python - How can I make a map screen in kivy, I only get black screen

Question

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.textinput import TextInput
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import StringProperty
from kivy.properties import ObjectProperty
from kivy.uix.label import Label
from kivy.core.window import Window
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.boxlayout import BoxLayout
from kivy_garden.mapview import MapView

kv = '''

<Login>
    ben: benName.text
    pw: passwort.text
    knopf: btn
    knopff: btnn
 
    GridLayout:
        cols: 1
        size: root.width,root.height
        GridLayout:
            cols: 2
            Label:
                text: "Username"
                font_size: 25
            TextInput:
                id: benName
                multiline: False
                font_size: 30
            Label:
                text: "Password"
                font_size: 25
                bold: True
            TextInput:
                password: True
                id: passwort
                multiline: False
                font_size: 40
        Button:
            size_hint: (1.,1.10)
            text:" Start "
            id: btn
            font_size: 40
            on_release:
                root.manager.current = "Map" if passwort.text == "1" and benName.text == "1" else "login"
                root.manager.transition.direction = "down"
        Button:
            size_hint: (1.,1.10)
            text: " Exit "
            id: btnn
            font_size: 40
            on_release: app.stop()


<Map>
    BoxLayout:
        Button:
            text: " [back] "
            bold: True
            font_size: 17
            size_hint: (None,None)
            width: 100
            height: 40
            on_release: app.stop()

'''

MyApp class:

class Login(Screen):
    ben = StringProperty()
    pw = StringProperty()
    knopf = ObjectProperty()


class MyApp(App):
    Builder.load_string(kv)
 
    def build(self):
        ms = ScreenManager()
        ms.add_widget(Login(name='login'))
        ms.add_widget(Map(name='Map'))
        self.title = "MyApp"
        return ms






class Map(Screen):
    def build(self):
        mapview = MapView(zoom=11, lat=50.6394, lon=3.057)
        return mapview


if __name__ == '__main__':
    MyApp().run()

Answer 1

Your build() method in the Map class is never called (it is not called automatically). I suggest you change the build() method to an on_enter() method:

class Map(Screen):
    def on_enter(self, *args):
        mapview = MapView(zoom=11, lat=50.6394, lon=3.057)
        self.ids.map.add_widget(mapview)

And add the map id to the Map rule in the kv:

<Map>
    BoxLayout:
        id: map
        Button:
            text: " [back] "
            bold: True
            font_size: 17
            size_hint: (None,None)
            width: 100
            height: 40
            on_release: app.stop()