Gtk.DrawingArea blank when attached to Gtk.Grid - grid

Question

(This is my first post, sorry if I do something wrong...)
I am writing a program in Vala with which one can design a classroom.
I have decided to use GTK for the GUI (Vala integrates well with this),
and Cairo to draw the classroom diagram (GTK comes with this by default).
I have created a 'classroom' class (a subclass of Gtk.DrawingArea),
which currently should just display a square:
public class Classroom : DrawingArea
{
private delegate void DrawMethod();
public Classroom()
{
this.draw.connect((widget, context) => {
return draw_class(widget, context, context.stroke);
});
}
bool draw_class(Widget widget, Context context, DrawMethod draw_method)
{
context.set_source_rgb(0, 0, 0);
context.set_line_width(8);
context.set_line_join (LineJoin.ROUND);
context.save();
context.new_path();
context.move_to(10, 10);
context.line_to(30, 10);
context.line_to(30, 30);
context.line_to(10, 30);
context.line_to(10, 10);
context.close_path();
draw_method(); // Actually draw the lines in the buffer to the widget
context.restore();
return true;
}
}
I have also created a class for my application:
public class SeatingPlanApp : Gtk.Application
{
protected override void activate ()
{
var root = new Gtk.ApplicationWindow(this);
root.title = "Seating Plan";
root.set_border_width(12);
root.destroy.connect(Gtk.main_quit);
var grid = new Gtk.Grid();
root.add(grid);
/* Make our classroom area */
var classroom = new Classroom();
grid.attach(classroom, 0, 0, 1, 1);
//root.add(classroom);
root.show_all();
}
public SeatingPlanApp()
{
Object(application_id : "com.github.albert-tomanek.SeatingPlan");
}
}
And this is my main function:
int main (string[] args)
{
return new SeatingPlanApp().run(args);
}
I put my classroom widget into a Gtk.Grid, my layout widget of choice.
When I compile my code and run it, I get a blank window:
However, if I do not use Gtk.Grid, and just add my classroom using root.add() (which I have commented out), the classroom widget is displayed correctly:
Why does my widget not show up when adding it using Gtk.Grid?
What can I do to fix this?

Answer 1

The problem is that the cell is sized 0x0 pixels, because the grid doesn't know how much space your drawing area actually needs.
A simple solution is to just request some fixed size, try this:
var classroom = new Classroom();
classroom.set_size_request (40, 40);
PS: I got this idea by looking at other similar questions on SO, particularly this one.