iOS UITableView scroll position jumps when scrolling up - ios

Question

Here's what I'm trying to do.
I have a UITableViewCell lets say with fixed height of 300 (it is actually a variable size height but I'm trying to simplify the example)
What I want to achieve is that when I scroll back up - I will have a "thumbnailed" version of the cell - with height of 75
I managed to make it happen, but now the problem is that when I scroll up the previous cell heights are adjusted and the scroll position "jumps" once the cell sizes are smaller, which causes the view to "jump back down" when he scrolls up.
How can I adjust it?
The code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell;
if (indexPath.row < lastViewedChapter)
{
cell = [self generateChapterCell:tableView indexPath:indexPath collapsed:YES];
}
else
{
cell = [self generateChapterCell:tableView indexPath:indexPath collapsed:NO];
if (indexPath.row > lastViewedChapter)
{
lastViewedChapter = indexPath.row;
}
}
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.row < lastViewedChapter)
{
return 73;
}
else
{
return 300; //actually here is a code that calculates the height
}
}

Answer 1

You've reduced height of the upper cell and then other cells moved up to fill that space while you were still scrolling right?
Try to set new tableView.contentOffset when you change the cell's height.
In your case the contentOffset.y should be (old contentOffset.y - (300 - 73)) when you return the cell's height as 73.
I didn't test on this but I think it may help and you must calculate new contentOffset for other case too (when scroll down, when table reload data).
static NSInteger _lastRow = -1;
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
if (_lastRow == -1) {
_lastRow = indexPath.row;
return 300;
} else {
if (_lastRow > indexPath.row) {
_lastRow = indexPath.row;
if ([tableView rectForRowAtIndexPath:indexPath].size.height == 300) {
[tableView setContentOffset:CGPointMake(tableView.contentOffset.x, (tableView.contentOffset.y - (300 - 73)))];
}
return 73;
} else {
_lastRow = indexPath.row;
return 300;
}
}
}
This code work fine but still has some bugs (the first row height when first load data is like you have scroll up to it once, when you scroll up to top fast it bounced not normally) but I hope this should help you.

Answer 2

This is something that will definitely happen since you have changed cell heights.
The question is how to mitigate this kind of bad user experience.
UITableView are subclassed from UIScrollView. UIScrollViews provide delegate which is available in UITableView class as well.
Do the following.
self.tableView.delegate = self;
And then implement the following function. In the following, location is a CGPoint variable defined in your header.
-(void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
location = tableView.contentOffset;
}
-(void)tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
CGPoint newLocation = tableView.contentOffset;
if (CGPointEqualToPoint(location, newLocation))
{
NSLog(#"are equal");
tableView.contentOffset = CGPointMake(location.x, location.y-227);
}
}