The now defunct Cocos2D-ObjC project was a hugely successful games development library for developing iPhone games. Building on top of OpenGLES, it offered a more performant means of rendering graphics and manipulating visual objects than the UIKit based API offered by default. The scene graph API implemented in Cocos2D is now reflected in Apple's own SpriteKit framework.
After using the library for several years in a few mobile game projects, I got a position working as a full time maintainer of the library, making contributions to the core library as well as it's companion UI building app SpriteBuilder (analogous to Interface Builder for UIKit).
Project link: https://github.com/cocos2d/cocos2d-objc