GCD

1. dispatch_barrier_async 与dispatch_barrier_sync
   添加栅栏，保证并行队列中 barrier 前面的任务执行完毕，才会执行 barrier 后面的任务
   区别：两者的区别即是 async 与 sync 的区别。

• dispatch_barrier_async 所有的任务全部添加到队列中，barrier 中的队列执行完毕，再执行 barrier 任务，等待 barrier 任务执行完毕，最后执行 barrier 后面的任务(dispatch_barrier_async 结果中 dfafgda 与 ccccc 均先于 ******** barrier ****** 出现)
  dispatch_barrier_async 结果

dfafgda , currentTime: 592539267.981731
ccccc, currentTime: 592539267.981827
ffff 444444, currentTime: 592539267.981756
ffff 111111, currentTime: 592539267.981731
ffff 222222, currentTime: 592539267.981732
ffff 333333, currentTime: 592539267.981731
ffff 111111_bbbbbbbb, currentTime: 592539269.986992
******** barrier ******, currentTime: 592539269.987095
******** barrier_2 ******, currentTime: 592539271.992194
bbbb 5555555, currentTime: 592539271.992281
bbbb 6666666, currentTime: 592539271.992298
bbbb 7777777, currentTime: 592539271.992321
bbbb 8888888, currentTime: 592539271.992325

• dispatch_barrier_sync 先将 barrier 前面的任务添加到队列中，阻塞代码所在的队列，等待这些任务执行完毕，然后添加 barrier 任务(dispatch_barrier_sync 结果中根据时间可以看到 ffff 111111_bbbbbbbb 出现以后，才出现了 ******** barrier ******)，等待 barrier 任务执行完毕，然后运行 barrier 后面的代码(根据时间可以看到 dfafgda 与 ccccc 均在 ******** barrier_2 ****** 以后出现)

private func barrierTest() {
        let queue = DispatchQueue.init(label: "aaa", qos: .default, attributes: .concurrent)
        queue.async {
            print("ffff 111111, currentTime: \(CFAbsoluteTimeGetCurrent())")
            sleep(2)
            print("ffff 111111_bbbbbbbb, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        queue.async {
            print("ffff 222222, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        queue.async {
            print("ffff 333333, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        queue.async {
            print("ffff 444444, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
//        queue.async(qos: .default, flags: .barrier) {
//            print("******** barrier ******, currentTime: \(CFAbsoluteTimeGetCurrent())")
//            sleep(2)
//            print("******** barrier_2 ******, currentTime: \(CFAbsoluteTimeGetCurrent())")
//        }
        queue.sync(flags: .barrier) { () -> Int in
            print("******** barrier ******, currentTime: \(CFAbsoluteTimeGetCurrent())")
            sleep(2)
            print("******** barrier_2 ******, currentTime: \(CFAbsoluteTimeGetCurrent())")
            return 1
        }
        queue.async {
            print("bbbb 5555555, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        print("dfafgda , currentTime: \(CFAbsoluteTimeGetCurrent())")
        queue.async {
            print("bbbb 6666666, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        queue.async {
            print("bbbb 7777777, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        queue.async {
            print("bbbb 8888888, currentTime: \(CFAbsoluteTimeGetCurrent())")
        }
        print("ccccc, currentTime: \(CFAbsoluteTimeGetCurrent())")
    }

dispatch_barrier_sync 结果

ffff 111111, currentTime: 592538837.188629
ffff 222222, currentTime: 592538837.188665
ffff 444444, currentTime: 592538837.188673
ffff 333333, currentTime: 592538837.188669
ffff 111111_bbbbbbbb, currentTime: 592538839.193317
******** barrier ******, currentTime: 592538839.193441
******** barrier_2 ******, currentTime: 592538841.194509
dfafgda , currentTime: 592538841.194632
ccccc, currentTime: 592538841.194643
bbbb 5555555, currentTime: 592538841.194689
bbbb 6666666, currentTime: 592538841.194704
bbbb 7777777, currentTime: 592538841.194708
bbbb 8888888, currentTime: 592538841.194712