Working with Python threads inside a Kivy application

Summary

• author: dessant
• kivy: >= 1.8.0

OpenGL related operations (widget, canvas, property manipulation etc.) should be done only in the main thread. This example shows us how to use Clock or the mainthread decorator to perform these actions, and how to use threading events to stop hanging threads when the app is about to close.

Usage

python main.py

Files

main.py

import threading
import time

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory
from kivy.animation import Animation
from kivy.clock import Clock, mainthread
from kivy.uix.gridlayout import GridLayout


Builder.load_string("""
<AnimWidget@Widget>:
    canvas:
        Color:
            rgba: 0.7, 0.3, 0.9, 1
        Rectangle:
            pos: self.pos
            size: self.size
    size_hint: None, None
    size: 400, 30


<RootWidget>:
    cols: 1

    canvas:
        Color:
            rgba: 0.9, 0.9, 0.9, 1
        Rectangle:
            pos: self.pos
            size: self.size

    anim_box: anim_box
    but_1: but_1
    lab_1: lab_1
    lab_2: lab_2

    Button:
        id: but_1
        font_size: 20
        text: 'Start second thread'
        on_press: root.start_second_thread(lab_2.text)

    Label:
        id: lab_1
        font_size: 30
        color: 0.6, 0.6, 0.6, 1
        text_size: self.width, None
        halign: 'center'

    AnchorLayout:
        id: anim_box

    Label:
        id: lab_2
        font_size: 100
        color: 0.8, 0, 0, 1
        text: '3'
""")


class RootWidget(GridLayout):

    stop = threading.Event()

    def start_second_thread(self, l_text):
        threading.Thread(target=self.second_thread, args=(l_text,)).start()

    def second_thread(self, label_text):
        # Remove a widget, update a widget property, create a new widget,
        # add it and animate it in the main thread by scheduling a function
        # call with Clock.
        Clock.schedule_once(self.start_test, 0)

        # Do some thread blocking operations.
        time.sleep(5)
        l_text = str(int(label_text) * 3000)

        # Update a widget property in the main thread by decorating the
        # called function with @mainthread.
        self.update_label_text(l_text)

        # Do some more blocking operations.
        time.sleep(2)

        # Remove some widgets and update some properties in the main thread
        # by decorating the called function with @mainthread.
        self.stop_test()

        # Start a new thread with an infinite loop and stop the current one.
        threading.Thread(target=self.infinite_loop).start()

    def start_test(self, *args):
        # Remove the button.
        self.remove_widget(self.but_1)

        # Update a widget property.
        self.lab_1.text = ('The UI remains responsive while the '
                           'second thread is running.')

        # Create and add a new widget.
        anim_bar = Factory.AnimWidget()
        self.anim_box.add_widget(anim_bar)

        # Animate the added widget.
        anim = Animation(opacity=0.3, width=100, duration=0.6)
        anim += Animation(opacity=1, width=400, duration=0.8)
        anim.repeat = True
        anim.start(anim_bar)

    @mainthread
    def update_label_text(self, new_text):
        self.lab_2.text = new_text

    @mainthread
    def stop_test(self):
        self.lab_1.text = ('Second thread exited, a new thread has started. '
                           'Close the app to exit the new thread and stop '
                           'the main process.')

        self.lab_2.text = str(int(self.lab_2.text) + 1)

        self.remove_widget(self.anim_box)

    def infinite_loop(self):
        iteration = 0
        while True:
            if self.stop.is_set():
                # Stop running this thread so the main Python process can exit.
                return
            iteration += 1
            print('Infinite loop, iteration {}.'.format(iteration))
            time.sleep(1)


class ThreadedApp(App):

    def on_stop(self):
        # The Kivy event loop is about to stop, set a stop signal;
        # otherwise the app window will close, but the Python process will
        # keep running until all secondary threads exit.
        self.root.stop.set()

    def build(self):
        return RootWidget()

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

##Comments

Section for user comments

Why must OpenGL operations be done in the main thread? Is it possible to have a main control/adapter thread that opens a thread for kivy and a thread for model operations?

The @mainthread is not necessarily the main thread but the thread that creates the kivy app to run. Despite this unfavorable limitation, a companion thread would suffice for other non-kivy event processing and interacts with kivy UI through @mainthread or clock scheduling.