UITableView UIImageView image Flickering - iOS/Objective C - ios

Question

For some reason when I'm downloading my image using GCD, the image will randomly start flickering.
I'll reset the content settings in simulator and it'll work once, then it'll just start flickering again.
This is the code I am using. I've got the reloads in there because if I don't reload it, the image doesn't show until I tap on the cell.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {
NSURL *url = [NSURL URLWithString:self.entries.arrayimage];
NSData *imgData = [NSData dataWithContentsOfURL:url];
dispatch_sync(dispatch_get_main_queue(), ^(void) {
cell.imageView.image = nil;
UIImage *img = [UIImage imageWithData:imgData];
cell.imageView.image = img;
[self.tableView reloadData];
});
});
return cell;
[self.tableView reloadData];

Answer 1

That's because you are using dispatch_async(dispatch_get_global_queue to async load imageData from file. Using this style load image in cellForRow will make cell image should should previous image first. Then finish async load, will call dispatch_sync(dispatch_get_main_queue(), to load the image you want to. Therefore, whenever you reloadData or any other methods to call cellForRow, the cell image will flicker.
I know you want to load image without blocking main thread, but it's not a good way.

Answer 2

Check out apple sample code for Lazy Image loading. And also I checked your code and found that you always downloading image from URL. Instead of that its good to download and save image in caches and then load image from next time in cellForRowAtIndexPath method from local caches if available.

Answer 3

Is this code in cellForRowAtIndexPath:?
If that is the case, the problem here is that all the cells are infinitely reloading the table. You should not be calling reloadData in any of datasource methods that are triggered by reloadData.
What you have is basically an infinite loop of reloading. (reloadData triggers cellForRowAtIndexPath: which once again triggers reloadData).
My suggestion is to use an external component for this as aeskreis posted in his comment.
SDWebImage is probably the best one out there and will allow you to simplify all of the code you have there into simply:
NSURL *url = [NSURL URLWithString:self.entries.arrayimage];
[cell.imageView setImageWithURL:url];

Answer 4

Okay Guys I found out the problem. It was constantly reloading the data which caused te flickering. instead of [self.tableView reloadData] I replaced it with this method:
[cell setNeedsLayout];
I believe this method detects if anything has been changed, and then updates it (from my memory of a couple hours ago so it's probably not 100% accurate), but that fixed my problem.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
NSURL *url = [NSURL URLWithString:self.entries.arrayimage];
NSData *imgData = [NSData dataWithContentsOfURL:url];
dispatch_sync(dispatch_get_main_queue(), ^{
UIImage *img = [UIImage imageWithData:imgData];
cell.imageView.image = img;
//[self.tableView reloadData];
[cell setNeedsLayout];
});
});
return cell;