    fn migrator() -> Migrator<Self> {
        #[derive(Reflect)]
        #[type_path = "migrate"]
        #[type_name = "Pos"]
        struct PosV0_1 {
            x: f32,
            y: f32,
        }

        Migrator::new::<PosV0_1>("0.1.0")
            .version("0.2.0", |v1| {
                // Changing type paths and type names is supported
                #[derive(Reflect)]
                #[type_path = "migrate"]
                #[type_name = "Position"]
                struct PosV0_2 {
                    x: f32,
                    y: f32,
                }

                Some(PosV0_2 { x: v1.x, y: v1.y })
            })
            .version("0.3.0", |v2| {
                #[derive(Reflect)]
                #[type_path = "migrate"]
                #[type_name = "Position"]
                struct PosV0_3 {
                    x: f32,
                    y: f32,
                    z: f32,
                }

                // Fields can be re-mapped from version to version, added, or removed
                Some(PosV0_3 {
                    x: v2.x,
                    y: v2.y,
                    z: 0.0,
                })
            })
            // The final version will represent the current layout
            .version("0.4.0", |v2| {
                Some(Self {
                    xyz: (v2.x, v2.y, v2.z),
                })
            })
    }
}

Rollbacks and checkpoints

With the "checkpoints" feature enabled, this crate provides methods for creating checkpoints which are ordered and can be rolled back / forwards through.

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 Checkpoints resource also gives you fine-tuned control of the currently stored rollback checkpoints.

Type registration

No special traits or NewTypes necessary, bevy_save takes full advantage of Bevy's built-in reflection. As long as the type implements Reflect and it's not filtered out, it will automatically be included in Snapshots.

use bevy::prelude::*;
use bevy_save::prelude::*;

/// `#[reflect(Ignore)]` prevents the type from being included in any snapshots or checkpoints.
#[derive(Component, Reflect)]
#[reflect(Component, Ignore)]
struct NotIncluded {
    x: f32,
    y: f32,
    z: f32,
}

/// `#[reflect(IgnoreCheckpoint)]` prevents the type from being included in checkpoints (but they will still be in snapshots).
#[derive(Component, Reflect)]
#[reflect(Component, IgnoreCheckpoint)]
struct NotInCheckpoints {
    x: f32,
    y: f32,
    z: f32,
}

Pipeline

The Pipeline trait allows you to use multiple different configurations of Backend and Format in the same App.

Using Pipeline also lets you re-use Snapshot appliers and builders.

use bevy::prelude::*;
use bevy_save::prelude::*;

#[derive(Component, Reflect)]
#[reflect(Component)]
struct Head;

#[derive(Component, Reflect)]
#[reflect(Component)]
struct Player;

struct HeirarchyPipeline;

impl Pipeline for HeirarchyPipeline {
    type Backend = DefaultDebugBackend;
    type Format = DefaultDebugFormat;

    type Key<'a> = &'a str;

    fn key(&self) -> Self::Key<'_> {
        "examples/saves/heirarchy"
    }

    fn capture(&self, builder: BuilderRef) -> Snapshot {
        builder
            .extract_entities_matching(|e| e.contains::<Player>() || e.contains::<Head>())
            .build()
    }

    fn apply(&self, world: &mut World, snapshot: &Snapshot) -> Result<(), bevy_save::Error> {
        snapshot
            .applier(world)
            .despawn::<Or<(With<Player>, With<Head>)>>()
            .apply()
    }
}

Type Filtering and 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.

Builder allows you to manually create snapshots like DynamicSceneBuilder:

use bevy::prelude::*;
use bevy_save::prelude::*;

#[derive(Resource, Reflect)]
#[reflect(Resource)]
struct FancyMap {
    map: Vec<(u32, String)>,
}

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
fn build_snapshot_resource(world: &World) -> Snapshot {
    Snapshot::builder(world)
        // Extract the resource by the type
        .extract_resource::<FancyMap>()

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

// Additionally, explicit type filtering like `ApplierRef` is available when building snapshots
fn build_snapshot_filter(world: &World) -> Snapshot {
    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()
}

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:

use bevy::prelude::*;
use bevy_save::prelude::*;

fn apply_snapshot(world: &mut World, root: Entity, snapshot: Snapshot) {
    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::<ChildOf>() {
                cmds.insert(ChildOf(root));
            }
        })

        .apply();
}

Hooks may also despawn entities:

use bevy::prelude::*;
use bevy_save::prelude::*;

fn apply_snapshot(world: &mut World, snapshot: Snapshot) {
    snapshot
        .applier(world)

        .hook(|entity, cmds| {
            if entity.contains::<Transform>() {
                cmds.despawn();
            }
        })

        // Alternatively, you could do the following:
        // .despawn::<With<Transform>>()

        .apply();
}

Entity mapping

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

This is done transparently by utilizing MapEntities.

use bevy::{ecs::{entity::MapEntities, reflect::ReflectMapEntities}, prelude::*};
use bevy_save::prelude::*;
use std::collections::HashMap;

/// It is required to register `#[reflect(MapEntities)]`
#[derive(Component, Reflect)]
#[reflect(Component, MapEntities)]
struct ExampleMap {
    data: HashMap<u32, Entity>,
}

impl MapEntities for ExampleMap {
    fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
        self.data.values_mut().for_each(|e| {
            *e = entity_mapper.get_mapped(*e);
        });
    }
}

/// While `bevy_save` supports entity maps, it is not necessary even if you are using `MapEntities`.
fn apply_snapshot(world: &mut World, snapshot: Snapshot) {
    snapshot
        .applier(world)

        // An entity map may be desired if you are already doing something like using persistent uuid entity identifiers
        // .entity_map(entity_map)

        // Otherwise, you should just despawn existing entities
        .despawn::<With<ExampleMap>>()

        .apply();
}

Flows

When creating a complex application, snapshot builder and applier functions tend to get complex and unwieldy.

Flows are chains of systems used to modularize this process, allowing you to build snapshots and apply them in stages.

They are defined similar to Bevy systems, but they require an input and an output.

Additionally, the introduction of Flows allows reflection to become optional - bring your own serialization if you so wish!

use bevy::prelude::*;
use bevy_save::prelude::*;
use serde::{Serialize, Deserialize};

// Flow labels don't encode any behavior by themselves, only point to flows
#[derive(Hash, Debug, PartialEq, Eq, Clone, Copy, FlowLabel)]
pub struct CaptureFlow;

fn main() {
    App::new()
        .add_flows(CaptureFlow, capture_tiles);
}

#[derive(Component, Clone, Serialize, Deserialize)]
enum Tile {
    Air,
    Dirt,
    Stone,
}

// User-defined captures make reflection unnecessary
#[derive(Serialize, Deserialize)]
struct MyCapture {
    // ... but then you'll need to specify everything you extract
    tiles: Vec<(u64, Tile)>,
}

// Flow systems have full access to the ECS (even write access)
fn capture_tiles(In(mut cap): In<MyCapture>, tiles: Query<(Entity, &Tile)>) -> MyCapture {
    cap.tiles.extend(tiles.iter().map(|(e, t)| (e.to_bits(), t.clone())));
    cap
}

// Flow systems can be added to flows from anywhere in your application
struct PluginA;

impl Plugin for PluginA {
    fn build(&self, app: &mut App) {
        app.add_flows(CaptureFlow, capture_tiles);
    }
}

Pathway

Pathway is the more flexible version of Pipeline which allows you to specify your own capture type and use Flows.

use bevy::prelude::*;
use bevy_save::prelude::*;

// Pathways look very similar to pipelines, but there are a few key differences
pub struct SimplePathway;

impl Pathway for SimplePathway {
    // The capture type allows you to save anything you want to disk, even without using reflection
    type Capture = Snapshot;

    type Backend = DefaultDebugBackend;
    type Format = DefaultDebugFormat;
    type Key<'a> = &'a str;

    fn key(&self) -> Self::Key<'_> {
        "examples/saves/flows"
    }

    // Instead of capturing and applying directly, now these methods just return labels to user-defined flows
    // This allows for better dependency injection and reduces code complexity
    fn capture(&self, _world: &World) -> impl FlowLabel {
        CaptureFlow
    }

    fn apply(&self, _world: &World) -> impl FlowLabel {
        ApplyFlow
    }
}

// Flow labels don't encode any behavior by themselves, only point to flows
#[derive(Hash, Debug, PartialEq, Eq, Clone, Copy, FlowLabel)]
pub struct CaptureFlow;

#[derive(Hash, Debug, PartialEq, Eq, Clone, Copy, FlowLabel)]
pub struct ApplyFlow;

Backend

The Backend is the interface between your application and persistent storage.

Some example backends may include FileIO, sqlite, LocalStorage, or network storage.

use bevy_save::prelude::*;
use serde::{de::DeserializeSeed, Serialize};
use smol::{fs::{File, create_dir_all}, io::{AsyncReadExt, AsyncWriteExt}};

struct FileIO;

impl<K: std::fmt::Display + Send> Backend<K> for FileIO {
    async fn save<F: Format, T: Serialize>(&self, key: K, value: &T) -> Result<(), Error> {
        let path = get_save_file(format!("{key}{}", F::extension()));
        let dir = path.parent().expect("Invalid save directory");
        create_dir_all(dir).await?;
        let mut buf = Vec::new();
        F::serialize(&mut buf, value)?;
        let mut file = File::create(path).await?;
        Ok(file.write_all(&buf).await?)
    }

    async fn load<F: Format, S: for<'de> DeserializeSeed<'de, Value = T>, T>(
        &self,
        key: K,
        seed: S,
    ) -> Result<T, Error> {
        let path = get_save_file(format!("{key}{}", F::extension()));
        let mut file = File::open(path).await?;
        let mut buf = Vec::new();
        file.read_to_end(&mut buf).await?;
        F::deserialize(&*buf, seed)
    }
}

Format

Format is how your application serializes and deserializes your data.

Formats can either be human-readable like JSON or binary like MessagePack.

use bevy_save::prelude::*;
use io_adapters::WriteExtension;
use serde::{de::DeserializeSeed, Serialize};
use std::io::{Read, Write};

pub struct RONFormat;

impl Format for RONFormat {
    fn extension() -> &'static str {
        ".ron"
    }

    fn serialize<W: Write, T: Serialize>(writer: W, value: &T) -> Result<(), Error> {
        let mut ser = ron::Serializer::new(
            writer.write_adapter(),
            Some(ron::ser::PrettyConfig::default()),
        )
        .map_err(Error::saving)?;

        value.serialize(&mut ser).map_err(Error::saving)
    }

    fn deserialize<R: Read, S: for<'de> DeserializeSeed<'de, Value = T>, T>(
        reader: R,
        seed: S,
    ) -> Result<T, Error> {
        ron::options::Options::default()
            .from_reader_seed(reader, seed)
            .map_err(Error::loading)
    }
}

Prefabs

The Prefab trait allows you to easily spawn entities from a blueprint.

use bevy::prelude::*;
use bevy_save::prelude::*;

const BALL_STARTING_POSITION: Vec3 = Vec3::new(0.0, -50.0, 1.0);
const BALL_DIAMETER: f32 = 30.;
const BALL_SPEED: f32 = 400.0;
const BALL_COLOR: Color = Color::srgb(1.0, 0.5, 0.5);
const INITIAL_BALL_DIRECTION: Vec2 = Vec2::new(0.5, -0.5);

#[derive(Component, Default, Reflect)]
#[reflect(Component)]
struct Ball;

#[derive(Component, Reflect, Deref, DerefMut)]
#[reflect(Component)]
struct Velocity(Vec2);

#[derive(Reflect)]
struct BallPrefab {
    position: Vec3,
    velocity: Vec2,
}

impl Default for BallPrefab {
    fn default() -> Self {
        Self {
            position: BALL_STARTING_POSITION,
            velocity: INITIAL_BALL_DIRECTION.normalize() * BALL_SPEED,
        }
    }
}

impl Prefab for BallPrefab {
    type Marker = Ball;

    fn spawn(self, target: Entity, world: &mut World) {
        // Some entities will need initialization from world state, such as mesh assets.
        // We can do that here.
        let mesh = world.resource_mut::<Assets<Mesh>>().add(Circle::default());
        let material = world
            .resource_mut::<Assets<ColorMaterial>>()
            .add(BALL_COLOR);

        world.entity_mut(target).insert((
            Mesh2d(mesh),
            MeshMaterial2d(material),
            Transform::from_translation(self.position)
                .with_scale(Vec2::splat(BALL_DIAMETER).extend(1.)),
            Ball,
            Velocity(self.velocity),
        ));
    }

    fn extract(builder: BuilderRef) -> BuilderRef {
        // We don't actually need to save all of those runtime components.
        // Only save the translation and velocity of the Ball.
        builder.extract_prefab(|entity| {
            Some(BallPrefab {
                position: entity.get::<Transform>()?.translation,
                velocity: entity.get::<Velocity>()?.0,
            })
        })
    }
}

Stability

bevy_save attempts to provide stability guarantees for Snapshot serialization and deserialization between crate versions on a best-effort basis, enforced with unit tests.

If a breaking change is introduced, the number after the + on the crate version will be incremented and it will be supported via the version method on SnapshotDeserializer.

bevy_save relies on serialization to create save files and as such is exposed to internal implementation details for types. As a result, Bevy or other crate updates may break your save file format. It should be possible to mitigate this by defining ReflectMigrate 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.

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.

Compatibility

bevy_save follows Semantic Versioning 2.0.0, with additional metadata: MAJOR.MINOR.PATCH+SNAPSHOT

Major: Breaking API changes and/or Snapshot format changes
Minor: Backwards-compatible API changes
Patch: Backwards-compatible bug-fixes
Snapshot: Snapshot version, incremented when the wire format of Snapshot changes in a way that will break existing applications

Bevy

Bevy Version	Crate Version
`0.16`	`0.18+3`, `0.19+3`, `1.0+4`⁴, `2.0+4`, `3.0+4`
`0.15`	`0.16+3`³, `0.17+3`
`0.14`²	`0.15+2`
`0.13`	`0.14+1`
`0.12`	`0.10+1`, `0.11+1`, `0.12+1`, `0.13+1`
`0.11`	`0.9+1`
`0.10`	`0.4+0`, `0.5+0`, `0.6+1`¹, `0.7+1`, `0.8+1`
`0.9`	`0.1`, `0.2+0`⁰, `0.3+0`

Snapshot Version

bevy_save introduced serialization support
bevy_save introduced a new Snapshot format
bevy changed Entity's on-disk representation
bevy_save began using FromReflect when taking snapshots
bevy_save introduced a new Snapshot format, see below

Migrating

0.18+3 -> 0.19+3

This version introduced Pathway, which is effectively a superset of Pipeline.

In World::capture, World:apply, World::save, World::load methods and similar, add a & before your existing pipeline
Previously provided Commands extension traits and associated commands have been removed (since Pathway operates on references), you'll need to write your own or use events instead
If you're using default-features = false, you'll need to add the reflect and checkpoints features in order to get parity with the last version
SnapshotBuilder and SnapshotApplier have been renamed to BuilderRef and ApplierRef, respectively.

0.19+3 -> 1.0+4

This version introduced versioning and migrations.

Removed the checkpoints field from Snapshot, instead saving Checkpoints as a resource via Reflect.
SnapshotVersion::V3 can be used with the version method on SnapshotDeserializer to load a snapshot created in a previous version (since 0.16) if the snapshot had checkpoints.
Snapshots created in a previous version without checkpoints should load as expected.
The fields for all serializers and deserializers have been made private. Use the new methods to construct them.
Non self-describing formats such as postcard should now work as expected.
Deserialization of Snapshot will fail for this version if PartialReflect::reflect_clone is not implemented for all contained types. While this is typically automatically implemented, opaque types must now manually add the attribute #[reflect(Clone)] in order for Snapshot::from_reflect to succeed.

1.0+4 -> 2.0+4

This version made some ergonomic changes and fixed a few bugs.

Applier now respects ReflectMapEntities for Components.
BoxedPartialReflect is now DynamicValue.
The CloneReflect trait has been removed. Snapshot, Checkpoints, and all the wrapper types now implement Clone.
WorldCheckpointExt::rollback_with has been removed, use WorldCheckpointExt::rollback instead.
Deserialization of Snapshot will no longer fail if PartialReflect::reflect_clone is not implemented for any contained types.

2.0+4 -> 3.0+4 (latest)

This version improved support for Relationships and introduced ReflectIgnore and ReflectIgnoreCheckpoint.

Types with Relationships must be registered to act correctly. Register #[reflect(Relationship)] and #[reflect(RelationshipTarget)] accordingly.
Both the Relationship and RelationshipTarget types must be registered. A warning will be logged if this is not the case.
If registered, RelationshipTarget types will no longer be included in Snapshot or applied on load. The Relationship type is the "source of truth" and the "target" entities will be populated from those values.
CheckpointRegistry has been removed. Instead, register #[reflect(IgnoreCheckpoint)].
SavePlugin is now SaveCorePlugin to differentiate from SavePlugins.

Platforms

Platform	Support
Windows	Yes
MacOS	Yes
Linux	Yes
WASM	Yes
Android	No
iOS	No

Feature Flags

Feature flag	Description	Default?
`reflect`	Enables reflection-based snapshots	Yes
`checkpoints`	Enables reflection-based checkpoints	Yes
`log`	Enables logging of warnings	Yes
`bevy_asset`	Enables `bevy_asset` type registration	Yes
`bevy_render`	Enables `bevy_render` type registration	Yes
`bevy_sprite`	Enables `bevy_sprite` type registration	Yes
`brotli`	Enables `Brotli` compression middleware	No

License

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

Name		Name	Last commit message	Last commit date
Latest commit History 144 Commits
crates/bevy_save_macros		crates/bevy_save_macros
examples		examples
src		src
tests		tests
.gitignore		.gitignore
Cargo.toml		Cargo.toml
LICENSE-APACHE		LICENSE-APACHE
LICENSE-MIT		LICENSE-MIT
README.md		README.md
build.rs		build.rs
rustfmt.toml		rustfmt.toml

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hankjordan/bevy_save

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Bevy_save

Features

Save file management

Save directory location

Reflection-based Snapshots

Versioning and Migrations

Rollbacks and checkpoints

Type registration

Pipeline

Type Filtering and Partial Snapshots

Entity hooks

Entity mapping

Flows

Pathway

Backend

Format

Prefabs

Stability

Entity

Compatibility

Bevy

Snapshot Version

Migrating

Platforms

Feature Flags

License

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