Semaphore

A Synchronization Primitive for Swift Concurrency

Requirements: iOS 13.0+ / macOS 10.15+ / tvOS 13.0+ / watchOS 6.0+ • Swift 5.10+ / Xcode 15.3+

📖 Documentation

This package provides AsyncSemaphore, a traditional counting semaphore.

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:

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”:

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:

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 🚦. The article discusses DispatchSemaphore, but it can easily be ported to Swift concurrency: get inspiration from the above examples.