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

Since you may recall our original TwUI
, 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
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

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
    helped a lot, but
    still uses a cell.
  • Three-slice
    images are a pain to draw.
  • NSPopover
    doesn't support much appearance customization.
  • Animator

    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

and NSScrollView
. There are
Let Rebel figure out whether you're animating or

Have you seen the username autocompletion popover?

Username autocompletion popover

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.