Back in August 2009, Shufflegazine featured an article talking about what makes a good iPhone app. The article has a good discussion about what apps are popular and why, and ultimately concludes that a good app has to be simple, intuitive, responsive, and give users a compelling reason to use it.

Unfortunately, there’s no recipe for how to write a simple, intuitive and compelling app. Our best bet for getting our hands on that recipe is probably this guy, but until he makes that announcement, handling these points is left as an exercise for the reader.

Writing a responsive app, however, is much easier.

A good working definition for a “responsive” app is one that responds to user input quickly and doesn’t hang without telling the user what’s going on. The rule of thumb here is that GUIs should respond to input in no more than 1 second, so the bar is set pretty high, especially for web-enabled apps. This article describes the most common reason GUIs get unresponsive and what to do about it.

How Do These Newfangled GUIs Work, Again?

Cocoa Touch’s GUI (like most GUIs) is built on an “event + event loop” architecture. User interactions like taps and keypresses are translated to events, and these events are then processed one-by-one in the imaginatively named event loop:

As long as each event gets processed quickly, the GUI stays nice and zippy. But because events are processed serially, one event can back up the whole gravy train. If one event takes five second to process, then the GUI will be completely unresponsive for those five seconds until the event loop can start processing new events again.

But what if you have some GUI resources that take five seconds to load? We can’t load them all at app initialization time because we don’t always know what resources we’ll need when the app is starting up, especially for things like profile pictures. Also, the iPhone is an embedded platform, so memory for preloading is in short supply, anyway. How, then, can we appease the event loop tiki gods? The answer, young grasshopper, is to load such resources asynchronously, and then update the GUI when they’re done loading.