Question

I have several cells in a UITableview, each with their own UIScrollView. When a user scrolls in one of the cells I would like all of the other cells to follow. Is there any way to create this kind of interaction by passing the contentOffset to the other cells?

I’m thinking delegation but so far I can’t get this to work. Any ideas?

Answer 1

This is actually quite easy. Just implement the UIScrollViewDelegate method scrollViewDidScroll: in your viewController and there you set the contentOffset of all other cells. Make your viewController the delegate of the scrollview in each cell and you are done.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"MyFancyCell";
    MyFancyCell *cell = (MyFancyCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    cell.scrollView.delegate = self; // actually you would do this in the storyboard or inside (cell == nil) if you don't use storyboard
    // configure cell    
    return cell;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView 
{
    if (scrollView == self.tableView) {
        // each tableView is a UIScrollView too. 
        // they will call this method which will lead to strange results if you change your cells scrollView.
        // just ignore the scroll events of the tableView
        return;
    }
    CGPoint contentOffset = scrollView.contentOffset;
    for (MyFancyCell *cell in [self.tableView visibleCells]) {
        cell.scrollView.contentOffset = contentOffset;
    }
}

---

EDIT:
you could forward the delegate calls back to the cell, like this:

for (MyFancyCell *cell in [self.tableView visibleCells]) {
    cell.scrollView.contentOffset = contentOffset;
    [cell scrollViewDidScroll:cell.scrollView];
}

or an imho better way. Make the viewController a delegate of the cell and in the scrollViewDidScoll method inside the cell you tell the delegate (viewController) that the contentOffset has changed so that the view controller updates the cells.

// MyFancyCell.h
@protocol MyFancyCellDelegate 
- (void)fancyCell:(MyFancyCell *)cell didChangeContentOffset:(CGPoint)offset;
@end
@property (weak, nonatomic) id <MyFancyCellDelegate> delegate;

// MyFancyCell.m

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
     // do your work here
     [self.delegate fancyCell:self didChangeContentOffset:scrollView.contentOffset];
}

// ViewController.m

- (void)fancyCell:(MyFancyCell *)cell didChangeContentOffset:(CGPoint)offset {
    for (MyFancyCell *cell in [self.tableView visibleCells]) {
        cell.scrollView.contentOffset = offset;
    }
}


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"MyFancyCell";
    MyFancyCell *cell = (MyFancyCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    cell.delegate = self;
    // configure
}