Odd threading issue with UITableView images loaded from URL - ios

Question

First up I'm a UI designer getting into Xcode, so please bear with me if i'm a little vague in my description here.
Basically I have a UITableView full of contacts, and each contact has a photo image to the left of the name, pulled from the web via url.. I have implemented lazy loading with the images, but when you scroll through the list, i seem to be getting odd flickering and a really 'laggy' (ie. not smooth) scroll.
I have tried playing around with the setNeedsDisplay calls (i know there's a lot of them), but it still seems to wig out.
Have been trying to google some 'best practices' for this, but not sure what's going on here... Any help would be greatly appreciated! Thanks.
Target: iOS8 in Xcode 6.1
Here's my code for cellForRowAtIndexPath:
if([peep getImage].length > 4) // if URL smaller than 4, it's probably not a valid URL
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
if([peep getImage])
{
if(![imageCache objectForKey:indexPath])
{
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[peep getImage]]]];
if(image != nil) {
[imageCache setObject:image forKey:indexPath];
[cell.peepImage setImage:[imageCache objectForKey:indexPath]];
[[cell peepImage] setNeedsDisplay];
}
});
}
else
{
dispatch_async(dispatch_get_main_queue(), ^{
[[cell peepImage] setImage:[imageCache objectForKey:indexPath]];
[[cell peepImage] setNeedsDisplay];
});
}
}
[cell.peepImage setNeedsDisplay];
});
}
else
{
cell.peepImage.image = [UIImage imageNamed: #"defaultuser.jpg"];
}
[cell setNeedsDisplay];
return cell;
}

Answer 1

My guess is the creation of the image on the main thread. By moving it off the main thread your problem should be fixed.
if([peep getImage].length > 4) // if URL smaller than 4, it's probably not a valid URL
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
if([peep getImage])
{
if(![imageCache objectForKey:indexPath])
{
// Move image creation here
__block UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[peep getImage]]]];
dispatch_async(dispatch_get_main_queue(), ^{
if(image != nil) {
[imageCache setObject:image forKey:indexPath];
[cell.peepImage setImage:[imageCache objectForKey:indexPath]];
[[cell peepImage] setNeedsDisplay];
}
});
}
else
{
dispatch_async(dispatch_get_main_queue(), ^{
[[cell peepImage] setImage:[imageCache objectForKey:indexPath]];
[[cell peepImage] setNeedsDisplay];
});
}
}
[cell.peepImage setNeedsDisplay];
});
}
else
{
cell.peepImage.image = [UIImage imageNamed: #"defaultuser.jpg"];
}
[cell setNeedsDisplay];
return cell;