Authored by Olivier Poitrey

Remove outdated motivation section

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... ... @@ -19,49 +19,6 @@ It provides:
NOTE: The version 3.0 of SDWebImage isn't fully backward compatible with 1.0 and requires iOS 5.0
minimum. If you need iOS < 5.0 support, please use the last 2.0 version.
Motivation
----------
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 up with
their solution, most of the time based on asynchronous usage with NSURLConnection, but none provided
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 a 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 attempt to implement my own async image
grabber with caching support as my first steps in this new world. Thus, I asked for help from my good
friend Sebastien Flory ([Fraggle][]), who was working on his great iPhone game ([Urban Rivals][], a
future app-store hit) for almost a year. He spent quite an amount of time 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... On my own network, Youtube was 10
time faster than my own servers... WTF??
In fact, my servers were fine 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. Right then, I
understood something important, asynchronous NSURLConnections are tied to the main runloop in the
NSEventTrackingRunLoopMode. As explained in the documentation, this runloop mode is affected by
UI events:
> Cocoa uses this mode to restrict incoming events during mouse-dragging loops and other sorts of
> user interface tracking loops.
A simple test to recognize an application using NSURLConnection in its default mode to load
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,
you've got one (try with the Facebook app for instance). It took me quite some time to understand
the reason for this lagging issue. Actually I first used NSOperation to workaround this issue.
This technique combined with an image cache instantly gave a lot of responsiveness to my app.
I thought this library could benefit other Cocoa Touch applications so I open-sourced it.
Who Use It
----------
... ... @@ -275,10 +232,3 @@ Future Enhancements
-------------------
- LRU memory cache cleanup instead of reset on memory warning
\ No newline at end of file
[Dailymotion]: http://www.dailymotion.com
[Fraggle]: http://fraggle.squarespace.com
[Urban Rivals]: http://fraggle.squarespace.com/blog/2009/9/15/almost-done-here-is-urban-rivals-iphone-trailer.html
[Three20]: http://groups.google.com/group/three20
[Joe Hewitt]: http://www.joehewitt.com
[tutorial]: http://blog.carbonfive.com/2011/04/04/using-open-source-static-libraries-in-xcode-4
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