How to set minimum/preferred size in custom Gtk widget

Question

I'm trying to implement a custom Gtk widget, but I can't figure out how to request a specific minimum size and preferred size.

Here you can see that I (try to) request a minimum size of 300x300 and preferred size of 500x500, but Gtk never even calls any of those functions and creates a 200x200 window:

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk


class MyWidget(Gtk.DrawingArea):
    # None of these are ever called
    def do_get_preferred_width_for_height(self, height):
        print('do_get_preferred_width_for_height')
        return 300, 500
    
    def do_get_preferred_height_for_width(self, width):
        print('do_get_preferred_height_for_width')
        return 300, 500

    def do_get_preferred_size(self):
        print('do_get_preferred_size')
        min_size = Gtk.Requisition()
        min_size.width = min_size.height = 300

        pref_size = Gtk.Requisition()
        pref_size.width = pref_size.height = 500

        return min_size, pref_size
    
    def do_size_request(self, requisition):
        print('do_size_request')
        requisition.width = 500
        requisition.height = 500
    
    # Except for these two
    def do_get_request_mode(self):
        print('do_get_request_mode')
        return Gtk.SizeRequestMode.CONSTANT_SIZE
    
    def do_draw(self, context):
        print('Window size:', window.get_allocated_width(), window.get_allocated_height())
        width = self.get_allocated_width()
        height = self.get_allocated_height()
        
        context.set_source_rgb(0, 1, 0)
        context.rectangle(0, 0, width, height)
        context.fill()

window = Gtk.Window()
window.add(MyWidget())
window.show_all()
window.connect('destroy', Gtk.main_quit)
Gtk.main()

# Output:
# do_get_request_mode
# do_get_request_mode
# Window size: 200 200

What am I doing wrong?

I've been working on this on and off all day, and I've found a partial solution. If you return Gtk.SizeRequestMode.WIDTH_FOR_HEIGHT in do_get_request_mode(), the width of the window works the way you want it to. If you return Gtk.SizeRequestMode.HEIGHT_FOR_WIDTH, the height works the way you want it to. But I can't for the life of me figure out how to get both to work at the same time! I've tried many different things, including using a tuple, using |, and using CONSTANT_SIZE along with them, all to no avail. Maybe you'd have some ideas for how to get them both to work? — Sylvester is on codidact.com
– Sylvester is on codidact.com, Commented Jan 18, 2022 at 21:28
@SylvesterKruin That is an interesting observation! I almost went and said I still don't know how to make it work, but when re-examined the docs I noticed I had forgotten to implement the do_get_preferred_width and do_get_preferred_height methods, and adding those made it work! I have no idea why Gtk won't call my do_get_preferred_size method, but hey, I'll take it. I'll let you write that as an answer if you want, since I already have more than enough rep anyway. — Aran-Fey
– Aran-Fey, Commented Jan 18, 2022 at 22:12
I can't get it to work, even in the MRE. I wrote the two methods you named above, and they both return 500, and they're getting called, but I've still got the problem. Am I doing something wrong? I tried returning a Gtk.Requisition, but I got an error message. How did you fix it in your MRE? — Sylvester is on codidact.com
– Sylvester is on codidact.com, Commented Jan 18, 2022 at 22:22
@SylvesterKruin They have to return two ints, the minimum size and the preferred size. So return 300, 500. — Aran-Fey
– Aran-Fey, Commented Jan 18, 2022 at 22:24
The totally confusing Gtk-CRITICAL error: “gtk_widget_get_preferred_width_for_height: assertion 'height >= 0' failed” led me here. – My Gtk.Bin-derived class announces constant size mode and the do_get_preferred_width_for_height() implementation is never called either. I was able to get rid of that nonsense message by implementing do_get_preferred_height() alone. — Robert Siemer
– Robert Siemer, Commented Jan 3, 2023 at 6:03

Sylvester is on codidact.com · Accepted Answer · 2022-01-18 22:56:01Z

2

The problem here (see this comment by the OP) is that the do_get_preferred_height() and do_get_preferred_width() methods are not implemented. Here is the code you need to add to your class:

def do_get_preferred_height(self):
    print('get_preffered_height') # So you can see when it's called
    return 300, 500

def do_get_preferred_width(self):
    print('get_preffered_width') # So you can see when it's called
    return 300, 500

This should make the window have an initial size of 500x500, and a minimum size of 300x300.

answered Jan 18, 2022 at 22:56

Sylvester is on codidact.com

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