串行与并发调度队列

Swift 3

串行队列

func serialQueues () {
    let serialQueue = DispatchQueue(label: "com.example.serial") //default queue type is a serial queue
    let start = Date ()
    for i in 0...3 {                                             //launch a bunch of tasks
        serialQueue.async {                                      //run tasks on a background thread, using our serial queue
            sleep(2)                                             //do some long task eg webservice or database lookup
            let timeTaken = Date().timeIntervalSince(start)
            print("serial long task \(i) done! total time taken: \(timeTaken)")
        }
    }
}

输出示例：

serial long task 0 done! total time taken: 2.07241100072861
serial long task 1 done! total time taken: 4.16347700357437
serial long task 2 done! total time taken: 6.23209798336029
serial long task 3 done! total time taken: 8.30682599544525

并发队列

func concurrentQueues () {
    let concurrentQueue = DispatchQueue(label: "com.example.concurrent", attributes: .concurrent) //explicitly specify the queue to be a concurrent queue
    let start = Date ()
    for i in 0...3 {            //launch a bunch of tasks
        concurrentQueue.async { //run tasks on a background thread, using our concurrent queue
            sleep(2)            //do some long task eg webservice or database lookup
            let timeTaken = Date().timeIntervalSince(start)
            print("concurrent long task \(i) done! total time taken: \(timeTaken)")
        }
    }
}

输出示例：

concurrent long task 3 done! total time taken: 2.07092100381851
concurrent long task 0 done! total time taken: 2.07087397575378
concurrent long task 2 done! total time taken: 2.07086700201035
concurrent long task 1 done! total time taken: 2.07089096307755

讨论

从上面的示例中可以看出，串行队列将按照提交到队列的顺序完成每个任务。每个任务将在执行前等待上一个任务完成。对于并发队列，每个任务不等待队列中的其他任务并尽快执行; 优点是队列上的所有任务将在不同的线程上同时运行，这使得并发队列比串行队列花费的时间更少。

如果任务执行顺序不重要，请始终使用并发队列以获得最佳效率。