From dfdc9f8369a0e7dd7e6f78a950f687a5c3822c39 Mon Sep 17 00:00:00 2001
From: Testare <Testare.i9z@gmail.com>
Date: Sun, 1 Oct 2023 17:53:12 -0700
Subject: [PATCH] `as_deref_mut()` method for Mut-like types (#9912)

# Objective

Add a new method so you can do `set_if_neq` with dereferencing
components: `as_deref_mut()`!

## Solution

Added an as_deref_mut method so that we can use `set_if_neq()` without
having to wrap up types for derefencable components

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Joseph <21144246+JoJoJet@users.noreply.github.com>
---
 crates/bevy_ecs/src/change_detection.rs | 49 +++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/crates/bevy_ecs/src/change_detection.rs b/crates/bevy_ecs/src/change_detection.rs
index 0db970ba58..1c5697d337 100644
--- a/crates/bevy_ecs/src/change_detection.rs
+++ b/crates/bevy_ecs/src/change_detection.rs
@@ -381,6 +381,15 @@ macro_rules! impl_methods {
                     ticks: self.ticks,
                 }
             }
+
+            /// Allows you access to the dereferenced value of this pointer without immediately
+            /// triggering change detection.
+            pub fn as_deref_mut(&mut self) -> Mut<'_, <$target as Deref>::Target>
+                where $target: DerefMut
+            {
+                self.reborrow().map_unchanged(|v| v.deref_mut())
+            }
+
         }
     };
 }
@@ -907,6 +916,7 @@ mod tests {
     use bevy_ecs_macros::Resource;
     use bevy_ptr::PtrMut;
     use bevy_reflect::{FromType, ReflectFromPtr};
+    use std::ops::{Deref, DerefMut};
 
     use crate::{
         self as bevy_ecs,
@@ -929,6 +939,19 @@ mod tests {
     #[derive(Resource, PartialEq)]
     struct R2(u8);
 
+    impl Deref for R2 {
+        type Target = u8;
+        fn deref(&self) -> &u8 {
+            &self.0
+        }
+    }
+
+    impl DerefMut for R2 {
+        fn deref_mut(&mut self) -> &mut u8 {
+            &mut self.0
+        }
+    }
+
     #[test]
     fn change_expiration() {
         fn change_detected(query: Query<Ref<C>>) -> bool {
@@ -1143,6 +1166,32 @@ mod tests {
         );
     }
 
+    #[test]
+    fn as_deref_mut() {
+        let mut world = World::new();
+
+        world.insert_resource(R2(0));
+        // Resources are Changed when first added
+        world.increment_change_tick();
+        // This is required to update world::last_change_tick
+        world.clear_trackers();
+
+        let mut r = world.resource_mut::<R2>();
+        assert!(!r.is_changed(), "Resource must begin unchanged.");
+
+        let mut r = r.as_deref_mut();
+        assert!(
+            !r.is_changed(),
+            "Dereferencing should not mark the item as changed yet"
+        );
+
+        r.set_if_neq(3);
+        assert!(
+            r.is_changed(),
+            "Resource must be changed after setting to a different value."
+        );
+    }
+
     #[test]
     fn mut_untyped_to_reflect() {
         let last_run = Tick::new(2);