mirror of
https://github.com/bevyengine/bevy
synced 2024-12-23 11:33:06 +00:00
1af73624fa
# Objective
* Implementing a custom `SystemParam` by hand requires implementing three traits -- four if it is read-only.
* The trait `SystemParamFetch<'w, 's>` is a workaround from before we had generic associated types, and is no longer necessary.
## Solution
* Combine the trait `SystemParamFetch` with `SystemParamState`.
* I decided to remove the `Fetch` name and keep the `State` name, since the former was consistently conflated with the latter.
* Replace the trait `ReadOnlySystemParamFetch` with `ReadOnlySystemParam`, which simplifies trait bounds in generic code.
---
## Changelog
- Removed the trait `SystemParamFetch`, moving its functionality to `SystemParamState`.
- Replaced the trait `ReadOnlySystemParamFetch` with `ReadOnlySystemParam`.
## Migration Guide
The trait `SystemParamFetch` has been removed, and its functionality has been transferred to `SystemParamState`.
```rust
// Before
impl SystemParamState for MyParamState {
fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self { ... }
}
impl<'w, 's> SystemParamFetch<'w, 's> for MyParamState {
type Item = MyParam<'w, 's>;
fn get_param(...) -> Self::Item;
}
// After
impl SystemParamState for MyParamState {
type Item<'w, 's> = MyParam<'w, 's>; // Generic associated types!
fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self { ... }
fn get_param<'w, 's>(...) -> Self::Item<'w, 's>;
}
```
The trait `ReadOnlySystemParamFetch` has been replaced with `ReadOnlySystemParam`.
```rust
// Before
unsafe impl ReadOnlySystemParamFetch for MyParamState {}
// After
unsafe impl<'w, 's> ReadOnlySystemParam for MyParam<'w, 's> {}
```
131 lines
3.7 KiB
Rust
131 lines
3.7 KiB
Rust
use crate::MainWorld;
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use bevy_ecs::{
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prelude::*,
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system::{
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ReadOnlySystemParam, ResState, SystemMeta, SystemParam, SystemParamItem, SystemParamState,
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SystemState,
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},
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};
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use std::ops::{Deref, DerefMut};
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/// A helper for accessing [`MainWorld`] content using a system parameter.
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///
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/// A [`SystemParam`] adapter which applies the contained `SystemParam` to the [`World`]
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/// contained in [`MainWorld`]. This parameter only works for systems run
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/// during [`RenderStage::Extract`].
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///
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/// This requires that the contained [`SystemParam`] does not mutate the world, as it
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/// uses a read-only reference to [`MainWorld`] internally.
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///
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/// ## Context
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///
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/// [`RenderStage::Extract`] is used to extract (move) data from the simulation world ([`MainWorld`]) to the
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/// render world. The render world drives rendering each frame (generally to a [Window]).
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/// This design is used to allow performing calculations related to rendering a prior frame at the same
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/// time as the next frame is simulated, which increases throughput (FPS).
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///
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/// [`Extract`] is used to get data from the main world during [`RenderStage::Extract`].
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///
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/// ## Examples
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///
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/// ```rust
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/// use bevy_ecs::prelude::*;
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/// use bevy_render::Extract;
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/// # #[derive(Component)]
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/// # struct Cloud;
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/// fn extract_clouds(mut commands: Commands, clouds: Extract<Query<Entity, With<Cloud>>>) {
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/// for cloud in &clouds {
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/// commands.get_or_spawn(cloud).insert(Cloud);
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/// }
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/// }
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/// ```
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///
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/// [`RenderStage::Extract`]: crate::RenderStage::Extract
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/// [Window]: bevy_window::Window
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pub struct Extract<'w, 's, P>
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where
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P: ReadOnlySystemParam + 'static,
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{
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item: SystemParamItem<'w, 's, P>,
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}
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impl<'w, 's, P> SystemParam for Extract<'w, 's, P>
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where
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P: ReadOnlySystemParam,
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{
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type State = ExtractState<P>;
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}
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#[doc(hidden)]
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pub struct ExtractState<P: SystemParam + 'static> {
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state: SystemState<P>,
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main_world_state: ResState<MainWorld>,
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}
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// SAFETY: only accesses MainWorld resource with read only system params using ResState,
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// which is initialized in init()
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unsafe impl<P: SystemParam + 'static> SystemParamState for ExtractState<P>
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where
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P: ReadOnlySystemParam + 'static,
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{
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type Item<'w, 's> = Extract<'w, 's, P>;
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fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
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let mut main_world = world.resource_mut::<MainWorld>();
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Self {
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state: SystemState::new(&mut main_world),
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main_world_state: ResState::init(world, system_meta),
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}
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}
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unsafe fn get_param<'w, 's>(
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state: &'s mut Self,
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system_meta: &SystemMeta,
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world: &'w World,
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change_tick: u32,
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) -> Self::Item<'w, 's> {
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let main_world = ResState::<MainWorld>::get_param(
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&mut state.main_world_state,
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system_meta,
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world,
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change_tick,
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);
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let item = state.state.get(main_world.into_inner());
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Extract { item }
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}
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}
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impl<'w, 's, P: SystemParam> Deref for Extract<'w, 's, P>
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where
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P: ReadOnlySystemParam,
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{
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type Target = SystemParamItem<'w, 's, P>;
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#[inline]
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fn deref(&self) -> &Self::Target {
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&self.item
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}
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}
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impl<'w, 's, P: SystemParam> DerefMut for Extract<'w, 's, P>
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where
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P: ReadOnlySystemParam,
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{
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#[inline]
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fn deref_mut(&mut self) -> &mut Self::Target {
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&mut self.item
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}
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}
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impl<'a, 'w, 's, P: SystemParam> IntoIterator for &'a Extract<'w, 's, P>
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where
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P: ReadOnlySystemParam,
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&'a SystemParamItem<'w, 's, P>: IntoIterator,
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{
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type Item = <&'a SystemParamItem<'w, 's, P> as IntoIterator>::Item;
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type IntoIter = <&'a SystemParamItem<'w, 's, P> as IntoIterator>::IntoIter;
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fn into_iter(self) -> Self::IntoIter {
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(&self.item).into_iter()
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}
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}
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