gistfile1.txt

from kivy.lang import Builder
from kivy.uix.anchorlayout import AnchorLayout
from kivy.base import runTouchApp

class AspectRatioEnforcer(AnchorLayout):
    def on_size(self, instance, size):
        self.resize_child()

    def resize_child(self):

        if not self.children:
            # No child widget is present to resize
            return
        child = self.children[0]

        aspect_ratio = 16 / 9.

        if self.width > aspect_ratio * self.height:
            child_height = self.height
            child_width = aspect_ratio * child_height

        else:
            child_width = self.width
            child_height = child_width / aspect_ratio

        child.size_hint = (None, None)
        child.width = child_width
        child.height = child_height

if __name__ == "__main__":
    runTouchApp(Builder.load_string("""
AspectRatioEnforcer:
    Widget:
        canvas:
            Color:
                rgba: 1, 0, 0, 1
            Rectangle:
                pos: self.pos
                size: self.size
    """))

Thank you! That's very helpful. Is there place I can appropriately add in:

        anchor_x: 'center'
        anchor_y: 'center'

To get the entire AspectRatioEnforcer(AnchorLayout) to center itself?