The Granite framework is something people have cooking for developers to really make it easy to develop applications on elementary. Right now it's very much a work-in-progress, but in the future they hope it will provide useful libraries for storing settings and communicating with other applications, among other things. Basically, the lead elementary developers noticed a lot of redundancy and unnecessary complications in writing applications for elementary, so Granite is their solution to these issues. Right now it is an amalgamation of some widgets and libraries whipped up by elementary developers, and some taken from the fantastic Docky/Plank projects.
See its [https://valadoc.org/granite/index.htm API] and [https://valadoc.org/granite/Granite.Widgets.html custom