进一步理解GCD

之前转了一篇GCD的文章–iOS中的GCD，近日重新理解了一下GCD。
iOS中实现并行的方式有三种：操作队列(Operation Queue)，调度队列(Dispatch Queue)和线程。
注意：

Swift将GCD进行了对象化。我们可以使用DispatchQueue和DispatchGroup的实例方法操作GCD，更符合面向对象的思想。
文中代码在Swift4中运行良好。
GCD（调度队列）中的核心元素有：队列、任务、调度源等等。

队列（dispatch_queue_t）

typedef NSObject<OS_dispatch_queue> *dispatch_queue_t;

OS_dispatch_queue的定义并没有公开，猜测其中包括了对线程的相关操作。
调度队列按照先进先出的的顺序触发提交到自己的block。
- 串行队列：同时只能执行一个block。
- 并行队列：可同时触发多个block。
三种队列：
- 主队列
- 私有队列
- 全局并发队列

向队列添加任务

dispatch_async：异步的向队列中添加任务，不知任务何时开始执行。
dispatch_async_f：第二个参数是队列的上下文指针：unsafeBitCast(0, UnsafeMutablePointer.self)
dispatch_sync：同步的向队列中添加任务，需要等待任务完成后才能向下执行。
dispatch_sync_f

死锁：block等main执行完，main等block执行完
func testDeadLock() {
        let q = DispatchQueue.main
        print("1")
        q.sync {
            print("2")
        }
        print("3")
    }

不死锁：main已释放
func testDeadLock1() {
        let q = DispatchQueue.main
        let gq = DispatchQueue.global()
        gq.async {
            print("1")
            q.sync {
                print("2")
            }
            print("3")
        }
    }

在队列中循环任务（swift3.2中已被废弃）

dispatch_apply：
dispatch_apply_f：swift3.2中已被废弃

暂定和重启任务（swift3.2中已被废弃）

dispatch_suspend：暂停执行队列中的下一个任务。
dispatch_resume：继续执行。

任务分组

dispatch_group_async：将任务分为一组

dispatch_group_wait：某组任务完成后执行下面的操作
未分组：

func testGroup() {
    var count = 0
    let concurrentQueue = DispatchQueue.global()
    concurrentQueue.async() {
        print("Task1 in dispatchGroup. Current thread is \(Thread.current)")
        sleep(2)
        count += 1
    }

    concurrentQueue.async() {
        print("Task2 in dispatchGroup. Current thread is \(Thread.current)")
        sleep(3)
        count += 1
    }

    concurrentQueue.async() {
        print("Task3 in dispatchGroup. Current thread is \(Thread.current)")
        sleep(1)
        count += 1
    }
    print("I expect the count is 3, and the factual count is \(count)")
}

执行结果：

I expect the count is 3, and the factual count is 0
Task2 in dispatchGroup. Current thread is <NSThread: 0x604000276d80>{number = 3, name = (null)}
Task1 in dispatchGroup. Current thread is <NSThread: 0x604000276dc0>{number = 5, name = (null)}
Task3 in dispatchGroup. Current thread is <NSThread: 0x60000046dd00>{number = 4, name = (null)}

分组：

var count = 0
        let concurrentQueue = DispatchQueue.global()
        let dispatchGroup = DispatchGroup()
        concurrentQueue.async(group: dispatchGroup) {
            print("Task1 in dispatchGroup. Current thread is \(Thread.current)")
            sleep(2)
            count += 1
        }
        concurrentQueue.async(group: dispatchGroup) {
            print("Task2 in dispatchGroup. Current thread is \(Thread.current)")
            sleep(3)
            count += 1
        }
        concurrentQueue.async(group: dispatchGroup) {
            print("Task3 in dispatchGroup. Current thread is \(Thread.current)")
            sleep(1)
            count += 1
        }
        _ = dispatchGroup.wait(timeout: DispatchTime.distantFuture)
        print("I expect the count is 3, and the factual count is \(count)")

执行结果：

Task1 in dispatchGroup. Current thread is <NSThread: 0x604000471a80>{number = 4, name = (null)}
Task2 in dispatchGroup. Current thread is <NSThread: 0x60000027ebc0>{number = 3, name = (null)}
Task3 in dispatchGroup. Current thread is <NSThread: 0x604000471c00>{number = 5, name = (null)}
I expect the count is 3, and the factual count is 3

任务

调度源(Dispatch Source)

在Swift4（3.2？）已被废弃。

参考：
1，iOS并发编程指南:Dispatch Queue任务执行与Dispatch Source
2，Apple Doc: Dispatch Queue