mirror of
https://github.com/bevyengine/bevy
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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> {}
```
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|---|---|---|
| .. | ||
| entity_ref_mut_lifetime_safety.rs | ||
| entity_ref_mut_lifetime_safety.stderr | ||
| query_exact_sized_iterator_safety.rs | ||
| query_exact_sized_iterator_safety.stderr | ||
| query_iter_combinations_mut_iterator_safety.rs | ||
| query_iter_combinations_mut_iterator_safety.stderr | ||
| query_iter_many_mut_iterator_safety.rs | ||
| query_iter_many_mut_iterator_safety.stderr | ||
| query_lifetime_safety.rs | ||
| query_lifetime_safety.stderr | ||
| query_to_readonly.rs | ||
| query_to_readonly.stderr | ||
| system_param_derive_readonly.rs | ||
| system_param_derive_readonly.stderr | ||
| system_query_get_lifetime_safety.rs | ||
| system_query_get_lifetime_safety.stderr | ||
| system_query_get_many_lifetime_safety.rs | ||
| system_query_get_many_lifetime_safety.stderr | ||
| system_query_get_many_mut_lifetime_safety.rs | ||
| system_query_get_many_mut_lifetime_safety.stderr | ||
| system_query_iter_lifetime_safety.rs | ||
| system_query_iter_lifetime_safety.stderr | ||
| system_query_iter_many_mut_lifetime_safety.rs | ||
| system_query_iter_many_mut_lifetime_safety.stderr | ||
| system_query_set_get_lifetime_safety.rs | ||
| system_query_set_get_lifetime_safety.stderr | ||
| system_query_set_iter_lifetime_safety.rs | ||
| system_query_set_iter_lifetime_safety.stderr | ||
| system_state_get_lifetime_safety.rs | ||
| system_state_get_lifetime_safety.stderr | ||
| system_state_iter_lifetime_safety.rs | ||
| system_state_iter_lifetime_safety.stderr | ||
| system_state_iter_mut_overlap_safety.rs | ||
| system_state_iter_mut_overlap_safety.stderr | ||
| world_query_derive.rs | ||
| world_query_derive.stderr | ||