本文不涉及 GCD 的概念和理论，仅记录了 GCD 在一些开发场景下的应用。嗯，欢迎大家积极留言补充。

耗时操作

这是应用最广泛的场景，为了避免阻塞主线程，将耗时操作放在子线程处理，然后在主线程使用处理结果。比如读取沙盒中的一些数据，然后将读取的数据展示在 UI，这个场景还有几个细分：

执行一个耗时操作后回调主线程

/// 主线程需要子线程的处理结果
func handle<T>(somethingLong: @escaping () -> T, finshed: @escaping (T) -> ()) {
    globalQueue.async {
        let data = somethingLong()
        self.mainQueue.async {
            finshed(data)
        }
    }
}
/// 主线程不需要子线程的处理结果
func handle(somethingLong: @escaping () -> (), finshed: @escaping () -> ()) {
    let workItem = DispatchWorkItem {
        somethingLong()
    }
    globalQueue.async(execute: workItem)
    workItem.wait()
    finshed()
}

/////////////////////////////////////////////////////////////////////////////

GCDKit().handle(somethingLong: { [weak self] in
    self?.color = UIColor.red
    sleep(2)
}) { [weak self] in
    self?.view.backgroundColor = self?.color
}

GCDKit().handle(somethingLong: {
    let p = Person()
    p.age = 40
    print(Date(), p.age)
    sleep(2)
    return p
}) { (p: Person) in
    print(Date(), p.age)
}

串行耗时操作

每一段子任务依赖上一个任务完成，全部完成后回调主线程：

/// 向全局并发队列添加任务，添加的任务会同步执行
func wait(code: @escaping GCDKitHandleBlock) -> GCDKit {
    handleBlockArr.append(code)
    return self
}

/// 处理完毕的回调，在主线程异步执行
func finshed(code: @escaping GCDKitHandleBlock) {
    globalQueue.async {
        for workItem in self.handleBlockArr {
            workItem()
        }
        self.handleBlockArr.removeAll()
        self.mainQueue.async {
            code()
        }
    }
}

/////////////////////////////////////////////////////////////////////////////

GCDKit().wait {
        self.num += 1
    }.wait {
        self.num += 2
    }.wait {
        self.num += 3
    }.wait {
        self.num += 4
    }.wait {
        self.num += 5
    }.finshed {
        print(self.num, Thread.current)
}

并发耗时操作

每一段子任务独立，所有子任务完成后回调主线程：

/// 向自定义并发队列添加任务，添加的任务会并发执行
func handle(code: @escaping GCDKitHandleBlock) -> GCDKit {
    let queue = DispatchQueue(label: "", attributes: .concurrent)
    let workItem = DispatchWorkItem {
        code()
    }
    queue.async(group: group, execute: workItem)
    return self
}

/// 此任务执行时会排斥其他的并发任务，一般用于写入事务，保证线程安全。
func barrierHandle(code: @escaping GCDKitHandleBlock) -> GCDKit {
    let queue = DispatchQueue(label: "", attributes: .concurrent)
    let workItem = DispatchWorkItem(flags: .barrier) {
        code()
    }
    queue.async(group: group, execute: workItem)
    return self
}

/// 处理完毕的回调，在主线程异步执行
func allDone(code: @escaping GCDKitHandleBlock) {
    group.notify(queue: .main, execute: {
        code()
    })
}

/////////////////////////////////////////////////////////////////////////////

GCDKit().barrierHandle {
        self.num += 1
    }.barrierHandle {
        self.num += 2
    }.barrierHandle {
        self.num += 3
    }.handle {
        self.num += 4
    }.handle {
        self.num += 5
    }.allDone {
        self.num += 6
        print(self.num, Thread.current)
}

延时执行

延时一段时间后执行代码，一般见于打开 App 一段时间后，弹出求好评对话框。

func run(when: DispatchTime, code: @escaping GCDKitHandleBlock) {
    DispatchQueue.main.asyncAfter(deadline: when) {
        code()
    }
}

/////////////////////////////////////////////////////////////////////////////

GCDKit().run(when: .now() + .seconds(120)) {
  self.doSomething()
}

定时器

由于 Timer 的 Target 是强引用，对于 Timer 的销毁需要特别处理，此外，Timer 的运行依赖于 Runloop，在 Runloop 的一次循环中，Timer 也只会执行一次，这使得在 Runloop 负担比较重时，可能会跳过 Timer 的执行，因此，在用到定时器的地方，你也可以用 CGD 的 TimerSource 替代：

/// 计时器
///
/// - Parameters:
///   - start: 开始时间
///   - end: 结束时间
///   - repeating: 多久重复一次
///   - leeway: 允许误差
///   - eventHandle: 处理事件
///   - cancelHandle: 计时器结束事件
func timer(start: DispatchTime,
           end: DispatchTime,
           repeating: Double,
           leeway: DispatchTimeInterval,
           eventHandle: @escaping GCDKitHandleBlock,
           cancelHandle: GCDKitHandleBlock? = nil)
{
    let timer = DispatchSource.makeTimerSource()
    timer.setEventHandler {
        eventHandle()
    }
    timer.setCancelHandler {
        cancelHandle?()
    }
    timer.schedule(deadline: start, repeating: repeating, leeway: leeway)
    timer.resume()
    run(when: end) {
        timer.cancel()
    }
}

/////////////////////////////////////////////////////////////////////////////

GCDKit().timer(start: .now(),
               end: .now() + .seconds(10),
               repeating: 2,
               leeway: .milliseconds(1),
               eventHandle: {
    self.doSomething()
}) {
    print("timer cancel")
}

并发遍历

如果你需要更快的处理数据，可以用 concurrentPerform 让循环操作并发执行：

func map<T>(data: [T], code: (T) -> ()) {
    DispatchQueue.concurrentPerform(iterations: data.count) { (i) in
        code(data[i])
    }
}

func run(code: (Int) -> (), repeting: Int) {
    DispatchQueue.concurrentPerform(iterations: repeting) { (i) in
        code(i)
    }
}

/////////////////////////////////////////////////////////////////////////////

let data = [1, 2, 3]
var sum = 0

GCDKit().map(data: data) { (ele: Int) in
    sleep(1)
    sum += ele
}
print(sum)

GCDKit().run(code: { (i) in
    sleep(1)
    sum += data[i]
}, repeting: data.count)
print(sum)

控制并发数

有时我们需要并发处理一些任务，但是并不想同时开很多线程，GCD 并没有类似 NSOperation 最大并发数的概念，但可以借助信号量实现：

func doSomething(label: String, cost: UInt32, complete:@escaping ()->()){
    NSLog("Start task%@",label)
    sleep(cost)
    NSLog("End task%@",label)
    complete()
}

/////////////////////////////////////////////////////////////////////////////

let semaphore = DispatchSemaphore(value: 3)
let queue = DispatchQueue(label: "", qos: .default, attributes: .concurrent)

queue.async {
    semaphore.wait()
    self.doSomething(label: "1", cost: 2, complete: {
        print(Thread.current)
        semaphore.signal()
    })
}

queue.async {
    semaphore.wait()
    self.doSomething(label: "2", cost: 2, complete: {
        print(Thread.current)
        semaphore.signal()
    })
}

queue.async {
    semaphore.wait()
    self.doSomething(label: "3", cost: 4, complete: {
        print(Thread.current)
        semaphore.signal()
    })
}

queue.async {
    semaphore.wait()
    self.doSomething(label: "4", cost: 2, complete: {
        print(Thread.current)
        semaphore.signal()
    })
}

queue.async {
    semaphore.wait()
    self.doSomething(label: "5", cost: 3, complete: {
        print(Thread.current)
        semaphore.signal()
    })
}

时序管理

时序管理主要有几种组合情况：

子任务内是否开线程；
子任务是否依次执行；

子任务内不开线程依次执行

参照耗时操作小节。

子任务内开线程依次执行

一般见于网络请求，一个接口的请求参数是另一个接口的返回值，这种情况就需要对网络请求进行时序管理，以下代码表示一个网络请求的封装：

func networkTask(label:String, cost:UInt32, complete:@escaping ()->()){
    NSLog("Start network Task task%@",label)
    DispatchQueue.global().async {
        sleep(cost)
        NSLog("End networkTask task%@",label)
        DispatchQueue.main.async {
            complete()
        }
    }
}

在子线程可开线程的情况下，依次执行需要借助信号量控制：

let semaphore = DispatchSemaphore(value: 1)
let queue = DispatchQueue(label: "", qos: .default, attributes: .concurrent)
queue.async {
    semaphore.wait()
    self.networkTask(label: "1", cost: 2, complete: {
        semaphore.signal()
    })
    semaphore.wait()
    self.networkTask(label: "2", cost: 4, complete: {
        semaphore.signal()
    })
    semaphore.wait()
    self.networkTask(label: "3", cost: 3, complete: {
        semaphore.signal()
    })
    semaphore.wait()
    self.networkTask(label: "4", cost: 1, complete: {
        semaphore.signal()
    })
    semaphore.wait()
    print("all done")
    semaphore.signal()
}

/////////////////////////////////////////////////////////////////////////////
2017-12-19 14:02:33.297613+0800 Demo[11757:4946542] Start network Task task1
2017-12-19 14:02:35.301386+0800 Demo[11757:4946541] End networkTask task1
2017-12-19 14:02:35.301971+0800 Demo[11757:4946542] Start network Task task2
2017-12-19 14:02:39.306592+0800 Demo[11757:4946541] End networkTask task2
2017-12-19 14:02:39.306901+0800 Demo[11757:4946542] Start network Task task3
2017-12-19 14:02:42.307843+0800 Demo[11757:4946541] End networkTask task3
2017-12-19 14:02:42.308268+0800 Demo[11757:4946542] Start network Task task4
2017-12-19 14:02:43.310724+0800 Demo[11757:4946541] End networkTask task4
all done

子任务内开线程不依次执行

这种情况多见于需要请求多个接口，全部请求完毕后再进行某些操作，这可以借助 GCD 的任务组处理：

let group = DispatchGroup()
group.enter()
networkTask(label: "1", cost: 2, complete: {
    group.leave()
})

group.enter()
networkTask(label: "2", cost: 4, complete: {
    group.leave()
})

group.enter()
networkTask(label: "3", cost: 2, complete: {
    group.leave()
})

group.enter()
networkTask(label: "4", cost: 4, complete: {
    group.leave()
})

group.notify(queue: .main, execute:{
    print("All network is done")
})

/////////////////////////////////////////////////////////////////////////////
2017-12-19 14:10:33.876393+0800 Demo[16495:4973791] Start network Task task1
2017-12-19 14:10:33.878869+0800 Demo[16495:4973791] Start network Task task2
2017-12-19 14:10:33.879142+0800 Demo[16495:4973791] Start network Task task3
2017-12-19 14:10:33.879309+0800 Demo[16495:4973791] Start network Task task4
2017-12-19 14:10:35.883851+0800 Demo[16495:4974025] End networkTask task1
2017-12-19 14:10:35.883850+0800 Demo[16495:4974030] End networkTask task3
2017-12-19 14:10:37.883995+0800 Demo[16495:4974026] End networkTask task2
2017-12-19 14:10:37.883995+0800 Demo[16495:4974027] End networkTask task4
All network is done

// 你也可以这样进行简写
let downloadGroup = DispatchGroup()
GCDKit().run(code: { (i) in
    downloadGroup.enter()
    networkTask(label: "\(i)", cost: UInt32(i), complete: {
        downloadGroup.leave()
    })
}, repeting: 10)
downloadGroup.notify(queue: .main) {
    print("All network is done")
}

/////////////////////////////////////////////////////////////////////////////
2017-12-19 15:07:13.253428+0800 Demo[49319:5169745] Start network Task task3
2017-12-19 15:07:13.253428+0800 Demo[49319:5169743] Start network Task task2
2017-12-19 15:07:13.253428+0800 Demo[49319:5169744] Start network Task task0
2017-12-19 15:07:13.253479+0800 Demo[49319:5169474] Start network Task task1
2017-12-19 15:07:13.253946+0800 Demo[49319:5169744] Start network Task task6
2017-12-19 15:07:13.253947+0800 Demo[49319:5169743] Start network Task task4
2017-12-19 15:07:13.253947+0800 Demo[49319:5169745] Start network Task task5
2017-12-19 15:07:13.254119+0800 Demo[49319:5169763] End networkTask task0
2017-12-19 15:07:13.254193+0800 Demo[49319:5169474] Start network Task task7
2017-12-19 15:07:13.254339+0800 Demo[49319:5169744] Start network Task task8
2017-12-19 15:07:13.254343+0800 Demo[49319:5169743] Start network Task task9
2017-12-19 15:07:14.258061+0800 Demo[49319:5169764] End networkTask task1
2017-12-19 15:07:15.258071+0800 Demo[49319:5169762] End networkTask task2
2017-12-19 15:07:16.258189+0800 Demo[49319:5169742] End networkTask task3
2017-12-19 15:07:17.258100+0800 Demo[49319:5169745] End networkTask task4
2017-12-19 15:07:18.258196+0800 Demo[49319:5169766] End networkTask task5
2017-12-19 15:07:19.258171+0800 Demo[49319:5169765] End networkTask task6
2017-12-19 15:07:20.259119+0800 Demo[49319:5169763] End networkTask task7
2017-12-19 15:07:21.258239+0800 Demo[49319:5169767] End networkTask task8
2017-12-19 15:07:22.258280+0800 Demo[49319:5169744] End networkTask task9
All network is done

自定义数据监听

当需要监听某个数据的变化，但不需要频繁的调用其对应的回调处理，可以使用 DispatchSourceUserData 进行监听，它会自动合并更改，并在队列空闲时进行回调，以节省 CPU 开销。

extension GCDKit {
    
    convenience init(valueChanged: @escaping (T) -> ()) {
        self.init()
        userDataAddSource = DispatchSource.makeUserDataAddSource()
        userDataAddSource?.setEventHandler(handler: { [weak self] in
            guard let `self` = self else { return }
            guard let value = self.value else { return }
            valueChanged(value)
        })
        userDataAddSource?.resume()
    }
    
    func send(_ value: T) {
        self.value = value
        userDataAddSource?.add(data: 1)
    }
}

/////////////////////////////////////////////////////////////////////////////

GCD = GCDKit<Int> { (value: Int) in
    print(value)
}

let serialQueue = DispatchQueue(label: "com")
serialQueue.async {
    for i in 1...1000{
        self.GCD.send(i)
    }
    for i in 1000...9999 {
        self.GCD.send(i)
    }
}

/////////////////////////////////////////////////////////////////////////////

64
9999

监听进程

在 Mac 开发中，你可以监听其他进程的开启关闭情况：

let apps = NSRunningApplication.runningApplications(withBundleIdentifier: "com.apple.mail")
let processIdentifier = apps.first?.processIdentifier
let source = DispatchSource.makeProcessSource(identifier: pid, eventMask: .exit)
source.setEventHandler {
    print("mail quit")
}
source.resume()

监听目录结构

let folder = try? FileManager.default.url(for: .documentDirectory,
                                          in: .userDomainMask,
                                          appropriateFor: nil,
                                          create: false)
print(folder!.path)
let fd = open(folder!.path, O_CREAT, 0o644)
let queue = DispatchQueue(label: "m")
let source = DispatchSource.makeFileSystemObjectSource(fileDescriptor: fd,
                                                       eventMask: .all,
                                                       queue: queue)
source.setEventHandler {
    print("folder changed")
}
source.resume()

let result = FileManager.default.createFile(atPath: folder!.path + "/abc", contents: nil, attributes: nil)
if result {
    print(0)
}
else {
    print(1)
}

线程安全

你可以在资源读写时对其所在线程进行一些限制，而不必使用线程锁，比如：

/// .barrier 保证执行时会排斥其他的并发任务，一般用于写入事务，保证线程安全。
func barrierHandle(code: @escaping GCDKitHandleBlock) -> GCDKit {
    let queue = DispatchQueue(label: "", attributes: .concurrent)
    let workItem = DispatchWorkItem(flags: .barrier) {
        code()
    }
    queue.async(group: group, execute: workItem)
    return self
}

或者开启一个串行队列同步读写任务：

extension GCDKit {
    
    var data: T? {
        get {
            return readWriteQueue.sync { value }
        }
        set {
            readWriteQueue.sync { value = newValue }
        }
    }
}