[Merged by Bors] - Added the ability to get or set the last change tick of a system.

crates/bevy_ecs/src/system/function_system.rs

@@ -406,6 +406,14 @@ where
         out
     }
 
+    fn get_last_change_tick(&self) -> u32 {
+        self.system_meta.last_change_tick
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.system_meta.last_change_tick = last_change_tick;
+    }
+
     #[inline]
     fn apply_buffers(&mut self, world: &mut World) {
         let param_state = self.param_state.as_mut().expect(Self::PARAM_MESSAGE);

crates/bevy_ecs/src/system/system.rs

@@ -31,6 +31,7 @@ pub trait System: Send + Sync + 'static {
     fn archetype_component_access(&self) -> &Access<ArchetypeComponentId>;
     /// Returns true if the system is [`Send`].
     fn is_send(&self) -> bool;
+
     /// Runs the system with the given input in the world. Unlike [`System::run`], this function
     /// takes a shared reference to [`World`] and may therefore break Rust's aliasing rules, making
     /// it unsafe to call.
@@ -59,6 +60,14 @@ pub trait System: Send + Sync + 'static {
     fn default_labels(&self) -> Vec<SystemLabelId> {
         Vec::new()
     }
+    /// Gets the system's last change tick
+    fn get_last_change_tick(&self) -> u32;
+    /// Sets the system's last change tick
+    /// # Warning
+    /// This is a complex and error-prone operation, that can have unexpected consequences on any system relying on this code.
+    /// However, it can be an essential escape hatch when, for example,
+    /// you are trying to synchronize representations using change detection and need to avoid infinite recursion.
+    fn set_last_change_tick(&mut self, last_change_tick: u32);
 }
 
 /// A convenience type alias for a boxed [`System`] trait object.

crates/bevy_ecs/src/system/system_chaining.rs

@@ -106,6 +106,15 @@ impl<SystemA: System, SystemB: System<In = SystemA::Out>> System for ChainSystem
         self.system_a.check_change_tick(change_tick);
         self.system_b.check_change_tick(change_tick);
     }
+
+    fn get_last_change_tick(&self) -> u32 {
+        self.system_a.get_last_change_tick()
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.system_a.set_last_change_tick(last_change_tick);
+        self.system_b.set_last_change_tick(last_change_tick);
+    }
 }
 
 /// An extension trait providing the [`IntoChainSystem::chain`] method for convenient [`System`]

crates/bevy_time/src/fixed_timestep.rs

@@ -224,6 +224,14 @@ impl System for FixedTimestep {
     fn check_change_tick(&mut self, change_tick: u32) {
         self.internal_system.check_change_tick(change_tick);
     }
+
+    fn get_last_change_tick(&self) -> u32 {
+        self.internal_system.get_last_change_tick()
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.internal_system.set_last_change_tick(last_change_tick);
+    }
 }
 
 #[cfg(test)]