Kivy screenmanager current screen not switching (at least not visually). Using kivymd-CodePudding

I am working on a login system with python, kivymd, and kivy's screenmanager to switch, however, when I try to switch the screen from the login screen to another one nothing happens, no errors, and no visual changes. This is a reproducible example:

.py file

import kivy
from kivy.uix.image import Image
from kivy.uix.screenmanager import ScreenManager, Screen

from kivy.lang import Builder
from kivymd.app import MDApp

class Screen1(Screen):
    def SwitchScreen(self):
        scrMng.current = "screen2"

class Screen2(Screen):
    def SwitchScreen(self):
        scrMng.current = "screen1"

scrMng = ScreenManager()
scrMng.add_widget(Screen1(name = "screen1"))
scrMng.add_widget(Screen2(name = "screen2"))

class TestApp(MDApp):
        def build(self):
            return Builder.load_file("main.kv")


TestApp().run()

.kv file

ScreenManager:
    Screen1:
    Screen2:

<Screen1>
    name: "screen1"
    
    MDCard:
        size_hint: None, None
        size: 300, 350
        pos_hint: {"center_x": 0.5, "center_y": 0.5}
        elevation: 10
        padding: 25
        spacing: 25
        orientation: 'vertical'

        MDLabel:
            text: "Screen2"
            font_size: 40
            halign: "center"
            font_bold: True
            theme_text_color: "Custom"
            text_color: (0.20, 0.33, 1.00, 1)

        MDRoundFlatButton:
            text: "SWITCH"
            font_size: 12
            pos_hint: {"center_x": 0.5}
            on_press: self.parent.parent.SwitchScreen()

<Screen2>
    name: "screen2"
    
    MDCard:
        size_hint: None, None
        size: 300, 350
        pos_hint: {"center_x": 0.5, "center_y": 0.5}
        elevation: 10
        padding: 25
        spacing: 25
        orientation: 'vertical'

        MDLabel:
            text: "Screen1"
            font_size: 40
            halign: "center"
            font_bold: True
            theme_text_color: "Custom"
            text_color: (0.20, 0.33, 1.00, 1)

        MDRoundFlatButton:
            text: "SWITCH"
            font_size: 12
            pos_hint: {"center_x": 0.5}
            on_press: self.parent.parent.SwitchScreen()

Python version: 3.9.9 Kivy version: 2.0.0

Any help is appreaciated!

CodePudding user response：

You are creating a GUI using the lines:

scrMng = ScreenManager()
scrMng.add_widget(Screen1(name = "screen1"))
scrMng.add_widget(Screen2(name = "screen2"))

But the scrMng created above is never used, so those lines can be eliminated. And the reference to scrMng in the python code will do nothing, since scrMng is not actually part of your app.

Your GUI is actually created by the line:

return Builder.load_file("main.kv")

Then the Screen classes can become:

class Screen1(Screen):
    def SwitchScreen(self):
        self.manager.current = "screen2"

class Screen2(Screen):
    def SwitchScreen(self):
        self.manager.current = "screen1"

Note that self.manager in a Screen class is always a reference to its ScreenManager.