UITableViewCell load images and reused cells

Question

i need to load from web/files some UIImages. I was searching and i found in other question this code:

    if (![[NSFileManager defaultManager] fileExistsAtPa         


        

Answer 1

I suspect the incorrect images you see are a result of you not setting your place holder image in the event of you having a local copy of an image but still retrieving the local copy asynchronously. Also In the code you appended for loading the local copy you use UIImage a UIKit component on a background thread.

Also interestingly you seem to be doing some kind of UIImage caching. Adding the images to what I assume is an NSMutableArray property named imageFriends. But you seem to have commented out the cache add in the event you have a local copy of the file. Also your posted code never uses the cached UIImages.

While 2 levels of caching seems a bit overboard if you wanted to do this you could do something like this:

UIImage *userImage = [self.imageFriends objectForKey:[NSNumber numberWithInt:user.userId]];
if (userImage) { // if the dictionary of images has it just display it
    cell.imageView.image = userImage;
}
else {
    cell.imageView.image = [UIImage imageNamed:@"xger86x.jpg"]; // set placeholder image
    BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:user.image];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSData *imageData = nil;
        if (fileExists){
            imageData = [NSData dataWithContentsOfFile:user.image];
        }
        else {
            imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:user.imageURL]];
            [imageData writeToFile:user.image atomically:YES];
        }
        if (imageData){
           dispatch_async(dispatch_get_main_queue(), ^{ 
                    // UIKit, which includes UIImage warns about not being thread safe
                    // So we switch to main thread to instantiate image
               UIImage *image = [UIImage imageWithData:imageData];
               [self.imageFriends setObject:image forKey:[NSNumber numberWithInt:user.userId]];
               UITableViewCell *lookedUpCell = [tableView cellForRowAtIndexPath:indexPath];
               if (lookedUpCell){
                   lookedUpCell.imageView.image = image;
                   [lookedUpCell setNeedsLayout];
               }
           }); 
        }
    });
}

UIImages are part of UIKit and not thread safe. But you can load the NSData on another thread.