1. GCD 的两个核心：任务和队列

任务：就是执行操作的意思，换句话说就是你在线程中执行的那段代码。在 GCD 中是放在 block 中的。执行任务有两种方式：同步执行（sync）和异步执行（async）。两者的主要区别是：是否等待队列的任务执行结束，以及是否具备开启新线程的能力。

同步执行（sync）：
- 同步添加任务到指定的队列中，在添加的任务执行结束之前，会一直等待，直到队列里面的任务完成之后再继续执行。
- 只能在当前线程中执行任务，不具备开启新线程的能力。
异步执行（async）：
- 异步添加任务到指定的队列中，它不会做任何等待，可以继续执行任务。
  可以在新的线程中执行任务，具备开启新线程的能力。

队列（Dispatch Queue）：这里的队列指执行任务的等待队列，即用来存放任务的队列。队列是一种特殊的线性表，采用 FIFO（先进先出）的原则，即新任务总是被插入到队列的末尾，而读取任务的时候总是从队列的头部开始读取。每读取一个任务，则从队列中释放一个任务。

串行队列（Serial Dispatch Queue）：每次只有一个任务被执行。让任务一个接着一个地执行。只开启一个线程，一个任务执行完毕后，再执行下一个任务
并行队列（Concurrent Dispatch Queue）：可以让多个任务并发（同时）执行。可以开启多个线程，并且同时执行任务

个人理解： 队列（Serial和Concurrent） 决定是否具备开启线程的能力，sync和async决定是否开启另外的线程

开启线程的能力跟所在的线程没关系

截屏2020-11-19下午8.19.36.png

2. 代码解析

代码1

//创建串行队列
    dispatch_queue_t mySerialQueue = dispatch_queue_create("my_test_queue", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t myQueue = dispatch_queue_create("my_test_queue_2", DISPATCH_QUEUE_SERIAL);

    //异步执行+串行队列：开启一个线程
    dispatch_async(mySerialQueue, ^{
        //任务1
        NSLog(@"7-----%@",[NSThread currentThread]);
        NSLog(@"7 === start");
        //异步 不用等待 继续执行
        dispatch_async(myQueue, ^{
             //任务3
            NSLog(@"4 === start");
            for (int i = 0; i<2; i++) {
                [NSThread sleepForTimeInterval:2];
                NSLog(@"4-----%@",[NSThread currentThread]);
            }
            dispatch_async(mySerialQueue, ^{
                 //任务4
                NSLog(@"8 === start");
                for (int i = 0; i<2; i++) {
                    [NSThread sleepForTimeInterval:2];
                    NSLog(@"8-----%@",[NSThread currentThread]);
                }
                NSLog(@"8 === end");
            });
            NSLog(@"4 === end");
        });
        //异步 不用等待继续执行
        dispatch_async(myQueue, ^{
            //任务5
            NSLog(@"5 === start");
            for (int i = 0; i<2; i++) {
                [NSThread sleepForTimeInterval:2];
                NSLog(@"5-----%@",[NSThread currentThread]);
            }
        });
        NSLog(@"7 === end");
    });
    //串行队列 需一个一个执行 ：等待 NSLog(@"7 === end") 执行后再执行
    dispatch_async(mySerialQueue, ^{
        //任务2
        NSLog(@"9 === start");
        for (int i = 0; i<2; i++) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"9-----%@",[NSThread currentThread]);
        }
        NSLog(@"9 === end");
    });

执行后输出结果是：

2019-04-16 14:37:09.149808+0800 test123[95964:8582595] 7-----{number = 3, name = (null)}
2019-04-16 14:37:09.149973+0800 test123[95964:8582595] 7 === start
2019-04-16 14:37:09.150092+0800 test123[95964:8582595] 7 === end
2019-04-16 14:37:09.150102+0800 test123[95964:8582592] 4 === start
2019-04-16 14:37:09.150225+0800 test123[95964:8582595] 9 === start
2019-04-16 14:37:11.153315+0800 test123[95964:8582595] 9-----{number = 3, name = (null)}
2019-04-16 14:37:11.153338+0800 test123[95964:8582592] 4-----{number = 4, name = (null)}
2019-04-16 14:37:13.157628+0800 test123[95964:8582595] 9-----{number = 3, name = (null)}
2019-04-16 14:37:13.157649+0800 test123[95964:8582592] 4-----{number = 4, name = (null)}
2019-04-16 14:37:13.158006+0800 test123[95964:8582595] 9 === end
2019-04-16 14:37:13.158021+0800 test123[95964:8582592] 4 === end
2019-04-16 14:37:13.158315+0800 test123[95964:8582592] 5 === start
2019-04-16 14:37:13.158366+0800 test123[95964:8582595] 8 === start
2019-04-16 14:37:15.163083+0800 test123[95964:8582592] 5-----{number = 4, name = (null)}
2019-04-16 14:37:15.163111+0800 test123[95964:8582595] 8-----{number = 3, name = (null)}
2019-04-16 14:37:17.163967+0800 test123[95964:8582592] 5-----{number = 4, name = (null)}
2019-04-16 14:37:17.163967+0800 test123[95964:8582595] 8-----{number = 3, name = (null)}
2019-04-16 14:37:17.164365+0800 test123[95964:8582595] 8 === end

分析：任务3（4===start）比任务2（9===start）早输出原因：异步执行任务3 后，下边代码还有耗时操作（任务5、NSLog）所以任务2 需要等待，会先异步执行任务3（4 === start ）

代码2

dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"1 === start");
        for (int i = 0; i<2; i++) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"1-----%@",[NSThread currentThread]);
        }
        NSLog(@"1 === end");
    });
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"2 === start");
        for (int i = 0; i<2; i++) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"2-----%@",[NSThread currentThread]);
        }
        NSLog(@"2 === end");
        dispatch_group_leave(group);
    });
    dispatch_group_enter(group);
    //创建串行队列
    dispatch_queue_t mySerialQueue = dispatch_queue_create("my_test_queue", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t myQueue = dispatch_queue_create("my_test_queue_2", DISPATCH_QUEUE_SERIAL);

    //异步执行+串行队列：开启一个线程
    dispatch_async(mySerialQueue, ^{
        NSLog(@"7-----%@",[NSThread currentThread]);
        NSLog(@"7 === start");
        //异步 不用等待 继续执行
        dispatch_async(myQueue, ^{
            NSLog(@"4 === start");
            for (int i = 0; i<2; i++) {
                [NSThread sleepForTimeInterval:2];
                NSLog(@"4-----%@",[NSThread currentThread]);
            }
            dispatch_async(mySerialQueue, ^{
                NSLog(@"8 === start");
                for (int i = 0; i<2; i++) {
                    [NSThread sleepForTimeInterval:2];
                    NSLog(@"8-----%@",[NSThread currentThread]);
                }
                NSLog(@"8 === end");
            });
            NSLog(@"4 === end");
        });
        //异步 不用等待继续执行
        dispatch_async(myQueue, ^{
            NSLog(@"5 === start");
            for (int i = 0; i<2; i++) {
                [NSThread sleepForTimeInterval:2];
                NSLog(@"5-----%@",[NSThread currentThread]);
            }
        });
        NSLog(@"7 === end");
    });
    //串行队列 需一个一个执行 ：等待 NSLog(@"7 === end") 执行后再执行
    dispatch_async(mySerialQueue, ^{
        NSLog(@"9 === start");
        for (int i = 0; i<2; i++) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"9-----%@",[NSThread currentThread]);
        }
        NSLog(@"9 === end");
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"3 === start");
        for (int i = 0; i<2; i++) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"3-----%@",[NSThread currentThread]);
        }
    });

输出结果：

2019-04-16 14:49:41.670736+0800 test123[96172:8588712] 7-----{number = 3, name = (null)}
2019-04-16 14:49:41.670807+0800 test123[96172:8588710] 1 === start
2019-04-16 14:49:41.670865+0800 test123[96172:8588711] 2 === start
2019-04-16 14:49:41.670975+0800 test123[96172:8588712] 7 === start
2019-04-16 14:49:41.671074+0800 test123[96172:8588712] 7 === end
2019-04-16 14:49:41.671086+0800 test123[96172:8588709] 4 === start
2019-04-16 14:49:41.671177+0800 test123[96172:8588712] 9 === start
2019-04-16 14:49:43.675862+0800 test123[96172:8588710] 1-----{number = 4, name = (null)}
2019-04-16 14:49:43.675871+0800 test123[96172:8588712] 9-----{number = 3, name = (null)}
2019-04-16 14:49:43.675862+0800 test123[96172:8588711] 2-----{number = 5, name = (null)}
2019-04-16 14:49:43.675864+0800 test123[96172:8588709] 4-----{number = 6, name = (null)}
2019-04-16 14:49:45.680467+0800 test123[96172:8588711] 2-----{number = 5, name = (null)}
2019-04-16 14:49:45.680547+0800 test123[96172:8588712] 9-----{number = 3, name = (null)}
2019-04-16 14:49:45.680467+0800 test123[96172:8588710] 1-----{number = 4, name = (null)}
2019-04-16 14:49:45.680478+0800 test123[96172:8588709] 4-----{number = 6, name = (null)}
2019-04-16 14:49:45.680875+0800 test123[96172:8588711] 2 === end
2019-04-16 14:49:45.680874+0800 test123[96172:8588712] 9 === end
2019-04-16 14:49:45.680967+0800 test123[96172:8588709] 4 === end
2019-04-16 14:49:45.680874+0800 test123[96172:8588710] 1 === end
2019-04-16 14:49:45.681135+0800 test123[96172:8588712] 8 === start
2019-04-16 14:49:45.681224+0800 test123[96172:8588709] 5 === start
2019-04-16 14:49:45.681723+0800 test123[96172:8588662] 3 === start
2019-04-16 14:49:47.682394+0800 test123[96172:8588662] 3-----{number = 1, name = main}
2019-04-16 14:49:47.683973+0800 test123[96172:8588709] 5-----{number = 6, name = (null)}
2019-04-16 14:49:47.684043+0800 test123[96172:8588712] 8-----{number = 3, name = (null)}
2019-04-16 14:49:49.683874+0800 test123[96172:8588662] 3-----{number = 1, name = main}
2019-04-16 14:49:49.684750+0800 test123[96172:8588709] 5-----{number = 6, name = (null)}
2019-04-16 14:49:49.684799+0800 test123[96172:8588712] 8-----{number = 3, name = (null)}
2019-04-16 14:49:49.685113+0800 test123[96172:8588712] 8 === end

解析：1和2先后顺序不一定 1、2、4、9相互交替执行，5肯定在4结束后执行，8肯定在9执行完后执行，最后5和8交替执行

GCD 的理解

1. GCD 的两个核心：任务和队列

开启线程的能力跟所在的线程没关系

2. 代码解析

代码1

代码2

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GCD 的理解

1. GCD 的两个核心 ：任务 和 队列

开启线程的能力跟所在的线程没关系

2. 代码解析

代码1

代码2

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1. GCD 的两个核心：任务和队列