python - Building a simple progress bar or loading animation in Kivy?

Question

I am writing a Kivy UI for cmd line utility I have developed. Everything works fine, but some of the processes can take from a few seconds to a few minutes to process and I would like to provide some indication to the user that the process is running. Ideally, this would be in the form of a spinning wheel or loading bar or something, but even if I could update my display to show the user that a process is running, it would be better than what I have now.

Currently, the user presses a button in the main UI. This brings up a popup that verifies some key information with the user, and if they are happy with those options, they press a 'run' button. I have tried opening a new popup to tell them that the process is running, but because the display doesn't update until the process finishes, this doesn't work.

I have a lot of coding experience, but mostly in the context of math and engineering, so I am very new to the designing of UIs and having to handle events and threads. A simple self-contained example would be greatly appreciated.

Answer 1

I was recently tackling the problem you described: display doesn't update until the process finishes

Here is a complete example that I got working with the help of @andy_s in the #Kivy IRC channel:

My main.py:

from kivy.app import App
from kivy.uix.popup import Popup
from kivy.factory import Factory
from kivy.properties import ObjectProperty
from kivy.clock import Clock

import time, threading

class PopupBox(Popup):
    pop_up_text = ObjectProperty()
    def update_pop_up_text(self, p_message):
        self.pop_up_text.text = p_message

class ExampleApp(App):
    def show_popup(self):
        self.pop_up = Factory.PopupBox()
        self.pop_up.update_pop_up_text('Running some task...')
        self.pop_up.open()

    def process_button_click(self):
        # Open the pop up
        self.show_popup()

        # Call some method that may take a while to run.
        # I'm using a thread to simulate this
        mythread = threading.Thread(target=self.something_that_takes_5_seconds_to_run)
        mythread.start()

    def something_that_takes_5_seconds_to_run(self):
        thistime = time.time() 
        while thistime + 5 > time.time(): # 5 seconds
            time.sleep(1)

        # Once the long running task is done, close the pop up.
        self.pop_up.dismiss()

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

My example.kv:

AnchorLayout:
    anchor_x: 'center'
    anchor_y: 'center'
    Button:
        height: 40
        width: 100
        size_hint: (None, None)
        text: 'Click Me'
        on_press: app.process_button_click()

<PopupBox>:
    pop_up_text: _pop_up_text
    size_hint: .5, .5
    auto_dismiss: True
    title: 'Status'   

    BoxLayout:
        orientation: "vertical"
        Label:
            id: _pop_up_text
            text: ''

If you run this example, you can click the Click Me button, which should open up a "progress bar" in the form of a modal/pop-up. This pop up will remain open for 5 seconds without blocking the main window. After 5 seconds, the pop up will automatically be dismissed.