bevy-ecs Checked

fn main() {

App::new()

.add_plugins(DefaultPlugins) // window, input, render, time, ...

.add_systems(Startup, setup) // runs once at startup

.add_systems(Update, move_players) // runs every frame

.run();

}

### 2. Components, resources, and spawning

#[derive(Component)]

struct Player;

#[derive(Component)]

struct Velocity(Vec2);

#[derive(Resource)]

struct Score(u32);

fn setup(mut commands: Commands) {

commands.insert_resource(Score(0));

// Camera2d is a component with required components (bundles removed in 0.16);

// spawning it pulls in Transform, Camera, etc. automatically.

commands.spawn(Camera2d);

// Spawn an entity as a tuple of components.

commands.spawn((

Player,

Velocity(Vec2::new(150.0, 0.0)),

Transform::from_xyz(0.0, 0.0, 0.0),

));

}

### 3. A system with a query + the Time resource

// Iterate every entity that has BOTH Velocity and Transform; mutate Transform.

fn move_players(time: Res<Time>, mut query: Query<(&Velocity, &mut Transform)>) {

for (velocity, mut transform) in &mut query {

// delta_secs() is f32 seconds (renamed from delta_seconds() in 0.16).

transform.translation += velocity.0.extend(0.0) * time.delta_secs();

}

}

### 4. Query filters (With / Without / Changed)

// Only entities tagged Player (the Player component itself isn't read).

fn aim_player(mut q: Query<&mut Transform, With<Player>>) { /* ... */ }

// Disjoint two mutable Transform queries so they don't conflict at runtime.

fn separate(

mut players: Query<&mut Transform, With<Player>>,

mut enemies: Query<&mut Transform, Without<Player>>,

) { /* ... */ }

// React only when Health changed since last run (change detection).

fn on_health_change(q: Query<&Health, Changed<Health>>) {

for health in &q { /* update the HUD, etc. */ }

}

### 5. Resources: read and write

fn add_points(mut score: ResMut<Score>) {

score.0 += 10; // ResMut = write access

}

fn show_score(score: Res<Score>) {

info!("score: {}", score.0); // Res = read access

}

### 6. Ordering, run conditions, and plugins

fn main() {

App::new()

.add_plugins((DefaultPlugins, GameplayPlugin))

// .chain() forces order: damage resolves before death is checked.

.add_systems(Update, (apply_damage, check_deaths).chain())

// run_if gates a system on a condition each frame.

.add_systems(Update, spawn_wave.run_if(wave_timer_finished))

.run();

}

struct GameplayPlugin;

impl Plugin for GameplayPlugin {

fn build(&self, app: &mut App) {

app.insert_resource(Score(0))

.add_systems(Startup, setup)

.add_systems(Update, (move_players, add_points));

}

}

## Pitfalls

- **`delta_seconds()` not found** → it was renamed to `time.delta_secs()` (and
  `elapsed_secs()`) in 0.16. Using the old name fails to compile.
- **Movement speed scales with frame rate** → multiply per-frame changes by
  `time.delta_secs()`. Never assume a fixed frame time.
- **Panic `error[B0001]`: "... accesses component(s) Transform in a way that conflicts
  with a previous system parameter"** → two `Query`s in one system both write the same
  component, or one reads while another writes overlapping entities. Make them disjoint
  with `With`/`Without`, or use `ParamSet`.
- **`Camera2dBundle`/`SpriteBundle` not found** → bundles were deprecated in 0.15 and
  removed in 0.16.
  Spawn the components directly (`Camera2d`, `Sprite`, `Transform`); required
  components fill in the rest.
- **"trait `Component` is not implemented"** → you forgot `#[derive(Component)]`
  (or `#[derive(Resource)]` for a resource).
- **Spawned entity not visible to a later query in the same frame** → `Commands` are
  deferred and applied at the next sync point. Read the entity in a subsequent system,
  not the one that spawned it.
- **System order assumed but not enforced** → systems run in parallel by default.
  If `B` must follow `A`, add `(A, B).chain()` or an explicit ordering constraint.
  To find order dependencies that only work by accident, shuffle the schedule in tests
  with `ScheduleBuildSettings::shuffle_seed` (0.20, `debug` feature; see the reference).
- **"conflicting implementations of trait `Component`" after deriving both `Resource`
  and `Component`** → since 0.19 `Resource` extends `Component`; derive `Resource` alone.
- **"Despawn everything" panics, or `Query<EntityMut>` next to a `Res<T>` panics with
  `error[B0002]`** → since 0.19 every resource lives on an entity tagged `IsResource`,
  so `Query<Entity>`/`EntityRef`/`EntityMut` match those entities too. Query your own
  marker component, or add `Without<IsResource>` (`use bevy::ecs::resource::IsResource`).
- **`On<Add, Player>` fails with "struct takes 1 generic argument but 2 generic
  arguments were supplied"** → 0.20 moved the component into the lifecycle event:
  write `On<Add<Player>>` (likewise `Insert<T>`, `Discard<T>`, `Remove<T>`, `Despawn<T>`).
- **`OnExit`/`OnEnter` run again when "switching" to the current state** →
  `NextState::set` always runs the transition schedules. Use `set_if_different` to skip
  same-state transitions (renamed from `set_if_neq` in 0.20; calling `set_if_neq` on a
  `ResMut<NextState<_>>` picks up the unrelated change-detection method and fails with
  "can't compare `NextState`").
- **"`text` does not live long enough" from `Name::new(text.as_str())`** → since 0.20
  `Name` takes a `&'