Multiple Threads Using GCD while Fetching Photos - iphone

Question

I have to fetch contacts from the Address Book and show photo beside each if found in a UITableView.
I fetch all contacts using ABContactsHelper library and then asynchronously fetch photos for visible rows in the UITableView using GCD blocks.
I referred to an Apple Sample code which waits for the UITableView to finish scrolling, get Visible NSIndexPaths & created threads to fetch photos. My problem so far is two fold.
First, if user scrolls, stops, scrolls & stops and does it quite a few times, too many threads are generated for fetching photos which slows down the app.
Secondly, when the thread returns to set photo in cache as well as the UITableViewCell however, the reference to UIImageView is now being reused for another record in UITableViewCell, hence the photo is placed on wrong record which eventually gets replace by correct one, when thread for that particular record returns.
Here is the code I is used both in cellForRowAtIndexPath as well as when UITableView stops scrolling.
- (void)loadImagesLazilyForIndexPath:(NSIndexPath *)indexPath photo:(UIImageView *)photo contact:(ContactModel *)contact
{
if (!self.tableView.isDragging && !self.tableView.isDecelerating) {
UIImage *thePhoto = [self.imagesForContacts objectForKey:indexPath];
if (!thePhoto) {
// NSLog(#"Photo Not Found - Now Fetching %#", indexPath);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
#autoreleasepool {
UIImage *image = [[JMContactsManager sharedContactsManager] photoForContact:contact];
if (!image)
image = self.noProfilePhoto;
[self.imagesForContacts setObject:image forKey:indexPath];
dispatch_async(dispatch_get_main_queue(), ^{
// NSLog(#"Photo Fetched %#", indexPath);
#autoreleasepool {
NSArray *visiblePaths = [self.tableView indexPathsForVisibleRows];
BOOL visible = [visiblePaths indexOfObjectPassingTest:^BOOL(NSIndexPath * ip, NSUInteger idx, BOOL *stop) {
if (ip.row == indexPath.row && ip.section == indexPath.section) {
*stop = YES;
return 1;
}
return 0;
}];
if (visible)
photo.image = [self.imagesForContacts objectForKey:indexPath];
[[NSURLCache sharedURLCache] removeAllCachedResponses];
}
});
}
});
} else {
// NSLog(#"Photo Was Found %#", indexPath);
#autoreleasepool {
photo.image = [self.imagesForContacts objectForKey:indexPath];
}
}
}
}

Answer 1

For this kind of functionality I would go with an NSOperation and an NSOperationQueue, they are build on top of GCD, but it gives you the opportunity to cancel operations. You could check which operation aren't visible anymore and cancel them. In thi s way you can control reference "away".
I see also another issue that could lead into a "problem" it seems that you are caching images in an NSMutableDictionary, aren't you? Or are you using an NSCache? If it is an NScache is fine, but most of mutable object aren't thread safe "naturally"
Boost up the priority of the queue :-)

Answer 2

As mentioned by #Andrea, you should be using an NSOperationQueue, which gives you the ability to cancel queued tasks.
Indexing your image cache by indexPath into your table is not robust as an index path for a given element could change (although maybe not in your specific case). You might consider indexing your image cache by ABRecord.uniqueId instead.
In any case it will not solve the problem of your images being set twice or more for the same cell. This happens because UITableView does not assign a view for each item but manages a pool of UITableCellViews, which it re-uses each time. What you could do is something along the following lines:
// Assuming your "ContactCellView" inherits from UITableCellView and has a contact property
// declared as follows: #property (retain) ABRecord *contact.
- (void) setContact:(ABRecord*)contact
{
_contact = contact;
__block UIImage *thePhoto = [self.imagesForContacts objectForKey:contact.uniqueId];
if (thePhoto == nil) {
_loadImageOp = [NSBlockOperation blockOperationWithBlock:^(void) {
// Keep a local reference to the contact because it might change on us at any time.
ABRecord *fetchContact = contact;
// Fetch the photo as you normally would
thePhoto = [[JMContactsManager sharedContactsManager] photoForContact:fetchContact];
if (thePhoto == nil)
thePhoto = self.noProfilePhoto;
// Only assign the photo if the contact has not changed in the mean time.
if (fetchContact == _contact)
_contactPhotoView.image = thePhoto;
}];
} else {
_contactPhotoView.image = thePhoto;
}
}