FLTK: Event when a window gets focus on MacOS

Question

Using FLTK, I'm writing a desktop application which uses multiple windows. The application manages a list of open windows and shows them in the menu with these lines:

for( int i = 0; i < windows.size(); ++i ) {
    menu->add(("&Windows/"+windows[i].name).c_str(), 0, MyMenuCallback);
}

Now I want to set a checkmark in front of the name of the top-most window:

flags = FL_MENU_TOGGLE|FL_MENU_VALUE;
menu->add(("&Windows/"+windows[i].name).c_str(), 0, MyMenuCallback, 0, flags);

I'm stuck at installing an event handler that gets called whenever the top-most window changes. I was hoping Fl::add_handler( &genericEventHandler ); would get called whenever focus changes, but that is not the case. So, my question is: How do I get notified, when the focus of my windows changes?

Answer 1

You should subclass Fl_Window to override its handle method to monitor FL_FOCUS and FL_UNFOCUS events. Here is a sample:

class MyWindow : public Fl_Window {
public:
    MyWindow(int X,int Y,int W,int H, const char* title) : Fl_Window (X, Y, W, H, title) {}

    int handle(int e) {
        switch(e) {
            case FL_FOCUS:
                std::cout << "Window " << label() << " is focused" << std::endl;
                break;
            case FL_UNFOCUS:
                std::cout << "Window " << label() << " has lost focus" << std::endl;
                break;
        }
        return(Fl_Window::handle(e));
    }
};

int main() {
    MyWindow win1(100, 100, 200,200, "Window 1");
    win1.show();

    MyWindow win2(350, 100, 200,200, "Window 2");
    win2.show();

    return Fl::run();
}