iOS_等待多个异步操作回调完成后，执行响应代码

dispatch_group_t group = dispatch_group_create();

dispatch_group_enter(group);
NSLog(@"执行1");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
  dispatch_group_leave(group);
  NSLog(@"完成1");
});

dispatch_group_enter(group);
NSLog(@"执行2");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
  dispatch_group_leave(group);
  NSLog(@"完成2");
});

dispatch_group_enter(group);
NSLog(@"执行3");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
  dispatch_group_leave(group);
  NSLog(@"完成3");
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
  NSLog(@"都完成后，执行");
});