Custom GTK+ HeaderBar with fullscreen toggle button

header_bar.py

#!/usr/bin/env python

#  Copyright (c) 2017 Kurt Jacobson
#  License: https://kcj.mit-license.org/@2017

import os
import gi

gi.require_version('Gtk', '3.0')
gi.require_version('Gdk', '3.0')

from gi.repository import Gtk
from gi.repository import Gdk

# GtkHeaderBar source, useful for finding style class names etc.
#    https://git.gnome.org/browse/gtk+/tree/gtk/gtkheaderbar.c?h=3.22.19#n394

ICONSIZE = Gtk.IconSize.MENU

class HeaderBar(Gtk.HeaderBar):
    def __init__(self, window, *args, **kwargs):
        Gtk.HeaderBar.__init__(self, *args, **kwargs)

        self.window = window
        self.window_box = window.get_child()

        self.window_size = None
        self.window_position = None

        # Track window state
        self.window_fullscreen = False
        self.window_maximized = False
        self.window.connect('window-state-event', self.on_window_state_event)
        self.window.connect('map-event', self.on_map)

        GetIcon = Gtk.Image.new_from_icon_name

        # Close icon
        self.close_icon = GetIcon('window-close-symbolic', ICONSIZE)
        self.close_icon.show()

        # Maximize icon
        self.maximize_icon = GetIcon('window-maximize-symbolic', ICONSIZE)
        self.maximize_icon.show()

        # Restore (unaximize) icon
        self.restore_icon = GetIcon('window-restore-symbolic', ICONSIZE)
        self.restore_icon.show()

        # Minimize icon
        self.minimize_icon = GetIcon('window-minimize-symbolic', ICONSIZE)
        self.minimize_icon.show()

        # Fullscreen icon
        self.fullscreen_icon = GetIcon('view-fullscreen-symbolic', ICONSIZE)
        self.fullscreen_icon.show()

        # Unfullscreen icon
        self.unfullscreen_icon = GetIcon('view-restore-symbolic', ICONSIZE)
        self.unfullscreen_icon.show()

        # Control button box, set style class to match that the default box
        self.button_box = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL)
        style = self.button_box.get_style_context()
        style.add_class('right')

        # Add to the right side of the titlebar
        self.pack_end(self.button_box)

        # Add the separator
        self.separator = Gtk.Separator(orientation=Gtk.Orientation.VERTICAL)
        self.button_box.pack_start(self.separator, False, False, 3)
        self.separator.get_style_context().add_class('titlebutton')

        # Fullscreen button
        self.fullscreen_btn = Gtk.Button()
        self.fullscreen_btn.add(self.fullscreen_icon)

        # Add same style classes as used by default controls
        self.fullscreen_btn.get_style_context().add_class('titlebutton')
        self.fullscreen_btn.get_style_context().add_class('fullscreen')
        self.button_box.pack_start(self.fullscreen_btn, False, False, 3)
        self.fullscreen_btn.connect('clicked', self.on_fullscreen_clicked)


        # Minimize button
        self.minimize_btn = Gtk.Button()
        self.minimize_btn.add(self.minimize_icon)

        # Add same style classes as used by default controls
        self.minimize_btn.get_style_context().add_class('titlebutton')
        self.minimize_btn.get_style_context().add_class('minimize')
        self.button_box.pack_start(self.minimize_btn, False, False, 3)
        self.minimize_btn.connect('clicked', self.on_minimize_clicked)


        # Maximize button
        self.maximize_btn = Gtk.Button()
        self.maximize_btn.add(self.maximize_icon)

        # Add same style classes as used by default controls
        self.maximize_btn.get_style_context().add_class('titlebutton')
        self.maximize_btn.get_style_context().add_class('maximize')
        self.button_box.pack_start(self.maximize_btn, False, False, 3)
        self.maximize_btn.connect('clicked', self.on_maximize_clicked)


        # Close button
        self.close_btn = Gtk.Button()
        self.close_btn.add(self.close_icon)

        # Add same style classes as used by default controls
        self.close_btn.get_style_context().add_class('titlebutton')
        self.close_btn.get_style_context().add_class('close')
        self.button_box.pack_start(self.close_btn, False, False, 3)
        self.close_btn.connect('clicked', self.on_close_clicked)

    def on_map(self, widget, event):

        # Prevent error on start up
        if not self.window_size or not self.window_position:
            return

        # Restore size from before fullscreen
        w, h = self.window_size
        self.window.resize(w, h)

        # Restore position
        x, y = self.window_position
        self.window.move(x, y)

    def on_close_clicked(self, widget):
        self.window.close()

    def on_maximize_clicked(self, widget):
        if self.window_maximized:
            self.window.unmaximize()
        else:
            self.window.maximize()

    def on_minimize_clicked(self, widget):
        self.window.iconify()

    def on_fullscreen_clicked(self, widget):
        if self.window_fullscreen:
            self.window.unfullscreen()
        else:
            # Get the current size and post so can restore latter
            self.window_size = self.window.get_size()
            self.window_position = self.window.get_position()
            self.window.fullscreen()

    def on_maximized_state_changed(self, maximized):
        # Remove whatever icon is present
        image = self.maximize_btn.get_child()
        self.maximize_btn.remove(image)

        if maximized:
            # Change to the restore icon
            self.maximize_btn.add(self.restore_icon)
        else:
            # Change back to the maximize icon
            self.maximize_btn.add(self.maximize_icon)

    def on_fullscreen_state_changed(self, fullscreen):
        # Remove whatever icon is present
        image = self.fullscreen_btn.get_child()
        self.fullscreen_btn.remove(image)

        if fullscreen:
            # Remove the headerbar from the titlebar and add to top of window
            self.window.remove(self)
            self.window_box.pack_start(self, False, False, 0)

            # Will end up ad the bottom, so move to top
            self.window_box.reorder_child(self, 0)

            # Change to un-fullscreen icon
            self.fullscreen_btn.add(self.unfullscreen_icon)

            # Maximizing a fullscreen window does nothing, so hide the button
            self.maximize_btn.hide()

        else:

            # Remove headerbar from box and put back in titlebar
            self.window_box.remove(self)

            # Calling `set_titlebar` on a realized window causes this (harmless) warning
            #    Gtk-WARNING **: gtk_window_set_titlebar() called on a realized window
            self.window.set_titlebar(self)

            # Change the icon back
            self.fullscreen_btn.add(self.fullscreen_icon)

            # Re-show the maximize button
            self.maximize_btn.show()

    def on_window_state_event(self, widget, event):
        # Listen to state event and track window state
        if self.window_fullscreen != bool(event.new_window_state & Gdk.WindowState.FULLSCREEN):
            self.window_fullscreen = bool(event.new_window_state & Gdk.WindowState.FULLSCREEN)
            self.on_fullscreen_state_changed(self.window_fullscreen)
        if self.window_maximized != bool(event.new_window_state & Gdk.WindowState.MAXIMIZED):
            self.window_maximized = bool(event.new_window_state & Gdk.WindowState.MAXIMIZED)
            self.on_maximized_state_changed(self.window_maximized)


def main():
    window = Gtk.Window()

    box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
    window.add(box)

    title = "Custom HeaderBar"
    subtitle = "with fullscreen toggle button"
    header_bar = HeaderBar(window, title=title, subtitle=subtitle)
    window.set_titlebar(header_bar)

    window.set_default_size(600, 300)
    window.connect('destroy', Gtk.main_quit)

    window.show_all()
    Gtk.main()

if __name__ == "__main__":
    main()

That's super cool, thanks!