Hands-On Design Patterns with Swift

Example of a counting semaphore

In the following example, we'll use a semaphore to suspend the execution of the current thread until a long-running task has completed:

let semaphore = DispatchSemaphore(value: 0)let queue = DispatchQueue(label: "com.run.concurrent",                          attributes: .concurrent)// Run a block 1 second of the futurequeue.asyncAfter(deadline: DispatchTime.now() + 1) {    print("Will Signal")    semaphore.signal()    print("Did Signal")}print("Will Wait")// wait for the semaphore, this suspends the current threadsemaphore.wait()print("Done")

This will give us the following output:

Will WaitWill SignalDid SignalDone

As you can see, this code successfully waited till the long-running operation was done, and signal() has been called on the semaphore. ...