Implement len and is_empty for EventReaders (#2969)

# Objective

Provide a non-consuming method of checking if there are events in an `EventReader`.

Fixes #2967

## Solution

Implements the `len` and `is_empty` functions for `EventReader` and `ManualEventReader`, giving users the ability to check for the presence of new events without consuming any.

crates/bevy_ecs/src/event.rs

@@ -192,7 +192,8 @@ impl<T> Default for ManualEventReader<T> {
  }
 }
 
-impl<T> ManualEventReader<T> {
+#[allow(clippy::len_without_is_empty)] // Check fails since the is_empty implementation has a signature other than `(&self) -> bool`
+impl<T: Resource> ManualEventReader<T> {
  /// See [`EventReader::iter`]
  pub fn iter<'a>(&'a mut self, events: &'a Events<T>) -> impl DoubleEndedIterator<Item = &'a T> {
  internal_event_reader(&mut self.last_event_count, events).map(|(e, _)| e)
@@ -205,6 +206,16 @@ impl<T> ManualEventReader<T> {
  ) -> impl DoubleEndedIterator<Item = (&'a T, EventId<T>)> {
  internal_event_reader(&mut self.last_event_count, events)
  }
+
+ /// See [`EventReader::len`]
+ pub fn len(&self, events: &Events<T>) -> usize {
+ events.event_reader_len(self.last_event_count)
+ }
+
+ /// See [`EventReader::is_empty`]
+ pub fn is_empty(&self, events: &Events<T>) -> bool {
+ self.len(events) == 0
+ }
 }
 
 /// Like [`iter_with_id`](EventReader::iter_with_id) except not emitting any traces for read
@@ -253,6 +264,16 @@ impl<'w, 's, T: Resource> EventReader<'w, 's, T> {
  (event, id)
  })
  }
+
+ /// Determines the number of events available to be read from this EventReader without consuming any.
+ pub fn len(&self) -> usize {
+ self.events.event_reader_len(self.last_event_count.0)
+ }
+
+ /// Determines if are any events available to be read without consuming any.
+ pub fn is_empty(&self) -> bool {
+ self.len() == 0
+ }
 }
 
 impl<T: Resource> Events<T> {
@@ -365,6 +386,29 @@ impl<T: Resource> Events<T> {
  State::B => self.events_b.iter().map(map_instance_event),
  }
  }
+
+ /// Determines how many events are in the reader after the given `last_event_count` parameter
+ fn event_reader_len(&self, last_event_count: usize) -> usize {
+ let a_count = if last_event_count <= self.a_start_event_count {
+ self.events_a.len()
+ } else {
+ self.events_a
+ .len()
+ .checked_sub(last_event_count - self.a_start_event_count)
+ .unwrap_or_default()
+ };
+
+ let b_count = if last_event_count <= self.b_start_event_count {
+ self.events_b.len()
+ } else {
+ self.events_b
+ .len()
+ .checked_sub(last_event_count - self.b_start_event_count)
+ .unwrap_or_default()
+ };
+
+ a_count + b_count
+ }
 }
 
 impl<T> std::iter::Extend<T> for Events<T> {
@@ -566,4 +610,46 @@ mod tests {
  events.update();
  assert!(events.is_empty());
  }
+
+ #[test]
+ fn test_event_reader_len_empty() {
+ let events = Events::<TestEvent>::default();
+ assert_eq!(events.get_reader().len(&events), 0);
+ assert!(events.get_reader().is_empty(&events));
+ }
+
+ #[test]
+ fn test_event_reader_len_filled() {
+ let mut events = Events::<TestEvent>::default();
+ events.send(TestEvent { i: 0 });
+ assert_eq!(events.get_reader().len(&events), 1);
+ assert!(!events.get_reader().is_empty(&events));
+ }
+
+ #[test]
+ fn test_event_reader_len_current() {
+ let mut events = Events::<TestEvent>::default();
+ events.send(TestEvent { i: 0 });
+ let reader = events.get_reader_current();
+ assert!(reader.is_empty(&events));
+ events.send(TestEvent { i: 0 });
+ assert_eq!(reader.len(&events), 1);
+ assert!(!reader.is_empty(&events));
+ }
+
+ #[test]
+ fn test_event_reader_len_update() {
+ let mut events = Events::<TestEvent>::default();
+ events.send(TestEvent { i: 0 });
+ events.send(TestEvent { i: 0 });
+ let reader = events.get_reader();
+ assert_eq!(reader.len(&events), 2);
+ events.update();
+ events.send(TestEvent { i: 0 });
+ assert_eq!(reader.len(&events), 3);
+ events.update();
+ assert_eq!(reader.len(&events), 1);
+ events.update();
+ assert!(reader.is_empty(&events));
+ }
 }