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In our last blog post, we revealed Mantle, our Cocoa model framework. Today, we're announcing Rebel, a framework for improving AppKit.

Since you may recall our original TwUI announcement, the decision to start using AppKit again bears some explanation.

Farewell, TwUI

For a while now, we’ve been collaborators on Twitter’s TwUI, a popular UI framework for the Mac. TwUI made it easy to build a modern layer-based application for OS X.

However, the AppKit improvements in Lion and Mountain Lion include substantial fixes for layer-backed views. On Snow Leopard, layer-backed NSTextFields and NSTextViews were almost unusable – now, most standard views behave sanely. NSScrollView, in particular, no longer consumes an absurd amount of memory or performs asynchronous tiling (so content no longer fades in while scrolling).

These fixes make TwUI less necessary, so we're slowly migrating GitHub for Mac back to be 100% AppKit, freeing up our development time to work on GitHub for Mac instead of maintaining an entire UI framework alongside it.

As we move away from using TwUI, we will also become less active in its development. We want to leave the framework in good hands, though, so if you're interested in helping maintain TwUI, please open an issue and explain why you think you'd be a good fit.

It's Not All Peaches and Cream

Still, AppKit isn't perfect.

Some significant improvements are only available on Mountain Lion. Even then, there are still some bugs – silly things like horizontally scrolling NSTextFields ending up on half pixels, or NSScrollView being unbearably slow.

Not to mention that many of its APIs are often difficult to use:

  • NSCell is the perennial example. Support for views (instead of cells) in NSTableView helped a lot, but NSControl still uses a cell.
  • Three-slice and nine-slice images are a pain to draw.
  • NSPopover doesn't support much appearance customization.
  • Animator proxies don't immediately reflect changes, and always animate, even when outside of an explicit animation group. Together, these behaviors make it impossible to write a single code path that performs correct layout regardless of whether an animation is occurring.

Introducing Rebel

This is where Rebel comes in. Rebel aims to solve the above problems, and whatever else we may run into.

There are fixes to the NSTextField blurriness and NSScrollView performance. There are iOS-like resizable images. Let Rebel figure out whether you're animating or not.

Have you seen the username autocompletion popover?

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popover

That's RBLPopover at work!

We want to make AppKit easy and enjoyable to use without rewriting it from the ground up.

Getting Involved

Rebel is currently alpha quality. We're already using it in GitHub for Mac, but we may still make breaking changes occasionally.

So, check it out, enjoy, and please file any issues that you find!

Have feedback on this post? Let @github know on Twitter.

Need help or found a bug? Contact us.

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