Download content for cells only for visible - ios

Question

I have UICollectionViewCell with dynamic content download (image download). I have download in block in cell:
-(MainVCCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
static NSString *cellIdentifier = #"MainVCCell";
MainVCCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
Person *person = [self.fetchedResult objectAtIndex:indexPath.row];
[cell.login setText:person.login];
if(person.avatar) {
[cell.avatarImageView setImage:[UIImage imageWithData:person.avatar]];
} else {
[cell.avatarImageView setImage:[UIImage imageNamed:#"placeholder"]];
[AsyncUrl request:[NSString stringWithFormat:#"some SSL URL",person.login] completeBlock:^(NSData *data) {
dispatch_queue_t downloadQueue = dispatch_queue_create("Download queue", NULL);
dispatch_async(downloadQueue, ^{
dispatch_async(dispatch_get_main_queue(), ^{
MainVCCell *cellToUpdate = (MainVCCell*)[collectionView cellForItemAtIndexPath:indexPath];
if(cellToUpdate) {
[cellToUpdate.avatarImageView setImage:[UIImage imageWithData:data]];
}
person.avatar = data;
[[CoreDataController sharedInstance] saveContext];
});
});
} errorBlock:^(NSError *error) {
NSLog(#"%#",error);
}];
}
return cell;
}
And it work fine, but of course when i scroll several times, i get so many connections and download fire that some of them even timeout. I understand why is this happening. Is there a way to cancel connections in invisible cell blocks? I want to download only a visible content.
I'm familiar with SDWebImage but this library is not support SSL connections, so i can't use it.

Answer 1

Collection views have a delegate method that is called when the cell disappears
- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
Just make sure you have a method for stop the connection, then call that method.
UICollectionViewDelegate Protocol

Answer 2

I strong recommend you to use AFNetworking.
Then you create an array of NSOperation in your viewDidLoad:
self.operationQueue = [[NSMultableArray alloc]init];
In your -(MainVCCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath, make something similar to this:
AFHTTPRequestOperation *operation [[AFHTTPRequestOperation alloc] initWithRequest:posterURLRequest];
operation.responseSerializer = [AFImageResponseSerializer serializer];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image)
{
if(cellToUpdate) {
[cellToUpdate.avatarImageView setImage:[UIImage imageWithData:data]];
}
person.avatar = data;
[[CoreDataController sharedInstance] saveContext];
}
failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error)
{
// manage errors
}];
[self.operationQueue addObject:operation];
[operation start];
Them At - (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
[self.operationQueue[indexPath.row] cancel];

Answer 3

use NSURLConnection to start a download.
Create a subClass of NSObject which has one NSUrlConnection instance property , for this subclass you provide a link and it will download the image using NSUrlConnection.
Create instances of this class when ever you want to download an image and push it into an array ( say ConnectionsArray).
When you think, you dont want to download particular indexPaths images, just cancel those by using ConnectionsArray.
Get that particular download-instance using indexPath and ConnectionsArray,and call cancel method of NSURLConnection of that object.
NSURLConnection has cancel method, which cancels the ongoing operation.