原理是從tableview中取出某個NSindexPath ,比如取出第0個 NSIndexPath *topPath = [self.tableView indexPathForCell:self.tableView.visibleCells.firstObject];

設定新值

self.topSection = topPath.section;

用KVC 來重新整理tableview;

@property NSInteger topSection;

- (void)viewDidLoad {
    [super viewDidLoad];
    [self addObserver:self forKeyPath:@"topSection" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

}

- (CGFloat)tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section {    return 8;}
- (nullable UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section{
    UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, SWidth, 8)];
    
    if (section == self.topSection) {
        view.backgroundColor = [UIColor redColor];
    }else{
        view.backgroundColor = [UIColor blackColor];
    }
    return view;
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
 
    NSIndexPath *topPath = [self.tableView indexPathForCell:self.tableView.visibleCells.firstObject];
    self.topSection = topPath.section;
    DLog(@"self.topSection:%ld",self.topSection);
 
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if ([keyPath isEqualToString:@"topSection"]) {
        [self.tableView reloadData];

       //或者（就需要記錄新值和舊值）

        [self.tableView reloadSections:indexSet withRowAnimation:UITableViewRowAnimationNone];

    }

}

- (void)dealloc
{
    [self removeObserver:self forKeyPath:@"topSection"];
}