BoxLayout 中的两个 kivy 图像

I tried to make an kivy program wit two images , which are supposed to change when I click on them. But when I tred to add two Widgets with the containing Images to an BoxLayout, I got one image at the position 0, 0 Why dosen't the BoxLayout work with my images? Here is my code:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.lang import Builder
from kivy.core.window import Window
from kivy.properties import NumericProperty
from kivy.properties import StringProperty
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout

class Feld(Widget):
    droga = StringProperty('hinterg.png')
    kr = StringProperty('kreuz.png')
    ks = StringProperty('kreis.png')
    def on_touch_down(self, touch):
        if self.collide_point(*touch.pos):
            self.droga = self.kr

root = Builder.load_string('''
BoxLayout:
    orientation: 'horizontal'
    Feld:
        id: a1
    Feld:
        id: a2

<Feld>:
    Image:
        source: root.droga



''')

class app(App):
    def build(self):
        Window.clearcolor = (0, 0.54, 1, 1)
        return root

if __name__ == "__main__":
    app().run()

解决方案

您的 Feld 实例由 BoxLayout 定位，但图像显示在作为 Feld 子级的 Image 小部件中.由于 Feld 只是一个普通的 Widget，它不会对其子级施加任何自动位置或大小，因此两个实际图像的默认大小为 (100, 100)，位置为 (0, 0).