按下向上/向下按钮时，平滑地上下移动滚动视图。objective-c

UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
scrollView.contentSize = CGSizeMake(self.view.frame.size.width, contentHeight);

UIView *contentView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, contentHeight)];
[scrollView addSubview:contentView];

UIButton *upButton = [UIButton buttonWithType:UIButtonTypeSystem];
[upButton setTitle:@"向上" forState:UIControlStateNormal];
[upButton addTarget:self action:@selector(scrollUp) forControlEvents:UIControlEventTouchUpInside];

UIButton *downButton = [UIButton buttonWithType:UIButtonTypeSystem];
[downButton setTitle:@"向下" forState:UIControlStateNormal];
[downButton addTarget:self action:@selector(scrollDown) forControlEvents:UIControlEventTouchUpInside];

- (void)scrollUp {
    CGPoint contentOffset = scrollView.contentOffset;
    contentOffset.y -= scrollDistance;
    [scrollView setContentOffset:contentOffset animated:YES];
}

- (void)scrollDown {
    CGPoint contentOffset = scrollView.contentOffset;
    contentOffset.y += scrollDistance;
    [scrollView setContentOffset:contentOffset animated:YES];
}