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.


Co-authored-by: Carter Anderson <mcanders1@gmail.com>

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> {
@@ -355,7 +376,7 @@ impl<T: Resource> Events<T> {
 
     /// Iterates over events that happened since the last "update" call.
     /// WARNING: You probably don't want to use this call. In most cases you should use an
-    /// `EventReader`. You should only use this if you know you only need to consume events
+    /// [`EventReader`]. You should only use this if you know you only need to consume events
     /// between the last `update()` call and your call to `iter_current_update_events`.
     /// If events happen outside that window, they will not be handled. For example, any events that
     /// happen after this call and before the next `update()` call will be dropped.
@@ -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));
+    }
 }