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Best way to cache images on ios app?
@interface FirstViewController : UITableViewController{

/**/
NSCache *_imageCache;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"hallo";
CustomCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

if (cell == nil)
{
    cell = [[CustomCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}

NSMutableArray *marr = [hallo objectAtIndex:indexPath.section];
NSDictionary *dict = [marr objectAtIndex:indexPath.row];

NSString* imageName = [dict objectForKey:@"Image"];
//NSLog(@"url: %@", imageURL);

UIImage *image = [_imageCache objectForKey:imageName];

if(image)
{
    cell.imageView.image = image;
}
else
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSString* imageURLString = [NSString stringWithFormat:@"example.com/%@", imageName];
        NSURL *imageURL = [NSURL URLWithString:imageURLString];
        UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:imageURL]];

        if(image)
        {
            dispatch_async(dispatch_get_main_queue(), ^{
                CustomCell *cell =(CustomCell*)[self.tableView cellForRowAtIndexPath:indexPath];
                if(cell)
                {
                    cell.imageView.image = image;
                }
            });
            [_imageCache setObject:image forKey:imageName];
        }
    });
}

cell.textLabel.text = [dict objectForKey:@"Name"];

return cell;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"ilvcCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    // set the various cell properties

    // now update the cell image

    NSString *imagename = [self imageFilename:indexPath]; // the name of the image being retrieved

    UIImage *image = [_imageCache objectForKey:imagename];

    if (image)
    {
        // if we have an cachedImage sitting in memory already, then use it

        cell.imageView.image = image;
    }
    else
    {
        cell.imageView.image = [UIView imageNamed:@"blank_cell_image.png"];

        // the get the image in the background

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

            // get the UIImage

            UIImage *image = [self getImage:imagename];

            // if we found it, then update UI

            if (image)
            {
                dispatch_async(dispatch_get_main_queue(), ^{

                    // if the cell is visible, then set the image

                    UITableViewCell *cell = [self.tableView cellForRowAtIndexPath:indexPath];
                    if (cell)
                        cell.imageView.image = image;
                });

                [_imageCache setObject:image forKey:imagename];
            }
        });
    }

    return cell;
}