This chapter covers the drawing functions that are provided with FLTK.
There are only certain places you can execute drawing code in FLTK. Calling these functions at other places will result in undefined behavior!
The most common is inside the virtual method Fl_Widget::draw(). To write code here, you must subclass one of the existing Fl_Widget classes and implement your own version of draw().
You can also write boxtypes and labeltypes. These are small procedures that can be called by existing Fl_Widget draw() methods. These "types" are identified by an 8-bit index that is stored in the widget's box(), labeltype(), and possibly other properties.
You can call Fl_Window::make_current() to do incremental update of a widget. Use Fl_Widget::window() to find the window.