kivy MDRoundFlatButton

у меня есть много screen, но код не хочется засорять и писать в каждом screen, как мне мне написать сразу во всех с возможностью запуска функции при нажатии.Спасибо за любую помощь.

from kivy.lang import Builder
from kivy.properties import ObjectProperty
from kivy.uix.scrollview import ScrollView
#from kivy.uix.image import Image
from kivy.uix.boxlayout import BoxLayout
#from kivymd.uix.button import MDFlatButton
from kivymd.app import MDApp
from kivymd.uix.list import OneLineListItem
#from kivymd.uix.card import MDCardSwipe
#from kivy.properties import StringProperty
#from kivymd.uix.dialog import MDDialog
from kivymd.uix.button import MDRoundFlatButton

KV = '''

BoxLayout:
    orientation: "vertical"
    MDToolbar:
        id: toolbar
        title: "Задачи"
        pos_hint: {"top": 1}
        left_action_items: [['menu', lambda x: nav_drawer.set_state()]]
        md_bg_color: .2, .2, .2, 1
        specific_text_color: 1, 1, 1, 1

    NavigationLayout:

        ScreenManager:
            id: screen_manager
            
            Screen:
                name: "screen 1"
                BoxLayout:
                    orientation: "vertical"
                    padding: "4dp"
                    spacing: "4dp"
                    id: "screen_1"

                    MDLabel:
                        text: 'GG'

                    MDRoundFlatButton:
                        text: "MDROUNDFLATBUTTON"
                        font_size: "18sp"
                        md_bg_color: .2, .2, .2, 1
                        size_hint: 1,.1

                    MDRoundFlatButton:
                        text: "MDROUNDFLATBUTTON"
                        font_size: "18sp"
                        md_bg_color: .2, .2, .2, 1
                        size_hint: 1,.1

            Screen:
                name: "screen 2"
                MDLabel:
                    text: 'Screen 2'


            Screen:
                name: "screen 3"

                MDLabel:
                    text: 'Screen 3'

'''

class TestNavigationDrawer(MDApp):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.screen = Builder.load_string(KV)

    def build(self):
        return self.screen

    #def on_start(self): # стартовый запуск 
        #instance_content = self.root.ids.content
        
TestNavigationDrawer().run()