用例子看Swift4的GCD

  关于GCD基础知识的文章很多，但很多都停留在iOS早期版本的状态，随着iOS的升级GCD的使用也随之升级了，下面举了很多例子来看一看。

1、串行队列使用同步运行

////1、普通队列同步运行
let queue = DispatchQueue(label:"com.test")
queue.sync{
    for _ in 0..<6 {
        print("aaaaaaa(Thread.current)")
    }
}
for _ in 0..<6  {
    print("bbbbbbb(Thread.current)")
}　　

 运行结果：

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

bbbbbbb<NSThread: 0x6040002608c0>{number = 1, name = main}

 

结果分析：

串行队形完全阻断了主线程的运行，都使用main线程，这种写法是完全没有意义的。

2、串行队列使用异步运行

//2、普通队列异步运行
let queue = DispatchQueue(label:"com.test")
queue.async{
    for _ in 0..<6 {
        print("aaaaaaa(Thread.current)")
    }
}
for _ in 0..<6  {
    print("bbbbbbb(Thread.current)")
}

运行结果：

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

bbbbbbb<NSThread: 0x604000066580>{number = 1, name = main}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x600000071680>{number = 3, name = (null)}

 

运行结果分析：

串行队列任务与主线程并行，串行队列使用了新的线程3，感觉上就像自己启了一个线程在做事情，使用DispatchQueue会方便很多。

3、串行队列多次运行

//3、普通队列异步运行多次
let queue = DispatchQueue(label:"com.test",qos:DispatchQoS.unspecified)
queue.async{
    for _ in 0..<6 {
        print("aaaaaaa(Thread.current)")
    }
}
queue.async{
    for _ in 0..<6 {
        print("cccccc(Thread.current)")
    }
}
for _ in 0..<6  {
    print("bbbbbbb(Thread.current)")
}

运行结果：

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

bbbbbbb<NSThread: 0x60400007cc40>{number = 1, name = main}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

cccccc<NSThread: 0x604000264a80>{number = 3, name = (null)}

 

运行结果分析：

队形中任务与主线程并行且使用了新的线程3，这里可以看处理，串行队列只有等第一个任务运行完才执行第二个（所以aaaaaa打印完，才有cccccc），这里例子很适合做并发中互斥操作，不使用锁。

4、并行队列异步运行多次

//4、并行队列异步运行多次
let queue = DispatchQueue(label:"com.test",qos:DispatchQoS.unspecified,attributes:.concurrent)
queue.async{
    for _ in 0..<6 {
        print("aaaaaaa(Thread.current)")
    }
}
queue.async{
    for _ in 0..<6 {
        print("cccccc(Thread.current)")
    }
}
for _ in 0..<6  {
    print("bbbbbbb(Thread.current)")
}　　

运行结果：

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

bbbbbbb<NSThread: 0x604000079c00>{number = 1, name = main}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

cccccc<NSThread: 0x600000267380>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

aaaaaaa<NSThread: 0x6040002657c0>{number = 4, name = (null)}

 

运行结果分析：

并行队列的2个任务与主线程同时运行，并行队列中任务分别使用线程3和线程4，这是一个并发的场景，比如我们一边加载不同图片一边不会影响UI的响应；这里也可以看出串行队列与并行队列的不同之处。

5、多个串行队列异步运行

let queue = DispatchQueue(label:"com.test",qos:DispatchQoS.unspecified)
let queue2 = DispatchQueue(label:"com.test",qos:DispatchQoS.userInteractive)
queue.async{
    for _ in 0..<16 {
        print("aaaaaaa(Thread.current)")
    }
}
queue2.async{
    for _ in 0..<16 {
        print("cccccc(Thread.current)")
    }
}

运行结果：

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

cccccc<NSThread: 0x60400006ec80>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

aaaaaaa<NSThread: 0x60400006ecc0>{number = 4, name = (null)}

 

运行结果分析：

例子中队列2的优先级比队列1高，所以cccccc在aaaaaa之前打印完，2个不同的串行队列使用不同的线程3和4，并行队列的优先级比较也是一样的结果，这里就不发例子

6、使用DispatchGroup做任务依赖

queue先做一个短任务，并行的queue2做一个长任务，等2个任务都做完group通知queue接着做任务，任务C依赖与任务A和任务B同时完成。

let queue = DispatchQueue(label:"com.test",qos:DispatchQoS.unspecified)
let queue2 = DispatchQueue(label:"com.test",qos:DispatchQoS.userInteractive)
let group = DispatchGroup()
queue.async(group: group, execute: {
    for _ in 0..<3 {
        print("aaaaaaa(Thread.current)")
    }
})
queue2.async(group: group, execute: {
    for _ in 0..<6 {
        print("bbbbb(Thread.current)")
    }
})

//执行完上面的两个耗时操作, 回到queue队列中执行下一步的任务
group.notify(queue: queue) {
    print("回到queue该队列中执行")
    for _ in 0..<3 {
        print("bbbbb(Thread.current)")
    }
}

 运行结果：

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x604000078580>{number = 4, name = (null)}

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x604000078580>{number = 4, name = (null)}

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

aaaaaaa<NSThread: 0x604000078580>{number = 4, name = (null)}

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

bbbbb<NSThread: 0x604000078540>{number = 3, name = (null)}

回到queue该队列中执行

cccccc<NSThread: 0x604000078580>{number = 4, name = (null)}

cccccc<NSThread: 0x604000078580>{number = 4, name = (null)}

cccccc<NSThread: 0x604000078580>{number = 4, name = (null)}

 

运行结果分析：

2任务分别运行做线程3和4，等他们都做完再到queue中打印cccccc

7、使用DispatchGroup做任务等待

queue和queue2是2个并行的队列，queue2中sleep(UInt32(3))可以让queue2超时

 

let queue = DispatchQueue(label:"com.test",qos:DispatchQoS.unspecified)
let queue2 = DispatchQueue(label:"com.test",qos:DispatchQoS.userInteractive)
let group = DispatchGroup()
queue.async(group: group, execute: {
    for _ in 0..<3 {
        print("aaaaaaa(Thread.current)")
    }
})
queue2.async(group: group, execute: {
    for _ in 0..<6 {
        print("bbbbb(Thread.current)")
    }
    //sleep(UInt32(3))
})

//等待上面任务执行，会阻塞当前线程，超时就执行下面的，上面的继续执行。可以无限等待 .distantFuture
let result:DispatchTimeoutResult = group.wait(timeout: .now() + 2.0)
switch result {
case .success:
    print("不超时, 上面的两个任务都执行完")
case .timedOut:
    print("超时了, 上面的任务还没执行完执行这了")
}

print("接下来的操作")

　　

打开sleep(UInt32(3)) 的结果：

aaaaaaa<NSThread: 0x604000273140>{number = 4, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
aaaaaaa<NSThread: 0x604000273140>{number = 4, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
aaaaaaa<NSThread: 0x604000273140>{number = 4, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
bbbbb<NSThread: 0x6000004703c0>{number = 3, name = (null)}
超时了, 上面的任务还没执行完执行这了
接下来的操作

注解sleep(UInt32(3))结果：

bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
aaaaaaa<NSThread: 0x600000278e00>{number = 4, name = (null)}
aaaaaaa<NSThread: 0x600000278e00>{number = 4, name = (null)}
bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
aaaaaaa<NSThread: 0x600000278e00>{number = 4, name = (null)}
bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
bbbbb<NSThread: 0x604000273280>{number = 3, name = (null)}
不超时, 上面的两个任务都执行完
接下来的操作

结果分析：

可以用Group的wait方法来做超时判断。

ps：例子多了点，都是一个一个字码的，系统对学习Swift的GCD有用。