# Objective
- `Parallel::drain()` has an unused type parameter `B` than can be
removed.
- Caught [on
Discord](https://discord.com/channels/691052431525675048/692572690833473578/1259004180560085003)
by Andrew, thanks!
## Solution
- Remove it! :)
## Testing
- `Parallel::drain()` should still function exactly the same.
---
## Changelog
- Removed unused type parameter in `Parallel::drain()`.
## Migration Guide
The type parameter of `Parallel::drain()` was unused, so it is now
removed. If you were manually specifying it, you can remove the bounds.
```rust
// 0.14
// Create a `Parallel` and give it a value.
let mut parallel: Parallel<Vec<u8>> = Parallel::default();
*parallel.borrow_local_mut() = vec![1, 2, 3];
for v in parallel.drain::<u8>() {
// ...
}
// 0.15
let mut parallel: Parallel<Vec<u8>> = Parallel::default();
*parallel.borrow_local_mut() = vec![1, 2, 3];
// Remove the type parameter.
for v in parallel.drain() {
// ...
}
```
# Objective
- bevy usually use `Parallel::scope` to collect items from `par_iter`,
but `scope` will be called with every satifified items. it will cause a
lot of unnecessary lookup.
## Solution
- similar to Rayon ,we introduce `for_each_init` for `par_iter` which
only be invoked when spawn a task for a group of items.
---
## Changelog
- added `for_each_init`
## Performance
`check_visibility ` in `many_foxes `
~40% performance gain in `check_visibility`.
---------
Co-authored-by: James Liu <contact@jamessliu.com>
# Objective
There's a repeating pattern of `ThreadLocal<Cell<Vec<T>>>` which is very
useful for low overhead, low contention multithreaded queues that have
cropped up in a few places in the engine. This pattern is surprisingly
useful when building deferred mutation across multiple threads, as noted
by it's use in `ParallelCommands`.
However, `ThreadLocal<Cell<Vec<T>>>` is not only a mouthful, it's also
hard to ensure the thread-local queue is replaced after it's been
temporarily removed from the `Cell`.
## Solution
Wrap the pattern into `bevy_utils::Parallel<T>` which codifies the
entire pattern and ensures the user follows the contract. Instead of
fetching indivdual cells, removing the value, mutating it, and replacing
it, `Parallel::get` returns a `ParRef<'a, T>` which contains the
temporarily removed value and a reference back to the cell, and will
write the mutated value back to the cell upon being dropped.
I would like to use this to simplify the remaining part of #4899 that
has not been adopted/merged.
---
## Changelog
TODO
---------
Co-authored-by: Joseph <21144246+JoJoJet@users.noreply.github.com>
