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# Objective - `Extract` does not implement `ReadOnlySystemParam` even though it should. - Noticed by @hymm on discord: https://discord.com/channels/691052431525675048/749335865876021248/1063535818267963543 ## Solution Implement the trait.
124 lines
3.7 KiB
Rust
124 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::{ReadOnlySystemParam, SystemMeta, SystemParam, SystemParamItem, SystemState},
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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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#[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: <Res<'static, MainWorld> as SystemParam>::State,
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}
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// SAFETY: The only `World` access (`Res<MainWorld>`) is read-only.
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unsafe impl<P> ReadOnlySystemParam for Extract<'_, '_, P> where P: ReadOnlySystemParam {}
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// SAFETY: The only `World` access is properly registered by `Res<MainWorld>::init_state`.
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unsafe impl<P> SystemParam for Extract<'_, '_, 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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type Item<'w, 's> = Extract<'w, 's, P>;
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fn init_state(world: &mut World, system_meta: &mut SystemMeta) -> Self::State {
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let mut main_world = world.resource_mut::<MainWorld>();
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ExtractState {
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state: SystemState::new(&mut main_world),
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main_world_state: Res::<MainWorld>::init_state(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::State,
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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 = Res::<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> 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> 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> 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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