在 Kivy 中加载应用程序时元素的大小不会更新

I have on_size() event, it works when I resize window, but I can't make it work on application loading correctly (it works, but not like it must):

from random import random, randint

import kivy

kivy.require('1.8.0')

from kivy.config import Config

Config.set('graphics', 'fullscreen', '0')
Config.set('graphics', 'width', 640)
Config.set('graphics', 'height', 480)
Config.set('graphics', 'position', 'custom')
Config.set('graphics', 'top', 40)
Config.set('graphics', 'left', 40)

from kivy.app import App
from kivy.uix.layout import Layout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.graphics import Color, Line, Rectangle


class Main(BoxLayout):
    side_padding = 20

    def __init__(self, **kwargs):
        super(Main, self).__init__(**kwargs)
        self.size = (0, 0)  # Doesn't help.
        self.on_size()  # Doesn't help.

    def on_size(self, *args):
        left, middle, right = self.ids.left, self.ids.middle, self.ids.right
        left.width = max([x.texture_size[0] for x in left.children]) + self.side_padding
        right.width = max([x.texture_size[0] for x in right.children]) + self.side_padding
        available_space = self.width - left.width - right.width
        if available_space > self.height:
            middle.size = (self.height, self.height)
            extra = (self.width - self.height - left.width - right.width) // 2
            left.width += extra
            right.width += extra
        else:
            middle.size = (available_space, available_space)


class SidePanel(BoxLayout):
    def __init__(self, **kwargs):
        super(SidePanel, self).__init__(**kwargs)


class GameField(Layout):
    def __init__(self, **kwargs):
        super(GameField, self).__init__(**kwargs)
        self.array_size = randint(4, 8)
        self.bind(pos=self.update, size=self.update)
        self.update()

    def update(self, *args):
        s_padding = 1 / 16
        s_sq_size = 1 - 2 * s_padding
        self.canvas.clear()
        with self.canvas:
            block_size = self.width / self.array_size
            sq_size = block_size * s_sq_size
            padding = block_size * s_padding
            Color(1, 1, 1)
            for j in range(self.array_size):
                for i in range(self.array_size):
                    Rectangle(pos=(self.x + padding + i * block_size, self.y + padding + j * block_size),
                              size=(sq_size, sq_size))
            for i in range(self.array_size + 1):
                shift = self.width * i / self.array_size
                Line(points=(self.x, self.y + shift, self.x + self.width, self.y + shift))
                Line(points=(self.x + shift, self.y, self.x + shift, self.y + self.height))


    def on_touch_up(self, touch):
        self.array_size = randint(4, 8)
        self.update()


class Application(App):
    def build(self):
        return Main()


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

*.kv:

#:kivy 1.8.0

<Main>:
    orientation: 'horizontal'
    canvas:
        Color:
            rgb: 1, 1, .9
        Rectangle:
            pos: self.pos
            size: self.size
    SidePanel:
        id: left
        Label:
            size_hint_y: None
            color: 0, 0, 0, 1
            text: 'Score'
        Label:
            color: 0, 0, 0, 1
            text: 'Event log log log loooooooooog'
        Label:
            color: 0, 0, 0, 1
            text: 'Event log log log
123'
        Label:
            color: 0, 0, 0, 1
            text: '35mdbj65 qewr'
        Label:
            color: 0, 0, 0, 1
            text: '3qht6ju7ustju'
    GameField:
        id: middle
    SidePanel:
        id: right
        Button:
            size_hint_y: None
            text: 'Menu'
        Label:
            color: 0, 0, 0, 1
            text: 'Bonuses bonuses'
<SidePanel>:
    orientation: 'vertical'
    size_hint_x: None
    size: self.size

<GameField>:
    pos_hint: {'center_y': .5}
    size_hint: None, None
    canvas.before:
        Color:
            rgb: .9, .9, .9
        Rectangle:
            pos: self.pos
            size: self.size

也许有什么错误?

解决方案

当我根据里面的标签和按钮更新班级孩子的大小时，我已经向他们添加了 on_size 事件:

def on_size(self, *args):
    self.parent.parent.on_size()


class SideLabel(Label):
    on_size = on_size


class SideButton(Button):
    on_size = on_size