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 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 don't find this solution to be performant enough, keep reading to find out why.
As many others, I implemented the NSURLConnection solution exposed everywhere on the net and it worked. But when I compared 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 request 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 server was 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 epoll, when a single thread can handle 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. I thought this technic could benefits to a lot of other applications, thus I open-sourced its implementation and there it is!
### DMWebImageView as UIImageWeb Drop-In Replacement
Most common use is in conjunction with an UITableView:
- Place an UIImageView as a subview of your UITableViewCell in Interface Builder
- Set its class to DMImageView in the identity panel.
- Optionally set an image from your bundle to this UIImageView, it will be used as a placeholder image waiting for the real image to be downloaded.
- In your tableView:cellForRowAtIndexPath: UITableViewDataSource method, invoke the setImageWithURL: method of the DMWebImage view with the URL of the image to download
Your done, everything will be handled for you, from parallel downloads to caching management.
### Asynchronous Image Downloader
It is possible to use the NSOperation based image downloader independently. Just create an instance of DMWebImageDownloader using its convenience constructor downloaderWithURL:target:action:.
The download will by queued immediately and the downloadFinishedWithImage: method will be called as soon as the download of image will be completed (prepare not to be called from the main thread).
### Asynchronous Image Caching
It is also possible to use the NSOperation based image cache store independently. DMImageCache 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 DMImageCache 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.
By default DMImageCache 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:
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.
