Hybrid Kivy and GTK application

kivygtk.kv

#:kivy 1.8.0

<Root>:
    orientation: "vertical"

    Button:
        text: "Start GTK thread" if not app.gtk_started else "Stop GTK thread"
        on_release: app.start_gtk_thread()

main.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from kivy.app import App
from kivy.properties import BooleanProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.clock import mainthread

from gi.repository import Gtk, GObject
from threading import Thread

class MyWindow(Gtk.Window):
    def __init__(self):
        window = Gtk.Window.__init__(self, title="Secondary GTK thread")

        self.box = Gtk.VBox()
        self.add(self.box)

        button1 = Gtk.Button(label="Add button in Kivy app")
        button1.connect("clicked", self.add_kivy_button)
        self.box.add(button1)

        button2 = Gtk.Button(label="Remove button from Kivy app")
        button2.connect("clicked", self.remove_kivy_button)
        self.box.add(button2)

    def add_kivy_button(self, widget):
        App.get_running_app().add_button("Hello world!!!")

    def remove_kivy_button(self, widget):
        App.get_running_app().remove_button(0)


class GTKThread(Thread):
    def run(self):
        self.win = MyWindow()
        self.win.connect("delete-event", Gtk.main_quit)
        self.win.show_all()
        
        GObject.threads_init()
        Gtk.main()

    def stop(self):
        Gtk.main_quit()

class Root(BoxLayout):
    pass

class KivyGTKApp(App):
    gtk_started = BooleanProperty(False)
    
    def build(self):
        return Root()

    def start_gtk_thread(self, *args):
        if self.gtk_started:
            # Stopping the mainloop doesn't work reliably. Avoid doing it.
            self.gtk.stop()
            self.gtk_started = False
        else:
            self.gtk = GTKThread()

            # Set this if you want GTK to (abruptly) shut down with Kivy
            # daemon = True

            # You can probably start more than one thread, if you keep track
            # of every instance
            self.gtk.start()
            self.gtk_started = True

    # This works without the decorator. However you should always
    # use it if you plan to call a method that does GUI stuff from
    # another thread, as it's not thread-safe to do so.
    # Another way to do it is to use Clock to schedule a call to
    # the method.
    #@mainthread
    def add_button(self, text):
        b = Button(text=text, size_hint_y=None, height="30dp")
        self.root.add_widget(b)

    #@mainthread
    def remove_button(self, index):
        if len(self.root.children) > 1:
            self.root.remove_widget(self.root.children[index])

if __name__ == "__main__":
    KivyGTKApp().run()