Struct EventReader

pub struct EventReader<'w, 's, E>
where
    E: Event,
{ /* private fields */ }

Reads events of type T in order and tracks which events have already been read.

Concurrency

Unlike EventWriter<T>, systems with EventReader<T> param can be executed concurrently (but not concurrently with EventWriter<T> or EventMutator<T> systems for the same event type).

Implementations

impl<'w, 's, E> EventReader<'w, 's, E>

where E: Event,

pub fn read(&mut self) -> EventIterator<'_, E>

Iterates over the events this EventReader has not seen yet. This updates the EventReader’s event counter, which means subsequent event reads will not include events that happened before now.

Examples found in repository

examples/ecs/send_and_receive_events.rs (line 60)

fn read_and_write_different_event_types(mut a: EventWriter<A>, mut b: EventReader<B>) {
    for _ in b.read() {}
    a.write(A);
}

/// A dummy event type.
#[derive(Debug, Clone, Event)]
struct DebugEvent {
    resend_from_param_set: bool,
    resend_from_local_event_reader: bool,
    times_sent: u8,
}

/// A system that sends all combinations of events.
fn send_events(mut events: EventWriter<DebugEvent>, frame_count: Res<FrameCount>) {
    println!("Sending events for frame {}", frame_count.0);

    events.write(DebugEvent {
        resend_from_param_set: false,
        resend_from_local_event_reader: false,
        times_sent: 1,
    });
    events.write(DebugEvent {
        resend_from_param_set: true,
        resend_from_local_event_reader: false,
        times_sent: 1,
    });
    events.write(DebugEvent {
        resend_from_param_set: false,
        resend_from_local_event_reader: true,
        times_sent: 1,
    });
    events.write(DebugEvent {
        resend_from_param_set: true,
        resend_from_local_event_reader: true,
        times_sent: 1,
    });
}

/// A system that prints all events sent since the last time this system ran.
///
/// Note that some events will be printed twice, because they were sent twice.
fn debug_events(mut events: EventReader<DebugEvent>) {
    for event in events.read() {
        println!("{event:?}");
    }
}

/// A system that both sends and receives events using [`ParamSet`].
fn send_and_receive_param_set(
    mut param_set: ParamSet<(EventReader<DebugEvent>, EventWriter<DebugEvent>)>,
    frame_count: Res<FrameCount>,
) {
    println!(
        "Sending and receiving events for frame {} with a `ParamSet`",
        frame_count.0
    );

    // We must collect the events to resend, because we can't access the writer while we're iterating over the reader.
    let mut events_to_resend = Vec::new();

    // This is p0, as the first parameter in the `ParamSet` is the reader.
    for event in param_set.p0().read() {
        if event.resend_from_param_set {
            events_to_resend.push(event.clone());
        }
    }

    // This is p1, as the second parameter in the `ParamSet` is the writer.
    for mut event in events_to_resend {
        event.times_sent += 1;
        param_set.p1().write(event);
    }
}

examples/window/custom_user_event.rs (line 74)

fn handle_event(mut events: EventReader<CustomEvent>) {
    for evt in events.read() {
        info!("Received event: {evt:?}");
    }
}

examples/app/drag_and_drop.rs (line 13)

fn file_drag_and_drop_system(mut events: EventReader<FileDragAndDrop>) {
    for event in events.read() {
        info!("{:?}", event);
    }
}

examples/input/touch_input_events.rs (line 13)

fn touch_event_system(mut touch_events: EventReader<TouchInput>) {
    for event in touch_events.read() {
        info!("{:?}", event);
    }
}

examples/input/keyboard_input_events.rs (line 14)

fn print_keyboard_event_system(mut keyboard_input_events: EventReader<KeyboardInput>) {
    for event in keyboard_input_events.read() {
        info!("{:?}", event);
    }
}

examples/3d/clustered_decals.rs (line 388)

fn handle_selection_change(
    mut events: EventReader<WidgetClickEvent<Selection>>,
    mut app_status: ResMut<AppStatus>,
) {
    for event in events.read() {
        app_status.selection = **event;
    }
}

Additional examples can be found in:

• examples/math/cubic_splines.rs
• examples/window/window_resizing.rs
• examples/async_tasks/external_source_external_thread.rs
• examples/input/char_input_events.rs
• examples/ecs/event.rs
• examples/2d/texture_atlas.rs
• examples/2d/pixel_grid_snap.rs
• examples/audio/pitch.rs
• examples/app/log_layers_ecs.rs
• examples/transforms/align.rs
• examples/math/random_sampling.rs
• examples/3d/split_screen.rs
• examples/3d/tonemapping.rs
• examples/input/text_input.rs
• examples/testbed/full_ui.rs
• examples/ui/scroll.rs
• examples/3d/pcss.rs
• examples/input/mouse_input_events.rs
• examples/math/sampling_primitives.rs
• examples/input/gamepad_input_events.rs
• examples/3d/ssr.rs
• examples/3d/visibility_range.rs
• examples/ui/size_constraints.rs
• examples/3d/mixed_lighting.rs

pub fn read_with_id(&mut self) -> EventIteratorWithId<'_, E>

Like read, except also returning the EventId of the events.

pub fn par_read(&mut self) -> EventParIter<'_, E>

Returns a parallel iterator over the events this EventReader has not seen yet. See also for_each.

Example

#[derive(Event)]
struct MyEvent {
    value: usize,
}

#[derive(Resource, Default)]
struct Counter(AtomicUsize);

// setup
let mut world = World::new();
world.init_resource::<Events<MyEvent>>();
world.insert_resource(Counter::default());

let mut schedule = Schedule::default();
schedule.add_systems(|mut events: EventReader<MyEvent>, counter: Res<Counter>| {
    events.par_read().for_each(|MyEvent { value }| {
        counter.0.fetch_add(*value, Ordering::Relaxed);
    });
});
for value in 0..100 {
    world.send_event(MyEvent { value });
}
schedule.run(&mut world);
let Counter(counter) = world.remove_resource::<Counter>().unwrap();
// all events were processed
assert_eq!(counter.into_inner(), 4950);

pub fn len(&self) -> usize

Determines the number of events available to be read from this EventReader without consuming any.

pub fn is_empty(&self) -> bool

Returns true if there are no events available to read.

Example

The following example shows a useful pattern where some behavior is triggered if new events are available. EventReader::clear() is used so the same events don’t re-trigger the behavior the next time the system runs.

#[derive(Event)]
struct CollisionEvent;

fn play_collision_sound(mut events: EventReader<CollisionEvent>) {
    if !events.is_empty() {
        events.clear();
        // Play a sound
    }
}

Examples found in repository

examples/games/breakout.rs (line 400)

fn play_collision_sound(
    mut commands: Commands,
    mut collision_events: EventReader<CollisionEvent>,
    sound: Res<CollisionSound>,
) {
    // Play a sound once per frame if a collision occurred.
    if !collision_events.is_empty() {
        // This prevents events staying active on the next frame.
        collision_events.clear();
        commands.spawn((AudioPlayer(sound.clone()), PlaybackSettings::DESPAWN));
    }
}

examples/asset/processing/asset_processing.rs (line 254)

fn print_text(
    handles: Res<TextAssets>,
    texts: Res<Assets<Text>>,
    mut asset_events: EventReader<AssetEvent<Text>>,
) {
    if !asset_events.is_empty() {
        // This prints the current values of the assets
        // Hot-reloading is supported, so try modifying the source assets (and their meta files)!
        println!("Current Values:");
        println!("  a: {:?}", texts.get(&handles.a));
        println!("  b: {:?}", texts.get(&handles.b));
        println!("  c: {:?}", texts.get(&handles.c));
        println!("  d: {:?}", texts.get(&handles.d));
        println!("  e: {:?}", texts.get(&handles.e));
        println!("(You can modify source assets and their .meta files to hot-reload changes!)");
        println!();
        asset_events.clear();
    }
}

pub fn clear(&mut self)

Consumes all available events.

This means these events will not appear in calls to EventReader::read() or EventReader::read_with_id() and EventReader::is_empty() will return true.

For usage, see EventReader::is_empty().

Examples found in repository

examples/games/breakout.rs (line 402)

fn play_collision_sound(
    mut commands: Commands,
    mut collision_events: EventReader<CollisionEvent>,
    sound: Res<CollisionSound>,
) {
    // Play a sound once per frame if a collision occurred.
    if !collision_events.is_empty() {
        // This prevents events staying active on the next frame.
        collision_events.clear();
        commands.spawn((AudioPlayer(sound.clone()), PlaybackSettings::DESPAWN));
    }
}

examples/asset/processing/asset_processing.rs (line 265)

fn print_text(
    handles: Res<TextAssets>,
    texts: Res<Assets<Text>>,
    mut asset_events: EventReader<AssetEvent<Text>>,
) {
    if !asset_events.is_empty() {
        // This prints the current values of the assets
        // Hot-reloading is supported, so try modifying the source assets (and their meta files)!
        println!("Current Values:");
        println!("  a: {:?}", texts.get(&handles.a));
        println!("  b: {:?}", texts.get(&handles.b));
        println!("  c: {:?}", texts.get(&handles.c));
        println!("  d: {:?}", texts.get(&handles.d));
        println!("  e: {:?}", texts.get(&handles.e));
        println!("(You can modify source assets and their .meta files to hot-reload changes!)");
        println!();
        asset_events.clear();
    }
}

Trait Implementations

impl<'w, 's, E> Debug for EventReader<'w, 's, E>

where E: Debug + Event,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<E> SystemParam for EventReader<'_, '_, E>

where E: Event,

type State = FetchState<E>

Used to store data which persists across invocations of a system.

type Item<'w, 's> = EventReader<'w, 's, E>

The item type returned when constructing this system param. The value of this associated type should be Self, instantiated with new lifetimes.

fn init_state( world: &mut World, system_meta: &mut SystemMeta, ) -> <EventReader<'_, '_, E> as SystemParam>::State

Registers any World access used by this SystemParam and creates a new instance of this param’s State.

unsafe fn new_archetype( state: &mut <EventReader<'_, '_, E> as SystemParam>::State, archetype: &Archetype, system_meta: &mut SystemMeta, )

For the specified Archetype, registers the components accessed by this SystemParam (if applicable).a

fn apply( state: &mut <EventReader<'_, '_, E> as SystemParam>::State, system_meta: &SystemMeta, world: &mut World, )

Applies any deferred mutations stored in this SystemParam’s state. This is used to apply Commands during ApplyDeferred.

fn queue( state: &mut <EventReader<'_, '_, E> as SystemParam>::State, system_meta: &SystemMeta, world: DeferredWorld<'_>, )

Queues any deferred mutations to be applied at the next ApplyDeferred.

unsafe fn validate_param<'w, 's>( state: &'s <EventReader<'_, '_, E> as SystemParam>::State, _system_meta: &SystemMeta, _world: UnsafeWorldCell<'w>, ) -> Result<(), SystemParamValidationError>

Validates that the param can be acquired by the get_param.

unsafe fn get_param<'w, 's>( state: &'s mut <EventReader<'_, '_, E> as SystemParam>::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'w>, change_tick: Tick, ) -> <EventReader<'_, '_, E> as SystemParam>::Item<'w, 's>

Creates a parameter to be passed into a SystemParamFunction.

impl<'w, 's, E> ReadOnlySystemParam for EventReader<'w, 's, E>

where E: Event, Local<'s, EventCursor<E>>: ReadOnlySystemParam, Res<'w, Events<E>>: ReadOnlySystemParam,