Download content for cells only for visible

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.

Do you mean that the certificate is invalid and you need to work around that? But that you don't want to edit SDWebImage code? — Wain, Jan 29 '14 at 13:47
I don't want to use it 3-party for it because SDWebImage solved one problem and creating much bigger one. — Jakub, Jan 29 '14 at 13:52

score 0 · Answer 1 · answered Jan 29 '14 at 13:43

0

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

answered Jan 29 '14 at 13:43

A'sa Dickens

2,045
1
16
21

I am more accustomed to the NSURLConnection rather than the GDC, so i can't really provide example code. – A'sa Dickens Jan 29 '14 at 13:44
Thanks, but how can I use delegate to download image to cells? Where to set a delegate for it? – Jakub Jan 29 '14 at 14:09
Under cell for item at index path method, you call a method in the cell for [download Image] which does all the download actions – A'sa Dickens Jan 29 '14 at 14:44
So the download should be in the cell subclass? And to it attach the delegate. This is serious MVC violation – Jakub Jan 29 '14 at 15:31

Guilherme Torres Castro · Answer 2 · 2014-01-29T15:04:32.850

0

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];

edited Jan 29 '14 at 15:04

answered Jan 29 '14 at 13:53

Guilherme Torres Castro

15,135
7
59
96

I've just check out official docs: http://cocoadocs.org/docsets/AFNetworking/2.0.3/ and don't see `AFImageRequestOperation` class. – Jakub Jan 29 '14 at 14:04
Just edited my answer, they changed AFImageRequestOperation To a normal AFHTTPRequestOperation with AFImageResponseSerializer. https://github.com/AFNetworking/AFNetworking/issues/1434 – Guilherme Torres Castro Jan 29 '14 at 15:05
Yep, i read a docs, but this approach does not support SSL basic authentication, and there was a problem with blocks. – Jakub Jan 29 '14 at 15:49
AFNetworking does support SSL with basic authentication, and which problem with blocks are you talking about? – Guilherme Torres Castro Jan 29 '14 at 16:10
Of course you should do little research about using SSL and basic authentication with AFNetworking. – Guilherme Torres Castro Jan 29 '14 at 16:13
Is this working? I have a UicollectionView with 100 elements and when i scroll down i want to prioritize the download of Images for the cells that are visible – Gabriel Feb 10 '14 at 11:44
Gabriel, I didn't test the code. But it should work. Give a try and then tell me ;) – Guilherme Torres Castro Feb 10 '14 at 13:48

santhu · Answer 3 · 2014-01-29T13:58:38.090

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.

Download content for cells only for visible

3 Answers3