Add get_change_ticks method to EntityRef and EntityMut (#2539)

Direct access to the change ticks is useful for integrating the reliable change detection with external stuff.

Author: TheRawMeatball

crates/bevy_ecs/src/world/entity_ref.rs

@@ -4,7 +4,7 @@ use crate::{
     change_detection::Ticks,
     component::{Component, ComponentId, ComponentTicks, Components, StorageType},
     entity::{Entities, Entity, EntityLocation},
-    ptr::{OwningPtr, Ptr},
+    ptr::{OwningPtr, Ptr, UnsafeCellDeref},
     storage::{SparseSet, Storages},
     world::{Mut, World},
 };
@@ -72,6 +72,17 @@ impl<'w> EntityRef<'w> {
         }
     }
 
+    /// Retrieves the change ticks for the given component. This can be useful for implementing change
+    /// detection in custom runtimes.
+    #[inline]
+    pub fn get_change_ticks<T: Component>(&self) -> Option<&'w ComponentTicks> {
+        // SAFE: entity location is valid
+        unsafe {
+            get_ticks_with_type(self.world, TypeId::of::<T>(), self.entity, self.location)
+                .map(|ticks| ticks.deref())
+        }
+    }
+
     /// Gets a mutable reference to the component of type `T` associated with
     /// this entity without ensuring there are no other borrows active and without
     /// ensuring that the returned reference will stay valid.
@@ -169,6 +180,17 @@ impl<'w> EntityMut<'w> {
         unsafe { self.get_unchecked_mut::<T>() }
     }
 
+    /// Retrieves the change ticks for the given component. This can be useful for implementing change
+    /// detection in custom runtimes.
+    #[inline]
+    pub fn get_change_ticks<T: Component>(&self) -> Option<&ComponentTicks> {
+        // SAFE: entity location is valid
+        unsafe {
+            get_ticks_with_type(self.world, TypeId::of::<T>(), self.entity, self.location)
+                .map(|ticks| ticks.deref())
+        }
+    }
+
     /// Gets a mutable reference to the component of type `T` associated with
     /// this entity without ensuring there are no other borrows active and without
     /// ensuring that the returned reference will stay valid.
@@ -531,6 +553,31 @@ unsafe fn get_component_and_ticks(
     }
 }
 
+#[inline]
+unsafe fn get_ticks(
+    world: &World,
+    component_id: ComponentId,
+    entity: Entity,
+    location: EntityLocation,
+) -> Option<&UnsafeCell<ComponentTicks>> {
+    let archetype = &world.archetypes[location.archetype_id];
+    let component_info = world.components.get_info_unchecked(component_id);
+    match component_info.storage_type() {
+        StorageType::Table => {
+            let table = &world.storages.tables[archetype.table_id()];
+            let components = table.get_column(component_id)?;
+            let table_row = archetype.entity_table_row(location.index);
+            // SAFE: archetypes only store valid table_rows and the stored component type is T
+            Some(components.get_ticks_unchecked(table_row))
+        }
+        StorageType::SparseSet => world
+            .storages
+            .sparse_sets
+            .get(component_id)
+            .and_then(|sparse_set| sparse_set.get_ticks(entity)),
+    }
+}
+
 // TODO: move to Storages?
 /// Moves component data out of storage.
 ///
@@ -601,6 +648,18 @@ pub(crate) unsafe fn get_component_and_ticks_with_type(
     get_component_and_ticks(world, component_id, entity, location)
 }
 
+/// # Safety
+/// `entity_location` must be within bounds of an archetype that exists.
+pub(crate) unsafe fn get_ticks_with_type(
+    world: &World,
+    type_id: TypeId,
+    entity: Entity,
+    location: EntityLocation,
+) -> Option<&UnsafeCell<ComponentTicks>> {
+    let component_id = world.components.get_id(type_id)?;
+    get_ticks(world, component_id, entity, location)
+}
+
 fn contains_component_with_type(world: &World, type_id: TypeId, location: EntityLocation) -> bool {
     if let Some(component_id) = world.components.get_id(type_id) {
         contains_component_with_id(world, component_id, location)