Add new and map methods to Ref

crates/bevy_ecs/src/change_detection.rs

@@ -542,6 +542,44 @@ impl<'a, T: ?Sized> Ref<'a, T> {
     pub fn into_inner(self) -> &'a T {
         self.value
     }
+    /// Map `Ref` to a different type using `f`.
+    ///
+    /// This doesn't do anything else than call `f` on the wrapped value.
+    /// This is equivalent to [`Mut::map_unchanged`].
+    pub fn map<U>(self, f: impl FnOnce(&T) -> &U) -> Ref<'a, U> {
+        Ref {
+            value: f(self.value),
+            ticks: self.ticks,
+        }
+    }
+    /// Create a new `Ref` using provided values.
+    ///
+    /// This is an advanced feature, `Ref`s are designed to be _created_ by
+    /// engine-internal code and _consumed_ by end-user code.
+    ///
+    /// - `value` - The value wrapped by `Ref`.
+    /// - `added` - A [`Tick`] that stores the tick when the wrapped value was created.
+    /// - `changed` - A [`Tick`] that stores the last time the wrapped value was changed.
+    /// - `last_run` - A [`Tick`], occurring before `this_run`, which is used
+    ///    as a reference to determine whether the wrapped value is newly added or changed.
+    /// - `this_run` - A [`Tick`] corresponding to the current point in time -- "now".
+    pub fn new(
+        value: &'a T,
+        added: &'a Tick,
+        changed: &'a Tick,
+        last_run: Tick,
+        this_run: Tick,
+    ) -> Ref<'a, T> {
+        Ref {
+            value,
+            ticks: Ticks {
+                added,
+                changed,
+                last_run,
+                this_run,
+            },
+        }
+    }
 }
 
 impl<'w, 'a, T> IntoIterator for &'a Ref<'w, T>