import Foundation

let condition = NSCondition()
let sema = DispatchSemaphore(value: 0)
var writtenStarted = 0
var writtenEnded = 0
var events = [String]()
var lock = NSLock()

func performLocked(_ action: () -> Void) {
    lock.lock()
    action()
    lock.unlock()
}

let concurrent = {
    performLocked {
        print("Started")
        events.append("Started")
    }

    condition.lock()
    writtenStarted += 1
    condition.signal()
    while writtenStarted < 2 {
        condition.wait()
    }
    condition.unlock()

    performLocked {
        print("Ended")
        events.append("Ended")
    }

    condition.lock()
    writtenEnded += 1
    condition.signal()
    while writtenEnded < 2 {
        condition.wait()
    }
    condition.unlock()
    sema.signal()
}

let op1 = BlockOperation(block: concurrent)
let op2 = BlockOperation(block: concurrent)

let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 8

operationQueue.addOperation(op1)
operationQueue.addOperation(op2)
sema.wait()