iOS 的串行队列和并发队列中的任务是如何执行的

我们都知道队列有串行队列和并发队列，主队列就属于串行队列，串行队列里面的任务是按顺序执行，并发队列里的任务是并发执行，至于并发几个线程去执行，这就交给GCD 了。
但是有个问题需要思考一下，我们都知道任务要执行，先要从队列中取出，然后再分配到线程中执行，也就是分三步：1.从队列中取出任务，2.执行任务，3.任务执行完毕。所谓串行队列里面的任务按顺序执行，这个按顺序是指的按顺序取出，还是等前一个任务执行完毕再执行下一个任务呢？写个代码测试一下。

串行队列同步异步执行测试

1.串行队列中同步执行一个任务，异步执行一个任务

let seralQueue = DispatchQueue(label: "dispatchSeralQueue")
//任务一
seralQueue.sync {
            for i in 0...10 {
                debugPrint("queueTest seralQueue1: \(Thread.current), \(i)")
            }
        }
//任务二
        seralQueue.async {
            for i in 0...10 {
                debugPrint("queueTest seralQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 0"
"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 1"
"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 2"
"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 3"
"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 4"
"queueTest seralQueue1: <_NSMainThread: 0x600002360540>{number = 1, name = main}, 5"
"queueTest seralQueue2: {number = 4, name = (null)}, 0"
"queueTest seralQueue2: {number = 4, name = (null)}, 1"
"queueTest seralQueue2: {number = 4, name = (null)}, 2"
"queueTest seralQueue2: {number = 4, name = (null)}, 3"
"queueTest seralQueue2: {number = 4, name = (null)}, 4"
"queueTest seralQueue2: {number = 4, name = (null)}, 5"

可以看到，第一个任务是在主线程执行，第二个任务在子线程执行，这符合代码逻辑，从输出顺序上来看，在串行队列中，虽然任务二用了异步执行的方式，也开了新的线程，但是顺序却是任务一执行完毕之后，任务二才会执行。

2.串行队列中异步执行两个任务

let seralQueue = DispatchQueue(label: "dispatchSeralQueue")
// 任务一
seralQueue.async {
            for i in 0...5 {
                debugPrint("queueTest seralQueue1: \(Thread.current), \(i)")
            }
        }
//任务 二
        seralQueue.async {
            for i in 0...5 {
                debugPrint("queueTest seralQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest seralQueue1: {number = 6, name = (null)}, 0"
"queueTest seralQueue1: {number = 6, name = (null)}, 1"
"queueTest seralQueue1: {number = 6, name = (null)}, 2"
"queueTest seralQueue1: {number = 6, name = (null)}, 3"
"queueTest seralQueue1: {number = 6, name = (null)}, 4"
"queueTest seralQueue1: {number = 6, name = (null)}, 5"
"queueTest seralQueue2: {number = 6, name = (null)}, 0"
"queueTest seralQueue2: {number = 6, name = (null)}, 1"
"queueTest seralQueue2: {number = 6, name = (null)}, 2"
"queueTest seralQueue2: {number = 6, name = (null)}, 3"
"queueTest seralQueue2: {number = 6, name = (null)}, 4"
"queueTest seralQueue2: {number = 6, name = (null)}, 5"

可以看到，两个任务都在子线程执行，而且是按顺序的，第一个任务执行完毕，才会轮到第二个任务执行。

3.串行队列同步执行两个任务

let seralQueue = DispatchQueue(label: "dispatchSeralQueue")
seralQueue.sync {
            for i in 0...5 {
                debugPrint("queueTest seralQueue1: \(Thread.current), \(i)")
            }
        }
        seralQueue.sync {
            for i in 0...5 {
                debugPrint("queueTest seralQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 0"
"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 1"
"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 2"
"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 3"
"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 4"
"queueTest seralQueue1: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 5"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 0"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 1"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 2"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 3"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 4"
"queueTest seralQueue2: <_NSMainThread: 0x600002fb0080>{number = 1, name = main}, 5"

同步执行顺序队列任务，没有开启新线程，任务按顺序执行。

并发队列同步异步执行测试

并发队列异步执行两个任务

let conCurrentQueue = DispatchQueue(label: "dispatchConcurrentQueue", attributes: .concurrent)
conCurrentQueue.async {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue1: \(Thread.current), \(i)")
            }
        }
        conCurrentQueue.async {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest conCurrentQueue2: {number = 5, name = (null)}, 0"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 0"
"queueTest conCurrentQueue2: {number = 5, name = (null)}, 1"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 1"
"queueTest conCurrentQueue2: {number = 5, name = (null)}, 2"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 2"
"queueTest conCurrentQueue2: {number = 5, name = (null)}, 3"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 3"
"queueTest conCurrentQueue2: {number = 5, name = (null)}, 4"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 4"
"queueTest conCurrentQueue2: {number = 5, name = (null)}, 5"
"queueTest conCurrentQueue1: {number = 6, name = (null)}, 5"

可以看到两个任务之间是交叉执行的，并没有顺序关系，而且也开辟了新的线程。

并发队列同步执行两个任务

conCurrentQueue.sync {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue1: \(Thread.current), \(i)")
            }
        }
        conCurrentQueue.sync {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 0"
"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 1"
"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 2"
"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 3"
"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 4"
"queueTest conCurrentQueue1: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 5"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 0"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 1"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 2"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 3"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 4"
"queueTest conCurrentQueue2: <_NSMainThread: 0x60000142c040>{number = 1, name = main}, 5"

可以看到并发队列同步执行，并没有开启新的线程，任务也是按顺序执行完毕的

并发队列中，同步执行一个任务，异步执行一个任务

let conCurrentQueue = DispatchQueue(label: "dispatchConcurrentQueue", attributes: .concurrent)
conCurrentQueue.sync {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue1: \(Thread.current), \(i)")
            }
        }
        conCurrentQueue.async {
            for i in 0...5 {
                debugPrint("queueTest conCurrentQueue2: \(Thread.current), \(i)")
            }
        }

"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 0"
"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 1"
"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 2"
"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 3"
"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 4"
"queueTest conCurrentQueue1: <_NSMainThread: 0x600000d6c000>{number = 1, name = main}, 5"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 0"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 1"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 2"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 3"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 4"
"queueTest conCurrentQueue2: {number = 7, name = (null)}, 5"

异步执行的任务开启了新的线程，但是两个任务之间还是顺序的关系，一个任务执行完毕，才会开始执行下一个任务。

结论

串行队列+同步执行=不开新线程，任务依次执行
串行队列+异步执行=开启新线程，任务依次执行
并发队列+同步执行=不开新线程，任务依次执行
并发队列+异步执行=开启新线程，任务并发执行