The Interface Kit holds all the classes you'll need to develop a GUI. Building on the messaging facilities provided by the Application Kit, the Interface Kit can be used to create a responsive and attractive graphical user interface.
The most important class in the Interface Kit is the BView class, which handles drawing and user interaction. Pointer and keyboard events are processed in this class.
A new addition Haiku has added over the BeOS API is the Layout API, which is based around the BLayoutItem and BLayout classes. These classes will take care of making sure all your GUI widgets end up where you want them, with enough space to be useful. You can start learning the Layout API by reading the introduction .