Bevy save and load

When to use

Load this when a design asks to persist player progress. Route asset and scene spawning to bevy-asset-scenes. Keep save bytes as data you serialize yourself until a current docs page names a format.

ECS shape

• What exists: app-wide progress is a Resource you serialize yourself. Level content loads through AssetServer and scene spawn (bevy-asset-scenes). A scene spawn is not a player slot.
• Format: progress structs use serde and RON. The file holds primitives and a version field. Entity ids and asset handles stay out of the file.
• Schedule: write the slot from an explicit request system. Do not write the file from every Update frame.

serde and ron sit beside Bevy. The progress type is a normal resource.

[dependencies]
bevy = "0.20"
serde = { version = "1", features = ["derive"] }
ron = "0.10"

use serde::{Deserialize, Serialize};

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
    level: String,
}

fn fresh_progress() -> Progress {
    Progress {
        version: 2,
        coins: 0,
        level: "town".to_string(),
    }
}

Encode and decode are plain functions. Call them from the request system, not from a per-frame query.

use serde::{Deserialize, Serialize};

#[derive(Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
    level: String,
}

fn encode_slot(progress: &Progress) -> String {
    ron::ser::to_string_pretty(progress, ron::ser::PrettyConfig::default())
        .expect("progress encodes")
}

fn decode_slot(text: &str) -> Progress {
    ron::from_str(text).expect("progress decodes")
}

fn round_trip() {
    let saved = Progress {
        version: 2,
        coins: 4,
        level: "town".to_string(),
    };
    let loaded = decode_slot(&encode_slot(&saved));
    assert_eq!(loaded.coins, 4);
}

Older files omit version. Migration builds the current struct before anything inserts it.

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
struct SaveV1 {
    coins: u32,
}

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
}

fn load_any(text: &str) -> Progress {
    if let Ok(current) = ron::from_str::<Progress>(text) {
        return current;
    }
    let legacy: SaveV1 = ron::from_str(text).expect("legacy slot");
    Progress {
        version: 2,
        coins: legacy.coins,
    }
}

The save system runs only while SaveRequest is present, then removes it. One system owns the resource and the disk write.

use serde::{Deserialize, Serialize};

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
}

#[derive(Resource)]
struct SaveRequest {
    path: String,
}

fn write_slot(progress: Res<Progress>, request: Res<SaveRequest>) -> std::io::Result<()> {
    let text = ron::ser::to_string(&*progress).expect("progress encodes");
    std::fs::write(&request.path, text)
}

fn save_when_asked(mut commands: Commands, progress: Res<Progress>, request: Option<Res<SaveRequest>>) {
    let Some(request) = request else {
        return;
    };
    let _ = write_slot(progress, request);
    commands.remove_resource::<SaveRequest>();
}

pub struct SavePlugin;

impl Plugin for SavePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Progress {
            version: 2,
            coins: 0,
        })
        .add_systems(Update, save_when_asked);
    }
}

What to persist

Need	Bevy idiom
Designer content	An asset Handle, or a Resource you own in code
Level instance	Scene spawn on bevy-asset-scenes
Player save slot	A versioned Resource written as RON when a SaveRequest exists

Scope

• The slot path, version field, and RON text live on Progress and SaveRequest above.
• Scene spawn is not a save game. Do not treat bsn! as a user save.
• A native std::fs write is the desktop slot. Web builds need a different storage story. Do not call std::fs and assume a browser has that path.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A free-floating struct outside the World
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No engine config-file types in the slot. WHY: the slot is a versioned Resource encoded as RON.

• No DynamicScene or scene-file writer this file does not list. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No third-party save crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20. Scene spawn is not a save format.

Official Documentation

• Resources - App-wide data the slot serializes.
• Examples - Example tree on the latest branch. Scene spawn lives with bevy-asset-scenes.

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-data-resources - Resource versus Component.
• bevy-asset-scenes - Loading content. Not a save slot.

Downstream / consumers

• None until a save format is opened.

Master

• bevy-master - Bevy router.