Running multiple Kivy apps at same time that communicate with each other

Question

I would like my Kivy application to be able to spawn multiple apps (i.e. new windows) on a Windows machine that can communicate with each other.

ScreenManager and Po

Answer 1

bj0's answer regarding subprocess was correct.

Even better, I figured out how to do this via multiprocessing, which allows better communication and passing of information between apps. It wasn't working before because I did multiprocessing.Process(target=ChildApp().run()).start() when it should be multiprocessing.Process(target=ChildApp().run).start(). The following works

# filename: test.py

from kivy.app import App
from kivy.uix.button import Button

from test2 import ChildApp

import multiprocessing


class MainApp(App):

    def build(self):
        b = Button(text='Launch Child App')
        b.bind(on_press=self.launchChild)
        return b

    def launchChild(self, button):
        app = ChildApp()
        p = multiprocessing.Process(target=app.run)
        p.start()

if __name__ == '__main__':
    MainApp().run()

---

# filename: test2.py

from kivy.app import App
from kivy.uix.label import Label


class ChildApp(App):
    def build(self):
        return Label(text='Child')

if __name__ == '__main__':
    ChildApp().run()

Answer 2

I'm not sure why it doesn't work with multiprocessing (I've never tried it), but it should at least work with subprocess. The reason your main window is locked is because subprocess.call blocks the thread that calls it while it waits for the subprocess to finish and return a result.

You want to use subprocess.Popen instead, which does not block.

Answer 3

I tried baconwichsand's code and can confirm with Python 3.6 and Windows 10 it does not work. Apparently only top level object classes can be pickled, and since both apps inherit from the App class python throws an error. However a top level definition that simply executes the ChildApp().run() command can be pickled and works. Here is my working code.

import multiprocessing
from kivy.app import App
from kivy.uix.label import Label

class MainApp(App):
    def build(self):
        return Label(text='Main App Window')

class OtherApp(App):
    def build(self):
        return Label(text='Other App Window')

def open_parent():
    MainApp().run()

def open_child():
    OtherApp().run()

if __name__ == '__main__':
    a = multiprocessing.Process(target=open_parent)
    b = multiprocessing.Process(target=open_child)
    a.start()
    b.start()

And here is the code I am using, including the Builder to use a shared .kv file for both windows.

import multiprocessing
from kivy.lang import Builder
from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.uix.widget import Widget

class MainRoot(Widget):
    pass

class OtherRoot(Widget):
    pass

class MainApp(App):
    def build(self):
        Builder.load_file('B:\Python_Codes\Testing Grounds\shared.kv')
        main = MainRoot()
        return main

class OtherApp(App):
    def build(self):
        Builder.load_file('B:\Python_Codes\Testing Grounds\shared.kv')
        other = OtherRoot()
        return other

def open_parent():
    MainApp().run()

def open_child():
    OtherApp().run()

if __name__ == '__main__':
    a = multiprocessing.Process(target=open_parent)
    b = multiprocessing.Process(target=open_child)
    a.start()
    b.start()