Key events

Detecting key presses in FLTK

Global shortcut handler

See section 6 of the FLTK online manual
Add an event handler function like this to your code:

int keyhandler(int event) {
    if (event == FL_SHORTCUT) {
        if (Fl::event_key() == 'y') {
            //do something because the y key was pressed
            return 1; // to indicate we used the key event
        }  // here you can test for other keys, as wanted
    }
    return 0;  // we had no interest in the event
}

Then hook this function into the event handling loop with a line like this, just before calling Fl::run()

Fl::add_handler(keyhandler);

There are other events you might look for: separate key up and key down events, for example. However, this global handler can't see all events, for example KeyUp events (when a key is released). There are other ways of doing this:

Other possibilities

Subclass your main window, or the widget inside it that you want to see the keys, and implement the handle() method in the derived class, see FLTK manual chapter 7 and appendix C
Provide a new dummy widget which doesn't appear on the screen, but has the job of handling all keyboard events. A simple example of this is provided in this file. It illustrates capturing y, n presses (for yes/no), cursor keys and key up events.
Use the shortcut() method of a button to cause it to be invoked when a key is pressed