Passing a function as an argument to another function : what is this sorcery ?

[Derix via Digitalmars-d-learn]

Hello everyone,

So I'm "Getting Started With Gtkd" [1] and the tuto includes this
piece of code :

...

        DrawingArea da = new DrawingArea(590, 200);
        da.addOnDraw(&onDraw);
        layout.put(da, 5, 30);

        add(layout);  // Add the layout to our main window
        showAll();
     }

     bool onDraw(Context c, Widget w)
     {
        //draw things
        return true;
     }
}

and I'm a bit puzzled by the line
        da.addOnDraw(&onDraw);

I'm pretty sure I've met this construct before along with
relevant explainations, but I find myself a bit forgetful and in
need of a refresher. Hence questions :

1) What is the generic name for this kind construct ?

2) Any hint to some reading about the supporting theory or
rationale ?

3) When the onDraw function is actually called, whence does it
takes its arguments from ? What is that Context ? Does it float
around as some sort of implicit global object ? When was it
instanciated ?

4) Is &onDraw a predefined event belonging to the class
DrawingArea ? Is the name of the onDraw function thus constrained
?

5) What is the chain of events leading to the "onDraw" event
occuring ? I'd guess it's the 'showAll()' line, but by the way
where does this 'showAll()' come from ? In the documentation [2],
I don't see it as a method belonging to the class DrawingArea. Is
it inherited from a superclass ?



Thank you for the enlightment !



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[1] http://britseyeview.com/software/articles/gsgtkd102.html
[2] http://api.gtkd.org/src/gtk/DrawingArea.html