Adding an element after clicking on a button - gtk

Question

I was trying to learn Vala by programming a very simple application and I stumbled over a problem, that I was unable to resolve on my own.
The program shows simply a button Init and on click it should add a Button X to the Grid container. Unfortunately, the contents of the Grid container remain empty and I don't know why.
Even more confusing is, that adding the Button right in the constructor works as expected.
So what I'm doing wrong here?
using Gtk;
class MyWindow: Gtk.Window {
private Gtk.Grid mGrid;
public MyWindow() {
var init=new Gtk.Button.with_label("Init");
init.clicked.connect((t)=>{
stdout.printf("Init");
mGrid.attach(new Gtk.Button.with_label("X"),0,0,1,1);
});
var box=new Gtk.Box(VERTICAL,0);
mGrid=new Gtk.Grid();
//mGrid.attach(new Gtk.Button.with_label("X"),0,0,1,1);
box.add(init);
box.add(mGrid);
this.add(box);
this.show_all();
}
}
int main(string[] args) {
Gtk.init(ref args);
new MyWindow();
Gtk.main();
return 0;
}

Answer 1

With the GTK+ toolkit widgets are hidden by default. Although you have this.show_all (), the button is created afterwards and is hidden. Changing the callback from:
init.clicked.connect((t)=>{
stdout.printf("Init");
mGrid.attach(new Gtk.Button.with_label("X"),0,0,1,1);
});
to something like:
init.clicked.connect((t)=>{
stdout.printf("Init");
var my_button = new Gtk.Button.with_label("X");
my_button.show_all ();
mGrid.attach(my_button,0,0,1,1);
});
now works.