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Behind the scenes with Apple's media browser
Mac Secrets Welcome my second “peek behind the scenes” for Cocoa developers. I enjoyed the responses to the first installment, both for and against, and especially the lively email exchanges, some of which I plan to pick up on in the near future!
For now, though, let me just emphasize a couple of important points.
First, this column is not for novice Cocoa developers. If you're new to Cocoa programming, then you definitely shouldn't be casually exploiting undocumented features of the platform. Instead, go check out some of the great Cocoa tutorials here.
Next, when working with undocumented classes and methods, be sure to bulletproof your code as much as possible. As one of my respondents said: “It's important to have your code fail gracefully if a wanted class/method doesn't exist, rather than... uhh... disgracefully!”
To put this in concrete terms, before calling an undocumented method, use the respondsToSelector: message to see if the method exists. Similarly, when checking for the presence of an undocumented class, use NSClassFromString to get a reference to the wanted class. If the returned reference is nil, then you know the class no longer exists.
You won't find these checks in my sample project because I don't want to obfuscate the basic intent of the code and we all have our own ideas about how error handling should work.
Things get slick
Having got the compulsory caveats out of the way, let's take a look at how to implement the so called "media browser" feature inside your own application. As the name suggests, a media browser is designed to let the end user browse for media - meaning things like photos, movies and audio files - before selecting a wanted item and incorporating it into their document.
Browse before selecting your media
At time of writing, a number of open-source media browser implementations exist. For example, type "iMediaBrowser" into Google, and you'll be pointed at various different projects. Heck, you can even buy a commercial implementation. The most popular browser, however, is totally free, includes full source code and it's available here. Once you've downloaded the code and looked through it, you'll see that it's quite a complex beast, using an extensible architecture to handle various different media types.