Kivy understanding

Question

I am a newbee with Kivy GUI framework and I have a few questions related to kvlang:

1.How can I add my custom widget class to root in kv file?(example) PS: I use here clear_widgets() then I try to add my custom widget but I get error after I click on button.

#:kivy 1.8

<HelloWorldForm@BoxLayout>:
    orientation: "vertical"
    Label:
        text:"Hello world"
    Button:
        text: "Go back"
        on_press: app.formGoBack()


<MainForm@BoxLayout>:
    orientation: "vertical"
    btnOpenForm: btnChangeForm
    BoxLayout:
        size_hint_y:None
        height:"40dp"
        Button:
            id:btnChangeForm
            text:"Go to hello world form"
            on_press: 
                root.clear_widgets()
                root.add_widget(HelloWorldForm)
        Button:
            id:btnExit
            text:"Exit"
            on_press: app.Exit()


MainForm:

How can I add HelloWorldForm widget class using add_widget method

2.How can I use add_widget and clear_widgets methods in python code?(for example)

main.kv

   <MainForm@BoxLayout>:
        orientation: "vertical"
        btnOpenForm: btnChangeForm
        BoxLayout:
            size_hint_y:None
            height:"40dp"
            Button:
                id:btnChangeForm
                text:"Go to hello world form"
                on_press: app.changeForm()

main.py

#!/usr/bin/python3.4


import kivy

kivy.require('1.8.0')

from kivy.app import App
from kivy.uix import *


class MainApp(App):
    def changeForm(self)
    /**
     TO-DO
    **/




app=MainApp()
app.run()

3.How can I access kvlang properties in python? For example i want to take the text from a button. How can I achieve that?

Answer 1

1. The problem is with this line: root.add_widget(HelloWorldForm). You are adding a class not a class instance. In particular, you probably want to add the same instance each time it is called rather than creating a new one, so you should not do root.add_widget(HelloWorldForm()).

I suggest that in your python code, add:

class MainApp(App):
    def build(self):
            self.helloworldform = HelloWorldForm()
            self.mainform = MainForm()
            return self.mainform

And in your kv replace root.add_widget(HelloWorldForm) with root.add_widget(app.helloworldform) This will add the instance of HelloWorldForm that you defined in the build function.

2. This ties into the first question, you can now access clear_widgets and add_widget functions through the references to helloworldform and mainform you saved in the build function.:

   self.mainform.clear_widgets()
   self.mainform.add_widget(self.helloworldform)
   

3. For example, to take text of btnChangeForm:

   self.mainform.btnOpenForm.text = 'This will change the text of the button.'
   

It is strange that you do btnOpenForm: btnChangeForm. This will save a reference to btnChagneForm but name is btnOpenForm. Why not have them the same name? btnChangeForm: btnChangeForm