JTTracker is a public domain sample that provides a framework for a UI tracking facility (grab handles). The code is public domain and free for use for all.
* Pardon my ignorance, but what is a ‘UI tracking facility’ or ‘grab handles’ for that matter? *
I’d guess that it’s for tracking the mouse. In Illustrator or Sketch or other graphical applications, little handles (usually squares or dots) are often displayed on the bounding box of the selected object(s). Clicking-and-dragging the handles around deforms the shape. This is just a guess. I second the guess!
As the developer, I will confirm all guesses! :-) Sorry for the poor explanation…