1. 单个队列

dispatch_queue_t serialQueue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t concurrentQueue = dispatch_queue_create("concur", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    NSLog(@"--主队列 %@", [NSThread currentThread]);
    
    //1. 同步串行(不会开启新线程)
    dispatch_sync(serialQueue, ^{
        NSLog(@"--同步串行 %@", [NSThread currentThread]);
    });
    
    //2. 同步并行(不会开启新线程)
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"--同步并行: %@", [NSThread currentThread]);
    });
    
    //3. 异步串行(会开启新线程)
    dispatch_async(serialQueue, ^{
        NSLog(@"--异步串行: %@", [NSThread currentThread]);
    });

    //4. 异步并行(会开启新线程)
    dispatch_async(concurrentQueue, ^{
        NSLog(@"--异步并行: %@", [NSThread currentThread]);
    });
    
    //5. 异步主队列(没有开启新线程)
    dispatch_async(mainQueue, ^{
        NSLog(@"--异步主队列: %@", [NSThread currentThread]);
    });
    
    //6. 主线程同步(死锁)
    dispatch_sync(mainQueue, ^{
        NSLog(@"--主线程同步: %@", [NSThread  currentThread]);
    });

2. 队列嵌套

    NSLog(@"--主线程: %@", [NSThread currentThread]);
    dispatch_queue_t serialQueue = dispatch_queue_create("com.objcc.serial1", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.objcc.concurrent1", DISPATCH_QUEUE_CONCURRENT);
    

    //1. 异步并发嵌套同一个同步并发(异步并发创建新线程，同步并发没有创建新线程)
    dispatch_async(concurrentQueue, ^{
        NSLog(@"--异步并发： %@", [NSThread currentThread]);
        dispatch_sync(concurrentQueue, ^{
            NSLog(@"--同步并发： %@", [NSThread currentThread]);
        });
    });

    //2. 异步并发嵌套同同一个异步并发队列(外层创建新线程，内层不创建新线程)
    dispatch_async(concurrentQueue, ^{
        NSLog(@"--异步并发1： %@",[NSThread currentThread]);
        dispatch_async(concurrentQueue, ^{
            NSLog(@"--异步并发2： %@",[NSThread currentThread]);
        });
    });
    
    
    //3. 同步并发嵌套同一个同步并发队列(都不会开启新线程)
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"--同步并发1： %@",[NSThread currentThread]);
        dispatch_sync(concurrentQueue, ^{
            NSLog(@"--同步并发1： %@",[NSThread currentThread]);
        });
    });
     
    
    //3. 同步并发嵌套异步并发(异步并发会开启新线程)
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"--同步并发1： %@",[NSThread currentThread]);
        dispatch_async(concurrentQueue, ^{
            NSLog(@"--异步并发2： %@",[NSThread currentThread]);
        });
    });
     
    //4. 异步串行，嵌套同一个同步串行(外环开启新线程，内环死锁卡崩溃)
    dispatch_async(serialQueue, ^{
        NSLog(@"--异步串行1： %@",[NSThread currentThread]);
        dispatch_sync(serialQueue, ^{
            NSLog(@"--同步串发2： %@",[NSThread currentThread]);
        });
    });
     
    
    //6. 异步串行嵌套同一个异步串行(外环开启新线程，内环在外环的线程中)
    //2021-05-25 21:54:17.658046+0800 GcdDemo[51591:6655010] --异步串行1： <NSThread: 0x6000033ad400>{number = 6, name = (null)}
    //2021-05-25 21:54:17.658177+0800 GcdDemo[51591:6655010] --异步串行2： <NSThread: 0x6000033ad400>{number = 6, name = (null)}
    dispatch_async(serialQueue, ^{
        NSLog(@"--异步串行1： %@",[NSThread currentThread]);
        dispatch_async(serialQueue, ^{
            NSLog(@"--异步串行2： %@",[NSThread currentThread]);
        });
    });
     
    
    //7. 同步串行，嵌套同一个同步串行队列(内环死锁卡崩溃)
    dispatch_sync(serialQueue, ^{
        NSLog(@"--同步串行1： %@",[NSThread currentThread]);
        dispatch_sync(serialQueue, ^{
            NSLog(@"--同步串行1： %@",[NSThread currentThread]);
        });
    });
    
    //8. 同步串行，嵌套异步串行队列(外环不开启线程，内环开启线程)
    //2021-05-25 21:58:05.277050+0800 GcdDemo[52128:6660403] --同步串行1： <NSThread: 0x600000a30180>{number = 1, name = main}
    //2021-05-25 21:58:05.277239+0800 GcdDemo[52128:6660631] --异步串行2： <NSThread: 0x600000a65fc0>{number = 7, name = (null)}
    dispatch_sync(serialQueue, ^{
        NSLog(@"--同步串行1： %@",[NSThread currentThread]);
        dispatch_async(serialQueue, ^{
            NSLog(@"--异步串行2： %@",[NSThread currentThread]);
        });
    });