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More methods to OnceCell #77
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I'm wondering if it's worth implementing a
get_mut_or_init. This method should accept a mutable reference so that it cannot be called concurrently.A typical use case for modifying a mutable reference is calling
get_or_initon anOnceCellwhose value is something likeArc<Mutex<T>>so that it can be modified concurrently.#[test] fn multi_init_mut() { let rt = tokio::runtime::Builder::new_multi_thread() .worker_threads(4) .enable_time() .build() .unwrap(); static CELL: OnceCell<AtomicUsize> = OnceCell::new(); rt.block_on(async { const N: usize = 100; let mut handles = vec![]; for i in 0..N { handles.push(rt.spawn(async move { let result = CELL .get_or_init(move || async move { AtomicUsize::new(1000 + i) }) .await; result.fetch_add(1, Ordering::Relaxed); tokio::time::sleep(Duration::from_millis(100)).await; result.load(Ordering::Relaxed) })); } let mut values = vec![]; for handle in handles { values.push(handle.await.unwrap()); } let value = CELL.get().unwrap().load(Ordering::Relaxed); assert!( value >= 1000 + N && value < 1000 + N + N, "final value: {}", value ); for v in values { assert_eq!(v, value); } }); }
You can see the linked issue.
While async primitives can be used to share cross threads, our OnceCell can be used as an analogy to std OnceLock while its pass-in function is an async fn.
Thus, this can be used as a cell that initialized only once and need not shared among threads.
Okay, it makes sense
get_or_try_initget_mutget_mut_or_initget_mut_or_try_initsettry_inserttakeYou can refer to:
Also, it's possible to add a
LazyCellprimitive that has a pre-configured initializer (closure).