iOS11下，凡是隐藏了导航栏的界面，具有ScrollView的子类的界面，都会在滑动或者pop后发生下沉。解决方法：
if (@available(iOS 11.0, *)) {
self.scrollView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;
} else {
self.automaticallyAdjustsScrollViewInsets = NO;
}
