https://github.com/groue/Semaphore

A Synchronization Primitive for Swift Concurrency
https://github.com/groue/Semaphore

async await concurrency counting-semaphore semaphore

Last synced: 4 months ago
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A Synchronization Primitive for Swift Concurrency

• Host: GitHub
• URL: https://github.com/groue/Semaphore
• Owner: groue
• License: mit
• Created: 2022-10-01T10:04:09.000Z (over 1 year ago)
• Default Branch: main
• Last Pushed: 2023-03-30T06:22:40.000Z (over 1 year ago)
• Last Synced: 2024-02-16T13:01:24.012Z (4 months ago)
• Topics: async, await, concurrency, counting-semaphore, semaphore
• Language: Swift
• Homepage:
• Size: 35.2 KB
• Stars: 385
• Watchers: 9
• Forks: 13
• Open Issues: 1
• Metadata Files:
  • Readme: README.md
  • Funding: .github/FUNDING.yml
  • License: LICENSE

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README

        
# Semaphore

**A Synchronization Primitive for Swift Concurrency**

**Requirements**: iOS 13.0+ / macOS 10.15+ / tvOS 13.0+ / watchOS 6.0+ • Swift 5.7+ / Xcode 14+

📖 **[Documentation](https://swiftpackageindex.com/groue/Semaphore/documentation)**

---

This package provides `AsyncSemaphore`, a [traditional counting semaphore](https://en.wikipedia.org/wiki/Semaphore_(programming)).

Unlike [`DispatchSemaphore`], it does not block any thread. Instead, Swift concurrency tasks are suspended "awaiting" for the semaphore.

### Usage

You can use a semaphore to suspend a task and resume it later:

```swift

let semaphore = AsyncSemaphore(value: 0)

Task {

  // Suspends the task until a signal occurs.

  await semaphore.wait()

  await doSomething()

}

// Resumes the suspended task.

semaphore.signal()

```

An actor can use a semaphore so that its methods can't run concurrently, avoiding the "actor reentrancy problem":

```swift

actor MyActor {

  private let semaphore = AsyncSemaphore(value: 1)

  

  func serializedMethod() async {

    // Makes sure no two tasks can execute

    // serializedMethod() concurrently. 

    await semaphore.wait()

    defer { semaphore.signal() }

    

    await doSomething()

    await doSomethingElse()

  }

}

```

A semaphore can generally limit the number of concurrent accesses to a resource:

```swift

class Downloader {

  private let semaphore: AsyncSemaphore

  /// Creates a Downloader that can run at most

  /// `maxDownloadCount` concurrent downloads. 

  init(maxDownloadCount: Int) {

    semaphore = AsyncSemaphore(value: maxDownloadCount) 

  }

  func download(...) async throws -> Data {

    try await semaphore.waitUnlessCancelled()

    defer { semaphore.signal() }

    return try await ...

  }

}

```

You can see in the latest example that the `wait()` method has a `waitUnlessCancelled` variant that throws `CancellationError` if the task is cancelled before a signal occurs.

For a nice introduction to semaphores, see [The Beauty of Semaphores in Swift 🚦](https://medium.com/@roykronenfeld/semaphores-in-swift-e296ea80f860). The article discusses [`DispatchSemaphore`], but it can easily be ported to Swift concurrency: get inspiration from the above examples. 

[`DispatchSemaphore`]: https://developer.apple.com/documentation/dispatch/dispatchsemaphore