Asynchronous image downloader with cache support with an UIImageView category
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Web Image

This library provides a category for UIImageVIew with support for remote images coming from the web.

It provides:

  • An UIImageView category adding web image and cache management to the Cocoa Touch framework
  • An asynchronous image downloader using threads (NSOperation)
  • An asynchronous memory + disk image caching with automatic cache expiration handling
  • A garantie that the same URL won't be downloaded several times
  • A garantie that bogus URLs won't be retried again and again
  • Performances!


As a dummy Objective-C developer working on my first iPhone application for my company (Dailymotion), I've been very frustrated by the lack of support in the Cocoa Touch framework for UITableView with remote images. After some googling, I found lot of forums and blogs coming with their solution, most of the time based on asynchronous usage have NSURLConnection, but none provides a simple library doing the work of async image grabbing + caching for you.

Actually there is one in the famous Three20 framework by Joe Hewitt, but it's yet massive and undocumented piece of code. You can't import just the the libraries you want without taking the whole framework (damn #import "TTGlobal.h"). Anyway, the Three20 implementation is based on NSURLConnection, and I soon discovered this solution wasn't ideal, keep reading to find out why.

As a hurried beginner in iPhone development, I couldn't admit to implement my own async image grabber with caching support as my first steps in this new world. Thus, I asked for help to my good friend Sebastien Flory (Fraggle), who was working on his great iPhone game (Urban Rivals, a future app-store hit) since almost a year. He spent quite an amount of time on implementing the very same solution for his needs, and was kind enough to give me his implementation for my own use. This worked quite well and allowed me to concentrate on other parts of my application. But when I started to compare my application with its direct competitor - the built-in Youtube application - I was very unhappy with the loading speed of the images. After some network sniffing, I found that every HTTP requests for my images was 10 times slower than Youtube's ones... On my own network, Youtube was 10 time faster than my own servers... WTF??

In fact, my servers were well but a lot of latency was added to the requests, certainly because my application wasn't responsive enough to handle the requests at full speed. At this moment, I understood something important, asynchronous NSURLConnections are tied to the main runloop (I guess). It's certainly based on the poll multiplexer system call, which allows a single thread to handle quite a huge number of simultaneous connections. It works well while nothing blocks in the loop, but in this loop, there is also the events handling. A simple test to recognize an application using NSURLConnection to load there remote images is to scroll the UITableView with your finger to disclose an unloaded image, and to keep your finger pressed on the screen. If the image doesn't load until you release you finger, the application is certainly using NSURLConnection (try with the Facebook app for instance). I'm not completely clear about the reason of this blocking, I thought the iPhone was running a dedicated run-loop for connections, but the fact is, NSURLConnection is affected by the application events and/or UI handling (or something else I'm not aware of).

Thus I explored another path and found this marvelous NSOperation class to handle concurrency with love. I ran some quick tests with this tool and I instantly got enhanced responsiveness of the image loading in my UITableView by... a lot. Thus I rewrote the Fraggle's implementation using the same concept of drop-in remplacement for UIImageView but with this new technic. I thought the result could benefits to a lot of other applications, thus we decided, with Fraggle, to open-sourced it, et voila!

How To Use It

Using UIImageView+WebCache category with UITableView

Just #import the UIImageView+WebCache.h header, and call the setImageWithURL:placeholderImage: method from the tableView:cellForRowAtIndexPath: UITableViewDataSource method. Everything will be handled for you, from parallel downloads to caching management.

#import "UIImageView+WebCache.h"


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
    static NSString *MyIdentifier = @"MyIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

    if (cell == nil)
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                       reuseIdentifier:MyIdentifier] autorelease];

    // Here we use the new provided setImageWithURL: method to load the web image
    [cell.imageView setImageWithURL:[NSURL URLWithString:@""]
                   placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

    cell.textLabel.text = @"My Text";
    return cell;

Using SDWebImageManager

The SDWebImageManager is the class behind the UIImageView+WebCache category. It ties the asynchronous downloader with the image cache store. You can use this classe directly to benefits from web image downloading with caching in another context than a UIView (ie: with Cocos).

Here is a simple example of how to use SDWebImageManager:

SDWebImageManager *manager = [SDWebImageManager sharedManager];

UIImage *cachedImage = [manager imageWithURL:url];

if (cachedImage)
    // Use the cached image immediatly
    // Start an async download
    [manager downloadWithURL:url delegate:self];

Your class will have to implement the SDWebImageManagerDelegate protocol, and to implement the imageHelper:didFinishWithImage: method from this protocol:

- (void)imageHelper:(SDWebImageManager *)imageHelper didFinishWithImage:(UIImage *)image
    // Do something with the downloaded image

Using Asynchronous Image Downloader Independently

It is possible to use the NSOperation based image downloader independently. Just create an instance of SDWebImageDownloader using its convenience constructor downloaderWithURL:target:action:.

downloader = [SDWebImageDownloader downloaderWithURL:url delegate:self];

The download will by queued immediately and the imageDownloader:didFinishWithImage: method from the SDWebImageDownloaderDelegate protocol will be called as soon as the download of image is completed (prepare not to be called from the main thread).

Using Asynchronous Image Caching Independently

It is also possible to use the NSOperation based image cache store independently. SDImageCache maintains a memory cache and an optional disk cache. Disk cache write operations are performed asynchronous so it doesn't add unnecessary latency to the UI.

The SDImageCache class provides a singleton instance for convenience but you can create your own instance if you want to create separated cache namespaces.

To lookup the cache, you use the imageForKey: method. If the method returns nil, it means the cache doesn't currently own the image. You are thus responsible of generating and caching it. The cache key is an application unique identifier for the image to cache. It is generally the absolute URL of the image.

UIImage *myCachedImage = [[SDImageCache sharedImageCache] imageFromKey:myCacheKey];

By default SDImageCache will lookup the disk cache if an image can't be found in the memory cache. You can prevent this from happening by calling the alternative method imageFromKey:fromDisk: with a negative second argument.

To store an image into the cache, you use the storeImage:forKey: method:

[[SDImageCache sharedImageCache] storeImage:myImage forKey:myCacheKey];

By default, the image will be stored in memory cache as well as on disk cache (asynchronously). If you want only the memory cache, use the alternative method storeImage:forKey:toDisk: with a negative third argument.

Future Enhancements

  • LRU memory cache cleanup instead of reset on memory warning