LIFOOperationQueue

A last-in-first-out NSOperation queue written in Objective-C.

What's it for?

A LIFO queue can help prioritize new operations over old ones. One such example would be loading images in a UITableView. The LIFO approach will ensure that images on screen take higher priority than those a user has already scrolled past.

How does it work?

LIFOOperationQueue is very much like NSOperationQueue. Just like the native implementation, you can configure the maximum number of concurrent operations. The only difference is that operations are added to the front of the queue and NSOperationQueuePriority has no effect. Initialization looks like this:

// initialize LIFOOperationQueue with a maximum thread count of 4
LIFOOperationQueue *operationQueue = [[LIFOOperationQueue alloc] initWithMaxConcurrentOperationCount:4];

Here are some quick examples of loading images in a UITableView with AFNetworking and LIFOOperationQueue.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    // create cell
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"CellIdentifier"];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"CellIdentifier"];
    }
    
    // initialize imageView.image
    cell.imageView.image = [UIImage imageNamed:@"blankImage.png"];
    
    // create image request
    NSURL *imageUrl = [NSURL URLWithString:@"http://www.i-love-cats.com/software/Adorable-Cats-Screensaver.jpg"];
    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:imageUrl];
    AFImageRequestOperation *imageRequestOperation = [AFImageRequestOperation imageRequestOperationWithRequest:urlRequest success:^(UIImage *image) {
        UITableViewCell updateCell = [tableView cellForRowAtIndexPath:indexPath];
        if (updateCell)
            updateCell.imageView.image = image;
    }];
    
    // add to LIFOOperationQueue
    [self.operationQueue addOperation:imageRequestOperation];
    
    return cell;
}

The code above would prioritize the latest cell's image over those that may not be on screen anymore. You can accomplish the same thing without AFNetworking by using blocks.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    // create cell
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"CellIdentifier"];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"CellIdentifier"];
    }
    
    // initialize imageView.image
    cell.imageView.image = [UIImage imageNamed:@"blankImage.png"];
    
    // add block to LIFOOperationQueue
    [self.operationQueue addOperationWithBlock:^{
        UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:url]]];
        
        // be sure to display image on main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            UITableViewCell updateCell = [tableView cellForRowAtIndexPath:indexPath];
            if (updateCell)
                updateCell.imageView.image = image;
        });
    }];
    
    return cell;
}

addOperationWithBlock: executes the block asynchronously by default.

License

LIFOOperationQueue is available under the MIT license. See the LICENSE file for more info.