Kivy:如何在下一个输入框上设置焦点？

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.textinput import TextInput

class MyBoxLayout(BoxLayout):
    def on_enter(self, instance):
        instance.focus = False  # 取消当前输入框的焦点
        next_input = self.ids[instance.next]  # 获取下一个输入框
        next_input.focus = True  # 设置下一个输入框的焦点

class MyApp(App):
    def build(self):
        return MyBoxLayout()

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