优点:

(1)易用：GCD比thread更简单易用。基于block的特效使它能极为简单地在不同代码作用域之间传递上下文。
(2)效率：GCD实现功能轻量，优雅，使得它在很多地方比专门创建消耗资源的线程更加实用且快捷。
(3)性能：GCD自动根据系统负载来增减线程数量，从而减少了上下文切换并增加了计算效率。
(4)安全：无需加锁或其他同步机制

多线程的四种方式

1,串行 同步

func test1() {
        let test1queue = DispatchQueue.init(label: "test1que")
        test1queue.sync {
            for i in 0..<2 {
                print("\(i),\(Thread.current)")
                
            }
            print("第一个结束\n-------------------------")
        }
        test1queue.sync {
            for i in 2..<4 {
                print("\(i),\(Thread.current)")
                
            }
            print("第二个结束\n-------------------------")
        }
        test1queue.sync {
            for i in 4..<6 {
                print("\(i),\(Thread.current)")
                
            }
            print("第三个结束\n-------------------------")
        }
        
    }
打印结果:
0,<NSThread: 0x280942040>{number = 1, name = main}
1,<NSThread: 0x280942040>{number = 1, name = main}
第一个结束
-------------------------
2,<NSThread: 0x280942040>{number = 1, name = main}
3,<NSThread: 0x280942040>{number = 1, name = main}
第二个结束
-------------------------
4,<NSThread: 0x280942040>{number = 1, name = main}
5,<NSThread: 0x280942040>{number = 1, name = main}
第三个结束
-------------------------

串行同步: 顺序执行 不开启新线程

2,串行 异步

func test2() {
        let test1queue = DispatchQueue.init(label: "test1que")
        test1queue.async {
            for i in 0..<2 {
                print("\(i),\(Thread.current)")
                
            }
            print("第一个结束\n-------------------------")
        }
        test1queue.async {
            for i in 2..<4 {
                print("\(i),\(Thread.current)")
                
            }
            print("第二个结束\n-------------------------")
        }
        test1queue.async {
            for i in 4..<6 {
                print("\(i),\(Thread.current)")
                
            }
            print("第三个结束\n-------------------------")
        }
        
    }
打印结果:
0,<NSThread: 0x283954bc0>{number = 6, name = (null)}
1,<NSThread: 0x283954bc0>{number = 6, name = (null)}
第一个结束
-------------------------
2,<NSThread: 0x283954bc0>{number = 6, name = (null)}
3,<NSThread: 0x283954bc0>{number = 6, name = (null)}
第二个结束
-------------------------
4,<NSThread: 0x283954bc0>{number = 6, name = (null)}
5,<NSThread: 0x283954bc0>{number = 6, name = (null)}
第三个结束
-------------------------

串行异步: 顺序执行 开启新线程(只开启一条线程)

3,并发 同步

func test3() {
        let test1queue = DispatchQueue(label: "test1que",  attributes: .concurrent)
        test1queue.sync {
            for i in 0..<2 {
                print("\(i),\(Thread.current)")
                
            }
            print("第一个结束\n-------------------------")
        }
        test1queue.sync {
            for i in 2..<4 {
                print("\(i),\(Thread.current)")
                
            }
            print("第二个结束\n-------------------------")
        }
        test1queue.sync {
            for i in 4..<6 {
                print("\(i),\(Thread.current)")
                
            }
            print("第三个结束\n-------------------------")
        }
    }
打印结果: 
0,<NSThread: 0x2817aadc0>{number = 1, name = main}
1,<NSThread: 0x2817aadc0>{number = 1, name = main}
第一个结束
-------------------------
2,<NSThread: 0x2817aadc0>{number = 1, name = main}
3,<NSThread: 0x2817aadc0>{number = 1, name = main}
第二个结束
-------------------------
4,<NSThread: 0x2817aadc0>{number = 1, name = main}
5,<NSThread: 0x2817aadc0>{number = 1, name = main}
第三个结束
-------------------------

并发同步: 顺序执行 不开新线程

4,并发 异步(也是用的最多的一种)

func test4() {
        let test1queue = DispatchQueue(label: "test1que",  attributes: .concurrent)
        for i in 0...50 {
            test1queue.async {
                print("\(i),\(Thread.current)----------------")
            }
        }
    }
打印结果:(最后10个)
40,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
41,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
42,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
43,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
44,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
45,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
46,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
47,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
48,<NSThread: 0x281b2f1c0>{number = 4, name = (null)}----------------
8,<NSThread: 0x281b32780>{number = 6, name = (null)}----------------
16,<NSThread: 0x281b1c140>{number = 7, name = (null)}----------------
17,<NSThread: 0x281b1c200>{number = 8, name = (null)}----------------
32,<NSThread: 0x281b18000>{number = 9, name = (null)}----------------
33,<NSThread: 0x281b180c0>{number = 10, name = (null)}----------------
49,<NSThread: 0x281b18900>{number = 11, name = (null)}----------------
50,<NSThread: 0x281b1ccc0>{number = 12, name = (null)}----------------

并发异步: 无序执行 开启n条线程

队列获取方式

//创建串行队列
let serial = DispatchQueue(label: "serialQueue")

//创建并行队列
let concurrent = DispatchQueue(label: "concurrentQueue", attributes: .concurrent)

//获取全局队列
let globalQueue = DispatchQueue.global(qos: .default)

//获取主线程队列
let mainQueue = DispatchQueue.main//一般跟UI相关的操作都放在主线程队列