Kivy: Popup not displaying

Question

Althoug my example code isn't one of the shortest this is the minimal code that presents the inner working of my App.

My Goal is to have saving popup dialog that updates progress bar (not in this example-not the issue). If you try to run this code you will see button "save" - after clicking it popup displays.

However I present update loop in the progress_bar() method suddenly the Popup is no longer displayed - WHY?

I was looking around the internet for the solution. At first I thought this has something to do with treading I was using to have popup updated while actually saving the file in another thread, but this example has no threading and the actual method is commented so the problem must be elsewhere.

import kivy
from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.popup import Popup
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from time import sleep

# Main app with save method
class MainApp(App):
  def build(self):
    sm = ScreenManager()
    self.save = save(app=self, name='save')
    sm.add_widget(self.save)
    self.saved_percent = 0.0
    return sm

  def save_file(self,file_path):
    for i in range(0,10):
      sleep(1)
      self.saved_percent = i * 10.0
      print("saved",i)
    self.save_finnished = True
    return 256 # check for written bytes

# one of many screens - this one is for saving
class save(Screen):
  def __init__(self, app, **kwargs):
    super(Screen,self).__init__(**kwargs)
    self.app = app
    lyo = BoxLayout(orientation='vertical')
    btn = Button(text="Save", size=(10,10))
    btn.bind(on_press=self.begin_save)
    lyo.add_widget(btn)
    self.add_widget(lyo) # basic layout to init the save

  def begin_save(self,*args):
    self.app.save_finnished = False
    self.cancel_save = False
    file_path = 'foo.txt' # debug static
    self.progress_bar(file_path) # TODO put in separate thread and continue
    #save_file(self,file_path)

  def progress_bar(self, file_path):
    btn = Button(text="Cancel")
    btn.bind(on_press=self.btn_cancel_save)

    pop = Popup(title="Saving", content=btn, auto_dismiss=False)
    pop.open() # does not open

    # this loop is somehow causing the Popup not to display
    #while (not self.app.save_finnished and not self.cancel_save):
    #    pass # progres bar is being updated here
    #pop.dismiss() # dismiss after exiting the loop

  def btn_cancel_save(self, *args):
    print("Cancel clicked")
    self.cancel_save = True


if __name__ == "__main__":
    top=MainApp()
    top.run()



















#
#import kivy
#from kivy.app import App
#from kivy.uix.screenmanager import ScreenManager, Screen
#from kivy.uix.boxlayout import BoxLayout
#from kivy.uix.progressbar import ProgressBar
#from kivy.uix.popup import Popup
#import threading
#from kivy.uix.label import Label
#from kivy.uix.button import Button
#from kivy.uix.boxlayout import BoxLayout
#from time import sleep
#from kivy.lang import Builder
#import queue
#
## Main app with save method
#class MainApp(App):
#  def build(self):
#    sm = ScreenManager()
#    self.save = save(app=self, name='save')
#    sm.add_widget(self.save)
#    self.saved_percent = 0.0
#    return sm
#
#  def save_file(self,file_path):
#    for i in range(0,10):
#      sleep(1)
#      self.saved_percent = i * 10.0
#      print("saved",i)
#    return 256 # check for written bytes
#
## one of many screens - this one is for saving
#class save(Screen):
#  def __init__(self, app, **kwargs):
#    super(Screen,self).__init__(**kwargs)
#    self.app = app
#    lyo = BoxLayout(orientation='vertical')
#    btn = Button(text="Save", size=(10,10))
#    btn.bind(on_press=self.begin_save)
#    lyo.add_widget(btn)
#    self.add_widget(lyo) # basic layout to init the save
#
#  def begin_save(self,*args):
#    self.save_finnished = False
#    self.cancel_save = False
#    file_path = 'foo.txt' # debug static
#    #written = self.poi.save_file(file_path)
#    #self.progress_bar(file_path)
#
#    # some black magic with threads to get ret value
#    # https://www.edureka.co/community/31966/how-to-get-the-return-value-from-a-thread-using-python
#    #que = queue.Queue()
#    #t = threading.Thread(target=lambda q, arg1: q.put(self.app.save_file(arg1)), args=(que, file_path))
#    #t.start()
#    self.progress_bar(file_path)
#    #progress_bar_thread = threading.Thread(target=self.progress_bar, args=(file_path,))
#    #progress_bar_thread.start()
#    #t.join()
#    #written = que.get()
#
##    self.save_finnished = True
##    size = os.path.getsize(file_path)
##    if(not self.btn_cancel_save):
##      if(written == size):
##        print("OK")
##        self.ok_dialog(True)
##      else:
##        print("Error saving to POI")
##        self.ok_dialog(False)
#
#  def progress_bar(self, file_path):
#    print("progress bar fnc ")
#    #lyo = BoxLayout(orientation='vertical')
#    #pb = ProgressBar(max=100.0)
#    #lbl = Label(text='0 %')
#    btn = Button(text="Cancel")
#    btn.bind(on_press=self.btn_cancel_save)
#    #lyo.add_widget(Label(text=file_path))
#    #lyo.add_widget(pb)
#    #lyo.add_widget(lbl)
#    #lyo.add_widget(btn)
#
#    #pop = Popup(title="Saving", content=lyo, auto_dismiss=False)
#    pop = Popup(title="Saving", content=btn, auto_dismiss=False)
#    pop.open() # does not open
#
##    print("start update loop")
##    print("finished?",self.save_finnished, "cancel?" ,self.cancel_save)
#    while (not self.save_finnished or not self.cancel_save):
#        pass # progres bar is being updated here
##      pb.value = self.app.saved_percent
##      s =  '%.12f' % self.app.saved_percent
##      i, p, d = s.partition('.')
##      str_percent = '.'.join([i, (d+'0'*2)[:2]])
##      lbl.text = str_percent +' %'
##      #print(" # ")
#    print("out of update loop - dismiss pop")
#    pop.dismiss()
#
#  def ok_dialog(self,success):
#    if(success):
#      title = "SUCCESS"
#    else:
#      title = "FAILED"
#    btn = Button(text="OK")
#
#    pop = Popup(title=title, content=btn, auto_dismiss=False)
#    btn.bind(on_press=pop.dismiss)
#    pop.open()
#
#
#  def btn_cancel_save(self, *args):
#    print("Cancel clicked")
#    self.cancel_save = True
#
#
#if __name__ == "__main__":
#    top=MainApp()
#    top.run()

Answer 1

without threads it can look like this (however if iteration time is long, it will be very unresponsive)

import time
from kivy.uix.progressbar import ProgressBar
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.uix.boxlayout import BoxLayout
from kivy.clock import Clock


class TestApp(App):
    def build(self):
        parent = Widget()
        btn = Button(text='copy file', on_press=self.start_copy_file)
        parent.add_widget(btn)
        return parent

    def start_copy_file(self, *args):
        self.copy_cancel_flag = False

        # open progressbar window
        self.progress_bar = ProgressBar(max=50)
        btn = Button(text="Cancel")
        btn.bind(on_press=self.copy_cancel)

        layout = BoxLayout()
        layout.add_widget(self.progress_bar)
        layout.add_widget(btn)

        self.pop = Popup(title="Saving", content=layout)
        self.pop.open()

        self.copy_context_i = 0
        Clock.schedule_once(self.copy_file_func, 1)

    def copy_cancel(self, *args):
        self.copy_cancel_flag = True
        self.pop.dismiss()

    def copy_file_func(self, dt):
        print('thread_copy_file_func i =', self.copy_context_i)
        self.progress_bar.value = self.copy_context_i
        time.sleep(0.1)
        if self.copy_cancel_flag:
            pass  # cancel condition
        elif self.copy_context_i > 50:
            pass  # normal operation end
        else:
            self.copy_context_i += 1
            Clock.schedule_once(self.copy_file_func)  # run again
            return  # exit befor closing popup and cleanup

        # end, close popup
        #   ... other cleanup ...
        self.pop.dismiss()


TestApp().run()

Answer 2

with threads I would use something like this; you have to exit the button callback, otherwise the eventloop does not redraw the widgets.

import time
from threading import Thread
from kivy.uix.progressbar import ProgressBar
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.uix.boxlayout import BoxLayout


class TestApp(App):
    def build(self):
        self.thread = None

        parent = Widget()
        btn = Button(text='copy file', on_press=self.start_thread_copy_file)
        parent.add_widget(btn)
        return parent

    def start_thread_copy_file(self, *args):
        if self.thread is None:
            # open progressbar window
            self.progress_bar = ProgressBar(max=5)
            btn = Button(text="Cancel")
            btn.bind(on_press=self.thread_copy_cancel)

            layout = BoxLayout()
            layout.add_widget(self.progress_bar)
            layout.add_widget(btn)

            self.pop = Popup(title="Saving", content=layout)
            self.pop.open()

            self.thread = Thread(target=self.thread_copy_file_func)
            self.thread.start()

    def thread_copy_cancel(self, *args):
        self.thread = None  # can be also done with a flag
        self.pop.dismiss()

    def thread_copy_file_func(self):
        try:
            for i in range(5):
                print('thread_copy_file_func i =', i)
                self.progress_bar.value = i
                time.sleep(1)
                if self.thread is None:  # cancel condition
                    break

        finally:
            self.thread = None
            self.pop.dismiss()


TestApp().run()