Collection view images are flickering while scrolling - ios

Question

Im using the following code to populate a collection view cells with images.
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *identifier = #"Cell";
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
UIImageView *recipeImageView = (UIImageView *)[cell viewWithTag:100];
recipeImageView.image = nil;
if ([imageArray count] >0){
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {
NSData *data0 = [NSData dataWithContentsOfURL: [NSURL URLWithString:[imageArray objectAtIndex:indexPath.row]]];
UIImage *image = [UIImage imageWithData: data0];
dispatch_sync(dispatch_get_main_queue(), ^(void) {
recipeImageView.image = image;
});
});
}
[spinnerShow stopAnimating];
return cell;
}
The problem is that, when Im scrolling the images are flickering and are flashing. Why is that so? How can I make those images to be stable without flickering?

Answer 1

Just a short overview, So you get your answer
UICollectionView is highly optimized, and thus only keep On-screen visible rows in memory. Now, All rows Cells are cached in Pool and are reused and not regenerated. Whenever, user scrolls the UICollectionView, it adds the just-hidden rows in Pool and reuses them for next to be visible rows.
So, now, coming to your answer
When you scroll your CollectionView, collectionView datasource method gets called again for every indexPath, coming in visible range and your image gets downloaded again
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
SOLUTION
Instantiate a instance NSMutableDictionary, outside of method.
Now in your code
#implementation ClassName{
NSMutableDictionary *cachedImage;
}
-(void)viewDidLoad(){
[super viewDidLoad];
cachedImage = [NSMutableDictionary new];
}
/*OLD CODE*/
UIImageView *recipeImageView = (UIImageView *)[cell viewWithTag:100];
recipeImageView.image = nil;
if ([imageArray count] >0){
//IF image is already downloaded, simply use it and don't download it.
if(cachedImage[[imageArray objectAtIndex:indexPath.row]] != nil){
recipeImageView.image = cachedImage[[imageArray objectAtIndex:indexPath.row]];
}
else{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {
NSData *data0 = [NSData dataWithContentsOfURL: [NSURL URLWithString:[imageArray objectAtIndex:indexPath.row]]];
UIImage *image = [UIImage imageWithData: data0];
dispatch_sync(dispatch_get_main_queue(), ^(void) {
recipeImageView.image = image;
//****SAVE YOUR DOWNLOADED IMAGE
cachedImage[[imageArray objectAtIndex:indexPath.row]] = image; //****SAVE YOUR DOWNLOADED IMAGE
});
});
}
}
/*OLD CODE*/

Answer 2

as per my knowledge, you are fetching image but not caching it that's why when your UICollectionViewCell gets reload, you get UIImageView's fresh instance so and this thing goes on and on in your code..
in this case, i recommend you to use SDWebImage OR AFNetworking Frameworks. because these frameworks does all the tricky stuff for you with the simple line of code (SDWebImage Framework),
NSURL* url = [NSURL URLWithString:str];
[yourImageView setBackgroundImageWithURL:url forState:UIControlStateNormal placeholderImage:kPlaceholder];