信号量处理事件并发 用来控制资源被多任务访问的情况 亲测有效

let semaphore = DispatchSemaphore(value: 1) 初始化信号量为1

semaphore.wait()  信号量-1 当前信号量为0 所有操作处于等待状态 不再执行

semaphore.signal() 当前信号量为1 所有操作处于可执行状态 开始执行

        let queue = DispatchQueue.global()
        let group = DispatchGroup()
        let semaphore = DispatchSemaphore(value: 1)
        
        queue.async(group: group) {
            semaphore.wait()
            self.semaphoreTask(label: "1", cost: 2, complete: {
                semaphore.signal()
            })
        }
        queue.async(group: group) {
            semaphore.wait()
            self.semaphoreTask(label: "2", cost: 2, complete: {
                semaphore.signal()
            })
        }
        queue.async(group: group) {
            semaphore.wait()
            self.semaphoreTask(label: "3", cost: 2, complete: {
                semaphore.signal()
            })
        }
        
        group.notify(queue: queue) {
            print("All task has finished")
        }

延迟方法

public func semaphoreTask(label:String, cost:UInt32, complete:@escaping ()->()){
        print("Start semaphore task \(label)")
        sleep(cost)
        print("End semaphore task  \(label)")
        complete()
    }

日志输出

Start semaphore task 1
End semaphore task 1
Start semaphore task 2
End semaphore task 2
Start semaphore task 3
End semaphore task 3
All task has finished