From 081b3d869d5ce6f0f790f61c870c20a6fcaa515b Mon Sep 17 00:00:00 2001
From: Aceeri <conmcclusk@gmail.com>
Date: Wed, 17 Aug 2022 20:28:25 -0700
Subject: [PATCH 1/3] Warning message for missing events

---
 crates/bevy_ecs/src/event.rs | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_ecs/src/event.rs b/crates/bevy_ecs/src/event.rs
index 04a7e6d74906e..a8d52b075f76b 100644
--- a/crates/bevy_ecs/src/event.rs
+++ b/crates/bevy_ecs/src/event.rs
@@ -2,7 +2,7 @@
 
 use crate as bevy_ecs;
 use crate::system::{Local, Res, ResMut, Resource, SystemParam};
-use bevy_utils::tracing::trace;
+use bevy_utils::tracing::{trace, warn};
 use std::ops::{Deref, DerefMut};
 use std::{
     fmt::{self},
@@ -149,6 +149,14 @@ impl<E: Event> Default for Events<E> {
     }
 }
 
+impl<E: Event> Events<E> {
+    pub fn oldest_event_count(&self) -> usize {
+        self.events_a
+            .start_event_count
+            .min(self.events_b.start_event_count)
+    }
+}
+
 #[derive(Debug)]
 struct EventSequence<E: Event> {
     events: Vec<EventInstance<E>>,
@@ -351,10 +359,18 @@ impl<E: Event> ManualEventReader<E> {
            + ExactSizeIterator<Item = (&'a E, EventId<E>)> {
         // if the reader has seen some of the events in a buffer, find the proper index offset.
         // otherwise read all events in the buffer
+        let missed = self.missed_events(events);
+        if missed > 0 {
+            let plural = if missed == 1 { "event" } else { "events" };
+            let type_name = std::any::type_name::<E>();
+            warn!("Missed {missed} `{type_name}` {plural}. Consider reading from the `EventReader` more often or updating the `Events` to update less. This is most likely due to run criteria/fixed timesteps or consuming events conditionally.");
+        }
+
         let a_index = (self.last_event_count).saturating_sub(events.events_a.start_event_count);
         let b_index = (self.last_event_count).saturating_sub(events.events_b.start_event_count);
         let a = events.events_a.get(a_index..).unwrap_or_default();
         let b = events.events_b.get(b_index..).unwrap_or_default();
+
         let unread_count = a.len() + b.len();
         // Ensure `len` is implemented correctly
         debug_assert_eq!(unread_count, self.len(events));
@@ -379,6 +395,11 @@ impl<E: Event> ManualEventReader<E> {
             .min(events.len())
     }
 
+    /// Amount of events we missed.
+    pub fn missed_events(&self, events: &Events<E>) -> usize {
+        events.oldest_event_count() - self.last_event_count
+    }
+
     /// See [`EventReader::is_empty`]
     pub fn is_empty(&self, events: &Events<E>) -> bool {
         self.len(events) == 0

From 90a619ea5ca33f258437131c833722312b00053e Mon Sep 17 00:00:00 2001
From: Aceeri <conmcclusk@gmail.com>
Date: Wed, 17 Aug 2022 20:39:01 -0700
Subject: [PATCH 2/3] Saturating sub for missed events

---
 crates/bevy_ecs/src/event.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_ecs/src/event.rs b/crates/bevy_ecs/src/event.rs
index a8d52b075f76b..54a482ac129b6 100644
--- a/crates/bevy_ecs/src/event.rs
+++ b/crates/bevy_ecs/src/event.rs
@@ -397,7 +397,9 @@ impl<E: Event> ManualEventReader<E> {
 
     /// Amount of events we missed.
     pub fn missed_events(&self, events: &Events<E>) -> usize {
-        events.oldest_event_count() - self.last_event_count
+        events
+            .oldest_event_count()
+            .saturating_sub(self.last_event_count)
     }
 
     /// See [`EventReader::is_empty`]

From 3abcfd81db862bf2588cc3e762247dfad0d274f6 Mon Sep 17 00:00:00 2001
From: Carter Anderson <mcanders1@gmail.com>
Date: Mon, 29 Aug 2022 19:12:41 -0700
Subject: [PATCH 3/3] clarify warning

---
 crates/bevy_ecs/src/event.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crates/bevy_ecs/src/event.rs b/crates/bevy_ecs/src/event.rs
index 54a482ac129b6..47972a4fa46ee 100644
--- a/crates/bevy_ecs/src/event.rs
+++ b/crates/bevy_ecs/src/event.rs
@@ -363,7 +363,7 @@ impl<E: Event> ManualEventReader<E> {
         if missed > 0 {
             let plural = if missed == 1 { "event" } else { "events" };
             let type_name = std::any::type_name::<E>();
-            warn!("Missed {missed} `{type_name}` {plural}. Consider reading from the `EventReader` more often or updating the `Events` to update less. This is most likely due to run criteria/fixed timesteps or consuming events conditionally.");
+            warn!("Missed {missed} `{type_name}` {plural}. Consider reading from the `EventReader` more often (generally the best solution) or calling Events::update() less frequently (normally this is called once per frame). This problem is most likely due to run criteria/fixed timesteps or consuming events conditionally. See the Events documentation for more information.");
         }
 
         let a_index = (self.last_event_count).saturating_sub(events.events_a.start_event_count);