How do I change the color of my widget in Kivy at run time?

Question

I\'m having trouble changing the color of a simple widget in Kivy. I can set the color when I create the widget, but I can\'t change it afterwards.

Here is the simpl

Answer 1

The answer by tshirtman is correct, here is the explanation of what's going on.

In your kv file when you set

:
    canvas:
        Color:
            rgba: self.r, 1, 1, 1
        Ellipse:
            pos: self.pos
            size: self.size

The line rgba: self.r, 1, 1, 1 tries to update the value of rgba whenever there is a change to the value of r. This is done implicitly in kv language by binding, which can be done on a kivy Property as it implements a Observer Pattern.

The variable r in your code was updated but it's just a variable that doesn't provide any indication that it's value has changed and can not be bound to. If you notice your changes to pos work because pos is a ReferenceListProperty.

General rule for programming in Kivy, if you want to change code depending on a property of a Widget/Object use a Kivy Property. It provides you the option to Observe Property changes and adjust your code accordingly either explicitly through bind/on_property_name events or implicitly through kv language as mentioned above.