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TraitPublishers-AsOperation.md

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TraitPublishers.AsOperation

TraitPublishers.AsOperation is a publisher that wraps the upstream single publisher in a Foundation Operation.

struct AsOperation<Upstream: SinglePublisher>: SinglePublisher {
    let upstream: Upstream
    let operationQueue: OperationQueue
    let queuePriority: Operation.QueuePriority
    
    /// Creates an `AsOperation` publisher
    init(
        upstream: Upstream,
        operationQueue: OperationQueue,
        queuePriority: Operation.QueuePriority)
}

Usage:

let operationQueue = OperationQueue()
let upstreamPublisher = ... // some single publisher
let publisher = upstreamPublisher.asOperation(in: operationQueue)
let publisher = upstreamPublisher.asOperation(in: operationQueue, queuePriority: .normal)

When it is subscribed, the publisher creates and schedules a new Operation in the OperationQueue. The subscription completes with the operation, when the uptream publisher completes.

Use subscribe(on:options:) when you need to control when the upstream publisher is subscribed:

let publisher = upstreamPublisher
    .subscribe(on: DispatchQueue.main)
    .asOperation(in: queue)

Use receive(on:options:) when you need to control when the returned publisher publishes its element and completion:

let publisher = upstreamPublisher
    .asOperation(in: queue)
    .receive(on: DispatchQueue.main)

When the operation queue has a maxConcurrentOperationCount of 1, TraitPublishers.AsOperation makes it possible to serialize publishers subscriptions.

See also SinglePublisherOperation.