Today, Facebook spent $2 billion and open-sourced a library for iOS developers.

Tweaks, available on GitHub, provides an interface for developers to make minor adjustments and tweak parameters of an app directly inside the app, in a few seconds. Those changes can be the color of a button or the speed of an animation, and Facebook says that Tweaks helped them build Paper, the highly praised alternative Facebook app.

Here’s TechCrunch’s Greg Kumparak on Tweaks:

For developers, it means being able to fine-tune applications faster and with less code. As an added bonus, it lets any of their designers who might not love to code help figure out the best settings without having to pop into the source or pester the dev team for a million new builds.

And Facebook, on the project’s page:

Occasionally, it’s perfect the first try. Sometimes, the idea doesn’t work at all. But often, it just needs a few minor adjustments. That last case is where Tweaks fits in. Tweaks makes those small adjustments easy: with no code changes and no computer, you can try out different options and decide which works best.

Some of the most useful parameters to adjust are animation timings, velocity thresholds, colors, and physics constants. At Facebook, we also use tweaks to temporarily disable new features during development. That way, the designers and engineers involved can enable it on just their devices, without getting in the way of others testing the app.

Tweaks looks like a handy solution for developers, designers, and, to an extent, even testers of apps. It’s available here.