how to center canvas? kivy

Question

How to place the white square to the center? I tried a lot of variants, but nothing worked. It work for Label and Buttons, but not for canvas. Or maybe I'm doing everything in wrong way. maybe you suggest the best solution for this task. I need window with background, label in left corner, label in right corner and a square in center

from kivy.app import App
from kivy.uix.widget import Widget

from kivy.uix.floatlayout import FloatLayout
from kivy.config import Config
from kivy.animation import Animation

from kivy.vector import Vector
from kivy.clock import Clock
from kivy.properties import NumericProperty, ReferenceListProperty,\
    ObjectProperty

Config.set('graphics', 'resizable', 'true')
Config.set('graphics', 'width', '900')
Config.set('graphics', 'height', '450')
Config.write()

class Helicopter(Widget):
    pass


class Background(Widget):
    pass


class Root(FloatLayout):
    #def on_touch_down(self, touch):
     #   Animation(center=touch.pos).start(self)
    pass

class FriendsApp(App):
    def build(self):

        return Root()

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

.kv file

#: kivy 1.10.0
<Root>
    AnchorLayout:
        canvas.before:
            Color:
                rgba: 1, 1, 1, 1  # white
            Rectangle:
                source: 'background.jpg'
                pos: self.pos
                size: self.size
        size: self.parent.size
        anchor_x: 'center'
        anchor_y: 'center'


    AnchorLayout:
        anchor_y:'top'
        anchor_x:'left'
        padding: 20
        Label:
            text: 'Lives: x2'
            size: self.texture_size
            size_hint: None, None

    AnchorLayout:
        anchor_x: 'right'
        anchor_y: 'top'
        padding: 20
        Label:
            text: 'Score: 0000000'
            size: self.texture_size
            size_hint: None,None

    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'center'
        canvas:
            Rectangle:
                size: 100, 100
                pos: self.pos

Answer 1

The AnchorLayout has its own canvas and you cannot aligns itself. There are two solutions to this problem. In the example, colors were added for visualization.

The AnchorLayout aligns its children to a border (top, bottom, left, right) or center.

Solution 1

Add a Widget as a children.

AnchorLayout:
    anchor_x: 'center'
    anchor_y: 'center'
    Widget:
        canvas.before:
            Color:
                rgba: 1, 1, 1, 1  # white
            Rectangle:
                size: 100, 100
                pos: self.pos
        size_hint: None,None

Solution 2

Replace the last AnchorLaoyout with Widget.

Widget:
    canvas:
        Rectangle:
            pos: self.center_x - 50, self.center_y - 50
            size: 100, 100

Example - Solution 1

kv file

#: kivy 1.10.0

<Root>
    AnchorLayout:
        anchor_y:'top'
        anchor_x:'left'
        padding: 20
        Label:
            canvas.before:
                Color:
                    rgba: 1, 0, 0, 1  # red
                Rectangle:
                    pos: self.pos
                    size: self.size
            text: 'Lives: x2'
            size: self.texture_size
            size_hint: None, None

    AnchorLayout:
        anchor_x: 'right'
        anchor_y: 'top'
        padding: 20
        Label:
            canvas.before:
                Color:
                    rgba: 0, 0, 1, 1  # blue
                Rectangle:
                    pos: self.pos
                    size: self.size
            text: 'Score: 0000000'
            size: self.texture_size
            size_hint: None,None

    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'center'
        Widget:
            canvas.before:
                Color:
                    rgba: 1, 1, 1, 1  # white
                Rectangle:
                    size: 100, 100
                    pos: self.pos
            size_hint: None,None

Output