Kivy ObjectProperty to update label text

Question

I am working on creating a kivy user interface to display the values generated by a data model I've written as a standard python object. In essence, I would like the user to be able to press a button, which would change the underlying data model and the results of this change would be automatically updated and displayed. It is my understanding that this can be implemented using kivy properties (in this case, ObjectProperty).

Here is some example code:

import kivy
kivy.require('1.7.0')

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.properties import ObjectProperty
from kivy.lang import Builder

Builder.load_string("""
<RootWidget>:
    cols: 2
    Label:
        text: "Attribute a:"
    Label:
        text: root.data_model.a
    Label:
        text: "Attribute b:"
    Label:
        text: root.data_model.b
    Label:
        text: "Attribute c:"
    Label:
        text: root.data_model.c
    Button:
        text: "Make data_model.a longer"
        on_press: root.button_press()
    Button:
        text: "Make data_model.b shorter"
        on_press: root.button_press2()
""")


class DataModel(object):
    def __init__(self):
        self.a = 'This is a'
        self.b ='This is b'

    @property
    def c(self):
        return self.a + ' and ' + self.b

class RootWidget(GridLayout):
    data_model = ObjectProperty(DataModel())

    def button_press(self, *args):
        self.data_model.a = 'This is a and it is really long now'
        print self.data_model.c

    def button_press2(self, *args):
        self.data_model.b = 'B'
        print self.data_model.c

class TestApp(App):
    def build(self):
        return RootWidget()

app = TestApp()
app.run()

The desired result is for when user presses either button, the labels would automatically update to show the new properties. As can be seen by the print statements, the data_model is getting updated correctly. However, none of the labels are updating. Can someone clarify how to do this?

Answer 1

However, none of the labels are updating. Can someone clarify how to do this?

The attributes you reference need to be Kivy properties, but the a, b and c you reference are all just python attributes so Kivy has no way to bind to their changes.

To work with properties you need your object to inherit from EventDispatcher (Kivy widgets do this automatically, which is why their properties work).

from kivy.event import EventDispatcher

class DataModel(EventDispatcher):
    a = StringProperty('')
    b = StringProperty('')
    c = StringProperty('')

    def __init__(self, *args, **kwargs):
        super(DataModel, self).__init__(*args, **kwargs)
        self.a = 'This is a'
        self.b ='This is b'
        self.bind(a=self.set_c)
        self.bind(b=self.set_c)

    def set_c(self, instance, value):
        self.c = self.a + ' and ' + self.b        

Note that this is not the only way (or even necessarily the best way) to get the behaviour you wanted for c. You could create the binding in kv language (I'd usually do it that way) or you can look at Kivy's AliasProperty for something more like your original definition.

Of course you could also set the values of a and b when the properties are declared.