enter和leave必须成对使用，若调用enter之后，没有相应的leave操作，那么这一组任务就永远执行不完。

dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    //成功或失败后
    dispatch_group_leave(group);
});
dispatch_group_enter(group);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    //成功或失败后
    dispatch_group_leave(group);
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
   
    NSLog(@"刷新UI");
});