只要是主队列就不会开线程

同步函数+（并发队列or主队列）的两种情况

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"11  %@",[NSThread currentThread]);
        });
    });
     
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"22  %@",[NSThread currentThread]);
        });
    });
     
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"33  %@",[NSThread currentThread]);
        });
    });
     
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"44  %@",[NSThread currentThread]);
        });
    });
     
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"55  %@",[NSThread currentThread]);
        });
    });

一个接一个执行

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"11  %@",[NSThread currentThread]);
    });
});
 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"22  %@",[NSThread currentThread]);
    });
});
 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"33  %@",[NSThread currentThread]);
    });
});
 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"44  %@",[NSThread currentThread]);
    });
});
 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"55  %@",[NSThread currentThread]);
    });
});

异步执行