# 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.
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## Changelog
TODO
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Co-authored-by: Joseph <21144246+JoJoJet@users.noreply.github.com>
