From 96a9d0405a9e93b4f6779b8fd9e04fbb2aa7a161 Mon Sep 17 00:00:00 2001
From: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
Date: Thu, 15 Jun 2023 08:57:47 -0400
Subject: [PATCH] Simplify the `ComponentIdFor` type (#8845)

# Objective

`ComponentIdFor` is a type that gives you access to a component's
`ComponentId` in a system. It is currently awkward to use, since it must
be wrapped in a `Local<>` to be used.

## Solution

Make `ComponentIdFor` a proper SystemParam.

---

## Changelog

- Refactored the type `ComponentIdFor` in order to simplify how it is
used.

## Migration Guide

The type `ComponentIdFor<T>` now implements `SystemParam` instead of
`FromWorld` -- this means it should be used as the parameter for a
system directly instead of being used in a `Local`.

```rust
// Before:
fn my_system(
    component_id: Local<ComponentIdFor<MyComponent>>,
) {
    let component_id = **component_id;
}

// After:
fn my_system(
    component_id: ComponentIdFor<MyComponent>,
) {
    let component_id = component_id.get();
}
```
---
 crates/bevy_ecs/src/component.rs         | 48 ++++++++++++++----------
 crates/bevy_ecs/src/removal_detection.rs |  6 +--
 2 files changed, 31 insertions(+), 23 deletions(-)

diff --git a/crates/bevy_ecs/src/component.rs b/crates/bevy_ecs/src/component.rs
index 27917d5fd6..2e5700f2fd 100644
--- a/crates/bevy_ecs/src/component.rs
+++ b/crates/bevy_ecs/src/component.rs
@@ -1,9 +1,10 @@
 //! Types for declaring and storing [`Component`]s.
 
 use crate::{
+    self as bevy_ecs,
     change_detection::MAX_CHANGE_AGE,
     storage::{SparseSetIndex, Storages},
-    system::{Local, Resource},
+    system::{Local, Resource, SystemParam},
     world::{FromWorld, World},
     TypeIdMap,
 };
@@ -795,47 +796,54 @@ impl ComponentTicks {
     }
 }
 
-/// Initialize and fetch a [`ComponentId`] for a specific type.
+/// A [`SystemParam`] that provides access to the [`ComponentId`] for a specific type.
 ///
 /// # Example
 /// ```rust
 /// # use bevy_ecs::{system::Local, component::{Component, ComponentId, ComponentIdFor}};
 /// #[derive(Component)]
 /// struct Player;
-/// fn my_system(component_id: Local<ComponentIdFor<Player>>) {
-///     let component_id: ComponentId = component_id.into();
+/// fn my_system(component_id: ComponentIdFor<Player>) {
+///     let component_id: ComponentId = component_id.get();
 ///     // ...
 /// }
 /// ```
-pub struct ComponentIdFor<T: Component> {
-    component_id: ComponentId,
-    phantom: PhantomData<T>,
-}
+#[derive(SystemParam)]
+pub struct ComponentIdFor<'s, T: Component>(Local<'s, InitComponentId<T>>);
 
-impl<T: Component> FromWorld for ComponentIdFor<T> {
-    fn from_world(world: &mut World) -> Self {
-        Self {
-            component_id: world.init_component::<T>(),
-            phantom: PhantomData,
-        }
+impl<T: Component> ComponentIdFor<'_, T> {
+    /// Gets the [`ComponentId`] for the type `T`.
+    #[inline]
+    pub fn get(&self) -> ComponentId {
+        **self
     }
 }
 
-impl<T: Component> std::ops::Deref for ComponentIdFor<T> {
+impl<T: Component> std::ops::Deref for ComponentIdFor<'_, T> {
     type Target = ComponentId;
     fn deref(&self) -> &Self::Target {
-        &self.component_id
+        &self.0.component_id
     }
 }
 
-impl<T: Component> From<ComponentIdFor<T>> for ComponentId {
+impl<T: Component> From<ComponentIdFor<'_, T>> for ComponentId {
+    #[inline]
     fn from(to_component_id: ComponentIdFor<T>) -> ComponentId {
         *to_component_id
     }
 }
 
-impl<'s, T: Component> From<Local<'s, ComponentIdFor<T>>> for ComponentId {
-    fn from(to_component_id: Local<ComponentIdFor<T>>) -> ComponentId {
-        **to_component_id
+/// Initializes the [`ComponentId`] for a specific type when used with [`FromWorld`].
+struct InitComponentId<T: Component> {
+    component_id: ComponentId,
+    marker: PhantomData<T>,
+}
+
+impl<T: Component> FromWorld for InitComponentId<T> {
+    fn from_world(world: &mut World) -> Self {
+        Self {
+            component_id: world.init_component::<T>(),
+            marker: PhantomData,
+        }
     }
 }
diff --git a/crates/bevy_ecs/src/removal_detection.rs b/crates/bevy_ecs/src/removal_detection.rs
index 81146371ad..5546f17eac 100644
--- a/crates/bevy_ecs/src/removal_detection.rs
+++ b/crates/bevy_ecs/src/removal_detection.rs
@@ -136,7 +136,7 @@ impl RemovedComponentEvents {
 /// ```
 #[derive(SystemParam)]
 pub struct RemovedComponents<'w, 's, T: Component> {
-    component_id: Local<'s, ComponentIdFor<T>>,
+    component_id: ComponentIdFor<'s, T>,
     reader: Local<'s, RemovedComponentReader<T>>,
     event_sets: &'w RemovedComponentEvents,
 }
@@ -180,7 +180,7 @@ impl<'w, 's, T: Component> RemovedComponents<'w, 's, T> {
 
     /// Fetch underlying [`Events`].
     pub fn events(&self) -> Option<&Events<RemovedComponentEntity>> {
-        self.event_sets.get(**self.component_id)
+        self.event_sets.get(self.component_id.get())
     }
 
     /// Destructures to get a mutable reference to the `ManualEventReader`
@@ -195,7 +195,7 @@ impl<'w, 's, T: Component> RemovedComponents<'w, 's, T> {
         &Events<RemovedComponentEntity>,
     )> {
         self.event_sets
-            .get(**self.component_id)
+            .get(self.component_id.get())
             .map(|events| (&mut *self.reader, events))
     }