Gtk.Widget.get_allocated_size() returns zero-size Gdk.Rectangle for object created after Gtk.Window.show_all() is called

olumide · July 5, 2023, 5:19pm

The following example creates a center-aligned button in a Window. Two seconds later this button is deleted and replaced by another button.

import gi

gi.require_version("Gtk", "3.0")
from gi.repository import Gtk, GLib


class ButtonWindow(Gtk.Window):
    def __init__(self):
        super().__init__()
        self.set_default_size(500, 200)
        
        button = Gtk.Button.new_with_label("one")
        button.set_halign(Gtk.Align.CENTER)
        button.set_valign(Gtk.Align.CENTER)
        self.add(button)
        
        self.timeout_id = GLib.timeout_add(2000, self.on_timeout, button)

    def on_timeout(self, button):
        rect, _ = button.get_allocated_size()
        print(rect.x, rect.y, rect.width, rect.height)
        
        self.remove(button)
        button = Gtk.Button.new_with_label("two")
        button.set_halign(Gtk.Align.CENTER)
        button.set_valign(Gtk.Align.CENTER)
        self.add(button)
        self.show_all()
        
        rect, _ = button.get_allocated_size()
        print(rect.x, rect.y, rect.width, rect.height)
    
win = ButtonWindow()
win.connect("destroy", Gtk.main_quit)
win.show_all()
Gtk.main()

Gtk.Widget.get_allocated_size() returns an incorrect size rect 0 0 500 200 (size of the Window) for the old button but 0 0 0 0 for the new button even after Gtk.Window.show_all() is called.

What am I doing wrong here?

Update

The behaviour is identical in the native GTK3 API

#include <gtk/gtk.h>

guint timeout_id;
GtkWidget *window;
GtkWidget *button;

static gboolean on_timeout(gpointer user_data)
{
	int baseline;
	GtkAllocation rect;
	GtkWidget *button2;
	
	gtk_widget_get_allocated_size(button, &rect, &baseline);
	printf("x: %i y: %i width: %i height: %i\n", rect.x, rect.y, rect.width, rect.height);
	
	gtk_container_remove(GTK_CONTAINER(window), button);
	//g_object_unref (button);  // segfault???
	
	button2 = gtk_button_new_with_label ("two");
	gtk_widget_set_size_request (button2, 200, 100);
	gtk_widget_set_halign (button2, GTK_ALIGN_CENTER);
	gtk_widget_set_valign (button2, GTK_ALIGN_CENTER);
	gtk_container_add (GTK_CONTAINER(window), button2);
	gtk_widget_show_all (window);
	
	gtk_widget_get_allocated_size(button2, &rect, &baseline);
	printf("x: %i y: %i width: %i height: %i\n", rect.x, rect.y, rect.width, rect.height);
	
	g_source_remove(timeout_id);
	return 0;
}

static void activate(GtkApplication* app, gpointer user_data)
{
  window = gtk_application_window_new (app);
  gtk_window_set_default_size (GTK_WINDOW (window), 500, 200);
  
  button = gtk_button_new_with_label ("one");
  gtk_widget_set_size_request (button, 100, 50);
  gtk_widget_set_halign (button, GTK_ALIGN_CENTER);
  gtk_widget_set_valign (button, GTK_ALIGN_CENTER);
  gtk_container_add (GTK_CONTAINER(window), button);
  gtk_widget_show_all (window);
  
  timeout_id = g_timeout_add(2000, on_timeout, NULL);
}

int main(int argc, char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

The output

x: 0 y: 0 width: 500 height: 200
x: 0 y: 0 width: 0 height: 0

olumide · July 6, 2023, 12:17pm

Inspired by the output of Accerciser’s displaying the correct extents

I tried the following:

def on_timeout(self, button):
	# Get dimensions of old button. WORKS!
	accessible = button.get_accessible()
	(x, y, width, height) = accessible.get_extents(Atk.CoordType.WINDOW)
	print(x, y, width, height)
	
	#rect, _ = button.get_allocated_size()
	#print(rect.x, rect.y, rect.width, rect.height)
	#print()
	
	self.remove(button)
	self.set_default_size(500, 200)
	button = Gtk.Button.new_with_label("two")
	button.set_size_request(200, 100)
	button.set_halign(Gtk.Align.CENTER)
	button.set_valign(Gtk.Align.CENTER)
	button.set_visible(True)
	self.add(button)
	self.show_all()
	
	#time.sleep(1) # Does not help
	
	# Get dimensions of new button the expected way. FAILS!
	# Probably too soon after object was created.
	accessible = button.get_accessible()
	(x, y, width, height) = accessible.get_extents(Atk.CoordType.WINDOW)
	print(x, y, width, height)
	
	# Getting the dimensions of new button via a timeout 1 millisecond later, SUCCEEDS
	GLib.timeout_add(1, self.on_timeout2, button)
	
def on_timeout2(self, button):
	accessible = button.get_accessible()
	(x, y, width, height) = accessible.get_extents(Atk.CoordType.WINDOW)
	print(x, y, width, height)
	print()

Result

200 75 100 50
-2147483648 -2147483648 1 1
150 50 200 100

Oddly, moving on_timeout2 forward as shown below

# FAILS
GLib.timeout_add(1, self.on_timeout2, button)

# SUCCEEDS
accessible = button.get_accessible()
(x, y, width, height) = accessible.get_extents(Atk.CoordType.WINDOW)
print(x, y, width, height)

Produced the same result. The first attempt failed, the second succeeded.

What is going on???

jfrancis · July 7, 2023, 1:05pm

Just some things about GTK you may want to know:

The size of any widget is determined recursively by its parent container(s)
The topmost container is usually a window
Windows cannot know their size until the window system gives GTK a surface to draw on
When creating/removing/updating widgets, GTK will try to queue any new window system events until at least the next return to the event loop
GTK is single threaded and uses a polling event loop, so calling sleep will stall the whole program, including stalling any events to/from the window system

With those in mind, in GTK3 you should be able to get the correct size by connecting to the size-allocate signal and getting the size in your signal handler. (Note that in GTK4 the signal no longer exists, so you have to use other methods there)

olumide · July 13, 2023, 5:35pm

Do you mean like this?

class MyButton(Gtk.Button):
    def __init__(self, label, width, height):
        super().__init__()
        self.set_label(label)
        self.set_size_request(width, height)
        self.set_halign(Gtk.Align.CENTER)
        self.set_valign(Gtk.Align.CENTER)
        self.connect("size-allocate", self.my_size)
        
    def my_size(self, obj, user_data):
        rect, _ = obj.get_allocated_size()
        print(rect.x, rect.y, rect.width, rect.height)
        
class ButtonWindow(Gtk.Window):
    def __init__(self):
        super().__init__()
        self.set_default_size(500, 200)
        
        button = MyButton("one", 100, 50)
        self.add(button)

    def on_timeout(self, button):       
        self.remove(button)
        self.set_default_size(500, 200)
        button = MyButton("two", 200, 100)
        self.add(button)

I get 0 0 500 200 (the window size) both times.

PeterB · July 13, 2023, 8:26pm

Looks like both buttons expand to fill the window; you could try setting their hexpand and vexpand to FALSE.

ebassi · July 13, 2023, 9:31pm

The real question is: why are you trying to get, or override, the size of widgets? What are you trying to achieve?

Querying the size of a widget outside the widget’s layout phase is typically an indication of some fundamental misunderstanding of how GTK works.

olumide · July 17, 2023, 7:59am

Both buttons do not expand to fill the window as you will see when you run either version of the program.

olumide · July 17, 2023, 8:17am

That may be the case.

What I am trying to do is to draw connector lines between a series buttons in an overriden Gtk.Widget.draw(ctx). Therefore I need to know the extents of the buttons in order to determine the starts and ends of the connectors.

olumide · August 1, 2023, 3:45pm

Solution.

The rect (dimensions) of a widget are passed in to the size-allocate signal hander.

system · August 31, 2023, 3:46pm

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