bevy-testing-patterns

Bevy testing patterns

When to use

Load this when a Bevy crate needs unit or integration tests. cargo test runs #[test] functions. A headless App with MinimalPlugins is the harness for schedule logic.

ECS shape

Command: cargo test compiles and runs unit tests, integration tests, and documentation tests. cargo test name_filter runs names that contain the filter. Arguments after -- go to libtest (cargo test -- --help).
What runs: functions annotated with #[test]. The harness runs them in multiple threads. The working directory for each test is the package root, so relative paths resolve from that root no matter where you invoked Cargo.
Scope: default cargo test covers the lib, bins as unit tests, integration tests, and doc tests, and it builds examples. --lib limits to the library. harness = false on a target means you supply main.
Profile: the test profile is the default. --profile=debug keeps debug info. -- --no-capture shows test output.
Bevy types inside a test: build an App, insert the resource, add the system, call update, then read world(). Keep the window out of the harness with MinimalPlugins.

How to run

Need	Command
Whole suite	`cargo test` from the package
One name	`cargo test name_filter`
Printed output	`cargo test -- --no-capture`

Pure logic stays a normal #[test] with no App.

fn score_after_pickup(score: u32, value: u32) -> u32 {
    score.saturating_add(value)
}

#[test]
fn pickup_adds_points() {
    assert_eq!(score_after_pickup(1, 2), 3);
}

Schedule tests own a headless app. world() is shared for the assert, so the query that writes Score has already finished.

#[derive(Resource, Default)]
struct Score(u32);

fn add_point(mut score: ResMut<Score>) {
    score.0 += 1;
}

#[test]
fn one_update_adds_a_point() {
    let mut app = App::new();
    app.add_plugins(MinimalPlugins)
        .insert_resource(Score(0))
        .add_systems(Update, add_point);
    app.update();
    assert_eq!(app.world().resource::<Score>().0, 1);
}

Spawn in Startup, then step Update once. The test reads the component through a one-off system result stored on a resource the test inserts, or through world().

#[derive(Component)]
struct Marker;

fn spawn_marker(mut commands: Commands) {
    commands.spawn(Marker);
}

#[test]
fn startup_spawns_one_marker() {
    let mut app = App::new();
    app.add_plugins(MinimalPlugins)
        .add_systems(Startup, spawn_marker);
    app.update();
    let count = app.world().query::<&Marker>().iter(app.world()).count();
    assert_eq!(count, 1);
}

Two writers of the same component are chained inside the test the same way they are chained in the game plugin.

#[derive(Resource)]
struct Wallet(i32);

fn earn(mut wallet: ResMut<Wallet>) {
    wallet.0 += 5;
}

fn spend(mut wallet: ResMut<Wallet>) {
    wallet.0 -= 2;
}

#[test]
fn earn_then_spend() {
    let mut app = App::new();
    app.add_plugins(MinimalPlugins)
        .insert_resource(Wallet(0))
        .add_systems(Update, (earn, spend).chain());
    app.update();
    assert_eq!(app.world().resource::<Wallet>().0, 3);
}

Scope

The harness above is MinimalPlugins, app.update(), and world(). It does not capture frames or inject window input.
Doctest working-directory rules differ from unit tests. Do not assume a doctest can read assets/ the same way a #[test] can. The package root is the unit-test working directory.

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 scene-tree test runner in this crate. WHY: schedule tests use MinimalPlugins, app.update(), and world().
No frame-capture or input-injection harness. WHY: this skill tests schedules with MinimalPlugins and app.update().
No third-party Bevy test crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.

Official Documentation

cargo test - #[test], filters, libtest args, package root as cwd.
Examples - Example tree on the latest branch. Examples are not the test harness.

Related Skills

Prerequisites

bevy-master - Who owns which piece of state.
bevy-project-foundations - The Cargo package cargo test runs against.

Downstream / consumers

None yet.

Master

bevy-master - Bevy router.