如何在kivymd中制作网格布局？

我正在尝试在 kivymd 中进行网格布局。已创建 GridLayout 但宽度未填满屏幕。kivymd中如何扩展列的宽度以适应屏幕？

我使用了 kivymd 文档中的示例并使用它创建了网格布局。

app.py

from kivymd.app import MDApp

from kivy.uix.screenmanager import Screen, ScreenManager

from kivy.lang import Builder

from main_screen_str import helper_string

from kivy.core.window import Window

Window.size = (300, 500)

class MainScreen(Screen):

pass

class MainApp(MDApp):

def __init__(self, **kwargs):

super().__init__(**kwargs)

self.sm = ScreenManager()

self.sm.add_widget(MainScreen(name="main_screen"))

self.main_str = Builder.load_string(helper_string)

def build(self):

screen = Screen()

screen.add_widget(self.main_str)

return screen

if __name__ == '__main__':

MainApp().run()

这是构建器字符串。网格布局已创建，但宽度不适合屏幕。如何扩展网格布局中单个列的宽度？

构建字符串

helper_string = """

ScreenManager:

MainScreen:

<MainScreen>:

name: 'main_screen'

MDGridLayout:

cols: 3

MDIconButton:

icon: "android"

theme_text_color: "Custom"