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Document ManualEventReader #3384

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Document ManualEventReader #3384

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46 changes: 29 additions & 17 deletions crates/bevy_ecs/src/event.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -50,7 +50,7 @@ struct EventInstance<T> {
}

#[derive(Debug)]
enum State {
enum DoubleBufferState {
A,
B,
}
Expand All @@ -74,6 +74,9 @@ enum State {
/// If events are not handled by the end of the frame after they are updated, they will be
/// dropped silently.
///
/// If you require more complex event management, consider working with `ResMut<Events>` directly,
/// and using a [ManualEventReader] to track which events your system has seen.
///
/// # Example
/// ```
/// use bevy_ecs::event::Events;
Expand Down Expand Up @@ -124,7 +127,7 @@ pub struct Events<T> {
a_start_event_count: usize,
b_start_event_count: usize,
event_count: usize,
state: State,
state: DoubleBufferState,
}

impl<T> Default for Events<T> {
Expand All @@ -135,7 +138,7 @@ impl<T> Default for Events<T> {
event_count: 0,
events_a: Vec::new(),
events_b: Vec::new(),
state: State::A,
state: DoubleBufferState::A,
}
}
}
Expand Down Expand Up @@ -173,6 +176,15 @@ impl<'w, 's, T: Resource> EventWriter<'w, 's, T> {
}
}

/// A simple event iterator which can be manually controlled.
///
/// In most cases, an [EventReader] will better meet your needs,
/// as it both tracks which events a system has seen and fetches the relevant data.
/// However, [EventReader] and [EventWriter] have conflicting accesses, and so cannot be used in the same system.
///
/// To get around this, you can store a [ManualEventReader] in a [Local] to help track events seen
/// when working directly with `ResMut<Events<T>>`,
/// allowing you to read and write events from within the same system.
Comment on lines +185 to +187
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Reading this, I find it confusing. Why something named ManualEventReader would work better than using EventReader?. Maybe a code sample in doc could help to show how ManualEventReader works with ResMut<Events<T>> to provide the same functionality as a EventReader and a EventWriter?

pub struct ManualEventReader<T> {
last_event_count: usize,
_marker: PhantomData<T>,
Expand Down Expand Up @@ -222,7 +234,7 @@ fn internal_event_reader<'a, T>(
};
*last_event_count = events.event_count;
match events.state {
State::A => events
DoubleBufferState::A => events
.events_b
.get(b_index..)
.unwrap_or_else(|| &[])
Expand All @@ -236,7 +248,7 @@ fn internal_event_reader<'a, T>(
.iter()
.map(map_instance_event_with_id),
),
State::B => events
DoubleBufferState::B => events
.events_a
.get(a_index..)
.unwrap_or_else(|| &[])
Expand Down Expand Up @@ -283,8 +295,8 @@ impl<T: Resource> Events<T> {
let event_instance = EventInstance { event_id, event };

match self.state {
State::A => self.events_a.push(event_instance),
State::B => self.events_b.push(event_instance),
DoubleBufferState::A => self.events_a.push(event_instance),
DoubleBufferState::B => self.events_b.push(event_instance),
}

self.event_count += 1;
Expand All @@ -311,14 +323,14 @@ impl<T: Resource> Events<T> {
/// called once per frame/update.
pub fn update(&mut self) {
match self.state {
State::A => {
DoubleBufferState::A => {
self.events_b.clear();
self.state = State::B;
self.state = DoubleBufferState::B;
self.b_start_event_count = self.event_count;
}
State::B => {
DoubleBufferState::B => {
self.events_a.clear();
self.state = State::A;
self.state = DoubleBufferState::A;
self.a_start_event_count = self.event_count;
}
}
Expand Down Expand Up @@ -355,12 +367,12 @@ impl<T: Resource> Events<T> {

let map = |i: EventInstance<T>| i.event;
match self.state {
State::A => self
DoubleBufferState::A => self
.events_b
.drain(..)
.map(map)
.chain(self.events_a.drain(..).map(map)),
State::B => self
DoubleBufferState::B => self
.events_a
.drain(..)
.map(map)
Expand All @@ -376,8 +388,8 @@ impl<T: Resource> Events<T> {
/// happen after this call and before the next `update()` call will be dropped.
pub fn iter_current_update_events(&self) -> impl DoubleEndedIterator<Item = &T> {
match self.state {
State::A => self.events_a.iter().map(map_instance_event),
State::B => self.events_b.iter().map(map_instance_event),
DoubleBufferState::A => self.events_a.iter().map(map_instance_event),
DoubleBufferState::B => self.events_b.iter().map(map_instance_event),
}
}
}
Expand All @@ -398,8 +410,8 @@ impl<T> std::iter::Extend<T> for Events<T> {
});

match self.state {
State::A => self.events_a.extend(events),
State::B => self.events_b.extend(events),
DoubleBufferState::A => self.events_a.extend(events),
DoubleBufferState::B => self.events_b.extend(events),
}

trace!(
Expand Down