Add an example demonstrating how to send and receive events in the same system (#11574)

# Objective

- Sending and receiving events of the same type in the same system is a
reasonably common need, generally due to event filtering.
- However, actually doing so is non-trivial, as the borrow checker
simultaneous hates mutable and immutable access.

## Solution

- Demonstrate two sensible patterns for doing so.
- Update the `ManualEventReader` docs to be more clear and link to this
example.

---------

Co-authored-by: Alice Cecile <alice.i.cecil@gmail.com>
Co-authored-by: Joona Aalto <jondolf.dev@gmail.com>
Co-authored-by: ickk <git@ickk.io>

Cargo.toml

@@ -1402,6 +1402,17 @@ description = "Illustrates event creation, activation, and reception"
 category = "ECS (Entity Component System)"
 wasm = false
 
+[[example]]
+name = "send_and_receive_events"
+path = "examples/ecs/send_and_receive_events.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.send_and_receive_events]
+name = "Send and receive events"
+description = "Demonstrates how to send and receive events of the same type in a single system"
+category = "ECS (Entity Component System)"
+wasm = false
+
 [[example]]
 name = "fixed_timestep"
 path = "examples/ecs/fixed_timestep.rs"

crates/bevy_ecs/src/event.rs

@@ -556,7 +556,46 @@ impl<'w, E: Event> EventWriter<'w, E> {
 }
 
 /// Stores the state for an [`EventReader`].
+///
 /// Access to the [`Events<E>`] resource is required to read any incoming events.
+///
+/// In almost all cases, you should just use an [`EventReader`],
+/// which will automatically manage the state for you.
+///
+/// However, this type can be useful if you need to manually track events,
+/// such as when you're attempting to send and receive events of the same type in the same system.
+///
+/// # Example
+///
+/// ```
+/// use bevy_ecs::prelude::*;
+/// use bevy_ecs::event::{Event, Events, ManualEventReader};
+///
+/// #[derive(Event, Clone, Debug)]
+/// struct MyEvent;
+///
+/// /// A system that both sends and receives events using a [`Local`] [`ManualEventReader`].
+/// fn send_and_receive_manual_event_reader(
+///     // The `Local` `SystemParam` stores state inside the system itself, rather than in the world.
+///     // `ManualEventReader<T>` is the internal state of `EventReader<T>`, which tracks which events have been seen.
+///     mut local_event_reader: Local<ManualEventReader<MyEvent>>,
+///     // We can access the `Events` resource mutably, allowing us to both read and write its contents.
+///     mut events: ResMut<Events<MyEvent>>,
+/// ) {
+///     // We must collect the events to resend, because we can't mutate events while we're iterating over the events.
+///     let mut events_to_resend = Vec::new();
+///
+///     for event in local_event_reader.read(&events) {
+///          events_to_resend.push(event.clone());
+///     }
+///
+///     for event in events_to_resend {
+///         events.send(MyEvent);
+///     }
+/// }
+///
+/// # bevy_ecs::system::assert_is_system(send_and_receive_manual_event_reader);
+/// ```
 #[derive(Debug)]
 pub struct ManualEventReader<E: Event> {
     last_event_count: usize,

examples/README.md

@@ -239,6 +239,7 @@ Example | Description
 [Parallel Query](../examples/ecs/parallel_query.rs) | Illustrates parallel queries with `ParallelIterator`
 [Removal Detection](../examples/ecs/removal_detection.rs) | Query for entities that had a specific component removed earlier in the current frame
 [Run Conditions](../examples/ecs/run_conditions.rs) | Run systems only when one or multiple conditions are met
+[Send and receive events](../examples/ecs/send_and_receive_events.rs) | Demonstrates how to send and receive events of the same type in a single system
 [Startup System](../examples/ecs/startup_system.rs) | Demonstrates a startup system (one that runs once when the app starts up)
 [State](../examples/ecs/state.rs) | Illustrates how to use States to control transitioning from a Menu state to an InGame state
 [System Closure](../examples/ecs/system_closure.rs) | Show how to use closures as systems, and how to configure `Local` variables by capturing external state

examples/ecs/send_and_receive_events.rs

@@ -0,0 +1,165 @@
+//! From time to time, you may find that you want to both send and receive an event of the same type in a single system.
+//!
+//! Of course, this results in an error: the borrows of [`EventWriter`] and [`EventReader`] overlap,
+//! if and only if the [`Event`] type is the same.
+//! One system parameter borrows the [`Events`] resource mutably, and another system parameter borrows the [`Events`] resource immutably.
+//! If Bevy allowed this, this would violate Rust's rules against aliased mutability.
+//! In other words, this would be Undefined Behavior (UB)!
+//!
+//! There are two ways to solve this problem:
+//!
+//! 1. Use [`ParamSet`] to check out the [`EventWriter`] and [`EventReader`] one at a time.
+//! 2. Use a [`Local`] [`ManualEventReader`] instead of an [`EventReader`], and use [`ResMut`] to access [`Events`].
+//!
+//! In the first case, you're being careful to only check out only one of the [`EventWriter`] or [`EventReader`] at a time.
+//! By "temporally" seperating them, you avoid the overlap.
+//!
+//! In the second case, you only ever have one access to the underlying  [`Events`] resource at a time.
+//! But in exchange, you have to manually keep track of which events you've already read.
+//!
+//! Let's look at an example of each.
+
+use bevy::core::FrameCount;
+use bevy::ecs::event::ManualEventReader;
+use bevy::prelude::*;
+
+fn main() {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins)
+        .add_event::<DebugEvent>()
+        .add_systems(Update, read_and_write_different_event_types)
+        .add_systems(
+            Update,
+            (
+                send_events,
+                debug_events,
+                send_and_receive_param_set,
+                debug_events,
+                send_and_receive_manual_event_reader,
+                debug_events,
+            )
+                .chain(),
+        );
+    // We're just going to run a few frames, so we can see and understand the output.
+    app.update();
+    // By running for longer than one frame, we can see that we're caching our cursor in the event queue properly.
+    app.update();
+}
+
+#[derive(Event)]
+struct A;
+
+#[derive(Event)]
+struct B;
+
+// This works fine, because the types are different,
+// so the borrows of the `EventWriter` and `EventReader` don't overlap.
+// Note that these borrowing rules are checked at system initialization time,
+// not at compile time, as Bevy uses internal unsafe code to split the `World` into disjoint pieces.
+fn read_and_write_different_event_types(mut a: EventWriter<A>, mut b: EventReader<B>) {
+    for _ in b.read() {}
+    a.send(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>) {
+    info!("Sending events for frame {:?}", *frame_count);
+
+    events.send(DebugEvent {
+        resend_from_param_set: false,
+        resend_from_local_event_reader: false,
+        times_sent: 1,
+    });
+    events.send(DebugEvent {
+        resend_from_param_set: true,
+        resend_from_local_event_reader: false,
+        times_sent: 1,
+    });
+    events.send(DebugEvent {
+        resend_from_param_set: false,
+        resend_from_local_event_reader: true,
+        times_sent: 1,
+    });
+    events.send(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>,
+) {
+    info!(
+        "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().send(event);
+    }
+}
+
+/// A system that both sends and receives events using a [`Local`] [`ManualEventReader`].
+fn send_and_receive_manual_event_reader(
+    // The `Local` `SystemParam` stores state inside the system itself, rather than in the world.
+    // `ManualEventReader<T>` is the internal state of `EventReader<T>`, which tracks which events have been seen.
+    mut local_event_reader: Local<ManualEventReader<DebugEvent>>,
+    // We can access the `Events` resource mutably, allowing us to both read and write its contents.
+    mut events: ResMut<Events<DebugEvent>>,
+    frame_count: Res<FrameCount>,
+) {
+    info!(
+        "Sending and receiving events for frame {} with a `Local<ManualEventReader>",
+        frame_count.0
+    );
+
+    // We must collect the events to resend, because we can't mutate events while we're iterating over the events.
+    let mut events_to_resend = Vec::new();
+
+    for event in local_event_reader.read(&events) {
+        if event.resend_from_local_event_reader {
+            // For simplicity, we're cloning the event.
+            // In this case, since we have mutable access to the `Events` resource,
+            // we could also just mutate the event in-place,
+            // or drain the event queue into our `events_to_resend` vector.
+            events_to_resend.push(event.clone());
+        }
+    }
+
+    for mut event in events_to_resend {
+        event.times_sent += 1;
+        events.send(event);
+    }
+}