ios8 UITableView scrolling jumps back when scrolling up - ios

Question

I have an odd situation with my UItableView.
I am loading images from the internet and presenting them in a uitableviewcell in an asynchronous manner. When I scroll down (i.e. row 1 to 6) the scroll scrolls down smoothly just as I expect it to.
However, then I scroll up (i.g. row 6 to 1) the scroll jumps back. for example if I scroll from row 6 to row 5, it will jump back to row 6. The second time I try to scroll up it lets me go up to row 4, but then it scrolls me back to row 5 or 6 but usually just 5.
What I don't understand is why this is happening in only one direction.
It seems to be effecting ios 8 but not ios 7.
Therefore it is an implementation difference in how ios8 and 7 handle the uitableview.
How can I fix this?
Here is some code to give you some context
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
CVFullImageTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"cell" forIndexPath:indexPath];
CVComicRecord *comicRecord = self.comicRecords[indexPath.row];
[cell.loaderGear stopAnimating];
[cell.text setText:comicRecord.title];
NSString *UUID = comicRecord.fullImagePageURL.absoluteString;
[cell setUUID:UUID];
//Check if image is in the cache
UIImage *fullImage = [self.contentViewCache objectForKey:UUID];
[cell setComicFullImage:fullImage];
if (fullImage) {
return cell;
}
[cell.loaderGear startAnimating];
[self requestImageForIndexPath:indexPath];
return cell;
}
- (void)requestImageForIndexPath:(NSIndexPath *)indexPath {
CVComicRecord *comicRecord = self.comicRecords[indexPath.row];
NSString *UUID = comicRecord.fullImagePageURL.absoluteString;
if ([self.contentViewCache objectForKey:UUID]) {
//if it is already cached, I do not need to make a request.
return;
}
id fd = CVPendingOperations.sharedInstance.fullDownloadersInProgress[UUID];
if (fd) {
//if it is in the queue you do no need to make a request
return;
}
comicRecord.failedFull = NO;
CVFullImageDownloader *downloader = [[CVFullImageDownloader alloc] initWithComicRecord:comicRecord withUUID:UUID];
[CVPendingOperations.sharedInstance.fullDownloaderOperationQueue addOperation:downloader];
//when operation completes it will post a notification that will trigger an observer to call fullImageDidFinishDownloading
}
- (void)fullImageDidFinishDownloading:(NSNotification *)notification {
CVComicRecord *comicRecord = notification.userInfo[#"comicRecord"];
NSString *UUID = notification.userInfo[#"UUID"];
UIImage *fullImage = notification.userInfo[#"fullImage"];
comicRecord.failedFull = NO;
[self.contentViewCache setObject:fullImage forKey:UUID];
dispatch_async(dispatch_get_main_queue(), ^{
for (NSIndexPath *indexPath in [self.tableView indexPathsForVisibleRows]) {
CVFullImageTableViewCell *cell = (id)[self.tableView cellForRowAtIndexPath:indexPath];
if (cell) {
if ([cell.UUID isEqualToString:UUID]) {
[cell.loaderGear stopAnimating];
[cell setComicFullImage:fullImage];
[cell layoutIfNeeded];
}
}
}
});
}
#pragma mark - Scroll
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
static int oldOffset = 0;
int newOffset = scrollView.contentOffset.y;
int dy = newOffset- oldOffset;
if (dy > 0) {
[self hideNavigationbar:YES animationDuration:0.5];
} else if (dy < 0) {
[self hideNavigationbar:NO animationDuration:0.5];
}
oldOffset = newOffset;
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
if (decelerate == NO) {
[self prioritizeVisisbleCells];
//currentPage is a property that stores that last row that the user has seen
[self setCurrentPage:[self currentlyViewedComicIndexPath].row];
}
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
[self prioritizeVisisbleCells];
//currentPage is a property that stores that last row that the user has seen
[self setCurrentPage:[self currentlyViewedComicIndexPath].row];
}
- (void)prioritizeVisisbleCells {
NSArray *ips = [self.tableView indexPathsForVisibleRows];
NSArray *activeIndexPaths = [CVPendingOperations.sharedInstance.fullDownloadersInProgress allKeys];
//add visible cells to queue first
NSSet *visible = [NSSet setWithArray:ips];
NSMutableSet *invisible = [NSMutableSet setWithArray:activeIndexPaths];
[invisible minusSet:visible];
for (NSIndexPath *ip in invisible) {
NSOperation *op = CVPendingOperations.sharedInstance.fullDownloadersInProgress[ip];
[op setQueuePriority:NSOperationQueuePriorityNormal];
}
for (NSIndexPath *ip in visible) {
NSOperation *op = CVPendingOperations.sharedInstance.fullDownloadersInProgress[ip];
[op setQueuePriority:NSOperationQueuePriorityHigh];
}
}

Answer 1

I found the answer in this post very helpful: Table View Cells jump when selected on iOS 8
Try to implement the tableView:estimatedHeightForRowAtIndexPath: in the UITableViewDelegate protocol. It works for me.

Answer 2

Implement the self-sizing and correct view constraints in your storyboard.
in your code put this together
tableView.rowHeight = UITableViewAutomaticDimension;
tableView.estimatedRowHeight = CGFloat value (the initial value)

Answer 3

This exact problem was happening to me while I was using the UITableViewAutomaticDimension and the estimatedRowHeight, as #nferocious76 suggested.
The issue was that my estimatedRowHeight was 400.0 while the actual row height was more like 80.0 (I copy/pasted it from another table view in my app with large cells). I never bothered to change it because I had read somewhere that the estimatedRowHeight didn't really matter as long as it was greater than the actual row height, but apparently this is not the case.

Answer 4

You should calculate your row height yourself and you should remove everything about estimated row height in your code.