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NOTE: The version 3.0 of SDWebImage isn't fully backward compatible with 1.0 and requires iOS 5.0
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minimum. If you need iOS < 5.0 support, please use the last 2.0 version.
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Motivation
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As a dummy Objective-C developer working on my first iPhone application for my company
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([Dailymotion][]), I've been very frustrated by the lack of support in the Cocoa Touch framework for
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UITableView with remote images. After some Googling, I found lot of forums and blogs coming up with
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their solution, most of the time based on asynchronous usage with NSURLConnection, but none provided
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a simple library doing the work of async image grabbing + caching for you.
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Actually there is one in the famous [Three20][] framework by [Joe Hewitt][], but it's a massive
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and undocumented piece of code. You can't import just the the libraries you want without taking the
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whole framework (damn #import "TTGlobal.h"). Anyway, the [Three20][] implementation is based on
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NSURLConnection, and I soon discovered this solution wasn't ideal. Keep reading to find out why.
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As a hurried beginner in iPhone development, I couldn't attempt to implement my own async image
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grabber with caching support as my first steps in this new world. Thus, I asked for help from my good
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friend Sebastien Flory ([Fraggle][]), who was working on his great iPhone game ([Urban Rivals][], a
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future app-store hit) for almost a year. He spent quite an amount of time implementing the very
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same solution for his needs, and was kind enough to give me his implementation for my own use. This
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worked quite well and allowed me to concentrate on other parts of my application. But when I started
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to compare my application with its direct competitor - the built-in Youtube application - I was very
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unhappy with the loading speed of the images. After some network sniffing, I found that every HTTP
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requests for my images was 10 times slower than Youtube's... On my own network, Youtube was 10
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time faster than my own servers... WTF??
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In fact, my servers were fine but a lot of latency was added to the requests, certainly because my
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application wasn't responsive enough to handle the requests at full speed. Right then, I
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understood something important, asynchronous NSURLConnections are tied to the main runloop in the
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NSEventTrackingRunLoopMode. As explained in the documentation, this runloop mode is affected by
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UI events:
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> Cocoa uses this mode to restrict incoming events during mouse-dragging loops and other sorts of
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> user interface tracking loops.
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A simple test to recognize an application using NSURLConnection in its default mode to load
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remote images is to scroll the UITableView with your finger to disclose an unloaded image, and to
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keep your finger pressed on the screen. If the image doesn't load until you release you finger,
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you've got one (try with the Facebook app for instance). It took me quite some time to understand
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the reason for this lagging issue. Actually I first used NSOperation to workaround this issue.
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This technique combined with an image cache instantly gave a lot of responsiveness to my app.
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I thought this library could benefit other Cocoa Touch applications so I open-sourced it.
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Who Use It
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@@ -275,10 +232,3 @@ Future Enhancements |
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-------------------
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- LRU memory cache cleanup instead of reset on memory warning |
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\ No newline at end of file |
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[Dailymotion]: http://www.dailymotion.com
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[Fraggle]: http://fraggle.squarespace.com
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[Urban Rivals]: http://fraggle.squarespace.com/blog/2009/9/15/almost-done-here-is-urban-rivals-iphone-trailer.html
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[Three20]: http://groups.google.com/group/three20
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[Joe Hewitt]: http://www.joehewitt.com
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[tutorial]: http://blog.carbonfive.com/2011/04/04/using-open-source-static-libraries-in-xcode-4 |
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