Kivy Dropdown 未在 ScreenManager 中打开但在屏幕中打开

Question

我无法让 kivy.DropDown 小部件与屏幕管理器一起使用。

我正在使用 kivy 文档提供的下拉代码，并将其添加到屏幕小部件，然后我将其添加到屏幕管理器以显示。以下代码应自行重现问题。

import kivy
kivy.require('1.10.1')

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.dropdown import DropDown
from kivy.uix.button import Button
from kivy.uix.anchorlayout import AnchorLayout

class MyScreen(Screen):
    def __init__(self, **kwargs):
        super(MyScreen, self).__init__(**kwargs)

        anchor = AnchorLayout()
        anchor.anchor_x = "center"
        anchor.anchor_y = "center"
        anchor.size = self.size
        anchor.pos = self.pos

        dropdown = DropDown()
        for index in range(10):
            # When adding widgets, we need to specify the height manually
            # (disabling the size_hint_y) so the dropdown can calculate
            # the area it needs.

            btn = Button(text='Value %d' % index, size_hint_y=None, height=44)

            # for each button, attach a callback that will call the select() method
            # on the dropdown. We'll pass the text of the button as the data of the
            # selection.
            btn.bind(on_release=lambda btn: dropdown.select(btn.text))

            # then add the button inside the dropdown
            dropdown.add_widget(btn)

        # create a big main button
        mainbutton = Button(text='Hello', size_hint=(None, None))

        # show the dropdown menu when the main button is released
        # note: all the bind() calls pass the instance of the caller (here, the
        # mainbutton instance) as the first argument of the callback (here,
        # dropdown.open.).
        mainbutton.bind(on_release=dropdown.open)

        # one last thing, listen for the selection in the dropdown list and
        # assign the data to the button text.
        dropdown.bind(on_select=lambda instance, x: setattr(mainbutton, 'text', x))
        anchor.add_widget(mainbutton)

        self.add_widget(anchor)

sm = ScreenManager() # transition = NoTransition())
sm.add_widget(MyScreen(name='screen'))

class MyApp(App):

    def build(self):
        return sm

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

为什么如果在 ScreenManager 中放入屏幕小部件，下拉小部件不起作用？欢迎澄清。

PS： 对于发现此问题的任何人，您可以使用微调器小部件来实现相同的功能。

Answer 1

我相信你的问题是由于垃圾收集。 __init__() 方法中的 dropdown 引用未保存（bind 使用 weakref，这不会阻止垃圾收集）。所以我认为您需要做的就是将您的 dropdown 局部变量替换为 self.dropdown 实例变量：

import kivy
kivy.require('1.10.1')

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.dropdown import DropDown
from kivy.uix.button import Button
from kivy.uix.anchorlayout import AnchorLayout

class MyScreen(Screen):
    def __init__(self, **kwargs):
        super(MyScreen, self).__init__(**kwargs)

        anchor = AnchorLayout()
        anchor.anchor_x = "center"
        anchor.anchor_y = "center"
        anchor.size = self.size
        anchor.pos = self.pos

        self.dropdown = DropDown()
        for index in range(10):
            # When adding widgets, we need to specify the height manually
            # (disabling the size_hint_y) so the dropdown can calculate
            # the area it needs.

            btn = Button(text='Value %d' % index, size_hint_y=None, height=44)

            # for each button, attach a callback that will call the select() method
            # on the dropdown. We'll pass the text of the button as the data of the
            # selection.
            btn.bind(on_release=lambda btn: self.dropdown.select(btn.text))

            # then add the button inside the dropdown
            self.dropdown.add_widget(btn)

        # create a big main button
        mainbutton = Button(text='Hello', size_hint=(None, None))

        # show the dropdown menu when the main button is released
        # note: all the bind() calls pass the instance of the caller (here, the
        # mainbutton instance) as the first argument of the callback (here,
        # dropdown.open.).
        mainbutton.bind(on_release=self.dropdown.open)

        # one last thing, listen for the selection in the dropdown list and
        # assign the data to the button text.
        self.dropdown.bind(on_select=lambda instance, x: setattr(mainbutton, 'text', x))
        anchor.add_widget(mainbutton)

        self.add_widget(anchor)

sm = ScreenManager() # transition = NoTransition())
sm.add_widget(MyScreen(name='screen'))

class MyApp(App):

    def build(self):
        return sm

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