How to pass arguments to draw() method - FLTK

Question

I´m new to FLTK and currently facing the following problem:
I have a class PointModel, which stores points with x- and y-coordinates, a class View, which needs to call update(), everytime the coordinates in the PointModel change (Observer Pattern) and draw them and finally a class MyBox, where the coordinates should be drawn in.
View is derived from Fl_Window. MyBox is derived from Fl_Box and part of View.
Therefore I need to know, how to pass the point coordinates from a member function (void update()) of View to the draw method of MyBox.

I´m trying to typecast the user_data pointer I get to View* in case to be able to get the PointModel, holding the point coordinates. But the window closes after calling the draw() method. Maybe I only get a NULL-Pointer here? Unfortunately I can´t check it through debugging, because somehow Eclipse doesn´t break at the breakpoints now..

Any solutions, hints what I´m doing wrong or possible alternatives?

Thanks in advance!

Here some pieces of my code:

View.cpp:

class MyBox : public Fl_Box {
    void draw() {
        Fl_Box::draw();
        View *v1 = (View*)this->parent();

        if(v1 != NULL) {
            int lastX = v1->getPointModel()->getLastX();
            int lastY = v1->getPointModel()->getLastY();
            int currentX = v1->getPointModel()->getCurrentX();
            int currentY = v1->getPointModel()->getCurrentY();

            fl_color(FL_WHITE);
            fl_line(lastX, lastY, currentX, currentY);
        }
    }

    public:
        MyBox(int X,int Y,int W,int H,const char*L=0) : Fl_Box(X,Y,W,H,L) {
        box(FL_FLAT_BOX);
    }
};

View::View(*arguments*) :Fl_Window(540,650,"View1") {
    begin();
    MyBox box(20,20,500,500," ");
    box.box(FL_UP_BOX);
    box.color(0x00000000);
    //more widgets
    end();
    show();
    Fl::add_timeout(3.0, Timer_CB, (void*)this);
    Fl::run();
}

edit: I updated the code to running version

Answer 1

Well, MyBox.draw() will be called before you call add_timeout(). user_data() will give you NULL most likely... You could add a check for NULL in there. Something like:

void draw() {
    Fl_Box::draw();
    void* ptr = user_data();
    if (ptr) {
        // do stuff here...
    }

Second possible problem is this->parent()->user_data(); . Parent of your MyBox object is the Fl_Window object (actually, your View object, which is a Fl_Window), and user_data() will always return NULL for it I think...

The easiest way to pass any object to your widget is to use user_data() method:

box.user_data(&myview); // after this box.user_data() will return a View*