How to get widget position in kivy

Question

I am learning to code with Python and Kivy. I want to get the position of a button (button1), which is in a boxlayout, relative to the window. However when I print button1.pos or button1.to_window(*button1.pos) I get [0, 0] which is clearly not the position of the button. I am assuming that the button position has not yet been changed at the time that I get the position but i'm not sure from where I can put this code so that it is executed after the button position has been changed (presumably by the layout), If this is the issue at all.

As you can see in my code, I tried to get the position from the on_start method, hoping this would solve my problem, but I got the same result.

Here is my code:

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

class Example(App):
    def build(self):
        return root

    def on_start(self):
        print("Position: " + str(button1.pos))
        print("Position2: " + str(button1.to_window(*button1.pos)))

root = BoxLayout()
root.orientation = 'vertical'
button1 = Button(text = "Button1")
button2 = Button(text = "Button2")
root.add_widget(button1)
root.add_widget(button2)

Example().run()

Thanks

Seems like duplicate https://stackoverflow.com/questions/25356210/how-to-get-a-widget-position-inside-its-layout-in-kivy — LOrD_ARaGOrN, Dec 10 '18 at 03:18
Possible duplicate of [How to get a widget position inside it's layout in Kivy?](https://stackoverflow.com/questions/25356210/how-to-get-a-widget-position-inside-its-layout-in-kivy) — U13-Forward, Dec 10 '18 at 03:26
@U9-Forward the problem is that it is calculating the position prematurely, that is, before the window and its components are rendered — eyllanesc, Dec 10 '18 at 03:30

score 1 · Answer 1 · answered Dec 10 '18 at 03:29

To understand the problem you should review the docs:

on_start()

Event handler for the on_start event which is fired after initialization (after build() has been called) but before the application has started running.

As noted start() is called before starting the application so the BoxLayout still does not do its job so you will get the default position of every widget that is (0, 0). For these cases you must use Clock.schedule_once() that calls the function when it finishes executing the synchronous task and the next frame is rendered.

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.clock import Clock

class Example(App):
    def build(self):
        Clock.schedule_once(lambda *args: self.print_pos())
        return root

    def print_pos(self):
        print("Position: " + str(button1.pos))
        print("Position2: " + str(button1.to_window(*button1.pos)))

root = BoxLayout()
root.orientation = 'vertical'
button1 = Button(text = "Button1")
button2 = Button(text = "Button2")
root.add_widget(button1)
root.add_widget(button2)

Example().run()

In my case I get the following:

Position: [0, 300.0]
Position2: (0, 300.0)

And that is valid since the position in kivy is with respect to the bottom left.

How to get widget position in kivy

1 Answers1