Python - Kivy：使用 ScreenManager 进行文本输入的 KeyError

Question

我正在使用 Kivy 和 ScreenManager（以及 KivyMD）进行一个项目。

使用它的原因是我需要制作一个具有多个屏幕的应用程序。我的目标是通过 (a string) ID 然后 从现有的 MDTextField 中获取 User Input打印出来。

但是，我不断收到此错误：File "<string>", line 37, in <module> File "c:/Users/admin/Desktop/myApp/Main.py", line 75, in loginFunction username = mainScreenInstance.ids["input_username"].text KeyError: 'input_username'。

我已经在整个互联网和 StackOverflow 上搜索了答案，但似乎没有任何解决方案适合我。也许有人可以帮助我解决我的问题并使我的代码正常工作。

无论如何，这是我简短的代码：

from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import Screen, ScreenManager
import sys

screen_helper = """
ScreenManager: 
    MenuScreen:
        name: 'menu'
    SecondScreen:
        name: 'second'

<MenuScreen>:
    MDToolbar:
        title: "Menu Screen"
        pos_hint: {"top": 1}
        anchor_title: "center"
        md_bg_color: (0/255, 0/255, 0/255, 1)
    Image:
        source: "Logo_h_black.png"
        pos_hint: {"center_x": 0.5, "center_y": 0.75}
        size_hint_x: (0.25)
        size_hint_y: (0.25)
    MDTextField:
        id: input_username
        hint_text: "Username"
        size_hint: (0.5, 0.1)
        pos_hint: {"center_x": 0.5, "center_y": 0.55}
        font_size: 20
        mode: "rectangle"
    MDTextField:
        id: input_password
        hint_text: "Password"
        size_hint: (0.5, 0.1)
        pos_hint: {"center_x": 0.5, "center_y": 0.45}
        font_size: 20
        mode: "rectangle"
    MDFillRoundFlatButton:
        text: "LOG IN"
        font_size: 17
        pos_hint: {"center_x": 0.5, "center_y": 0.25}
        on_press: app.loginFunction()
    
<SecondScreen>:
    MDLabel:
        text: 'Profile'
        halign: 'center'
    MDRectangleFlatButton:
        text: 'Back'
        pos_hint: {'center_x':0.5,'center_y':0.1}
        on_press: root.manager.current = 'menu'
        

"""

class MenuScreen(Screen):
    pass

class SecondScreen(Screen):
    pass

sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SecondScreen(name='second'))


class myApp(MDApp):

    def loginFunction(self, args=None):          
                                                 
        if args is None:
           args = sys.argv

        mainScreenInstance = sm.get_screen('menu')

        username = mainScreenInstance.ids["input_username"].text
        password = mainScreenInstance.ids["input_password"].text

        print (username)
        print (password)

    def build(self):
        screen = Builder.load_string(screen_helper)
        return screen

myApp().run()

Answer 1

问题是您的loginFunction 试图从sm 获取mainScreenInstance，但sm 是在screen_helper 加载之前创建的，因此它不知道您的ids已在您的screen_helper 中定义。此外，sm 实际上并未用作 GUI 的一部分，它只是被忽略（除了您在 loginFunction 中对它的引用）。

因此，我建议您完全消除以下几行：

sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SecondScreen(name='second'))

并通过替换将loginFunction 更改为不使用sm：

mainScreenInstance = sm.get_screen('menu')

与：

mainScreenInstance = self.root.get_screen('menu')

Answer 2

您正在复制小部件定义，然后没有在代码中使用它们。最后，您将使用新实例从 screen_helper 字符串覆盖屏幕管理器。当我开始学习 Kivy 时，我做了一些。 kv 语言起初令人困惑，但一旦你掌握它就很漂亮。这个版本应该可以工作（我没有测试过它，因为我还没有找到在我手机的开发环境中安装 kivymd 的简单方法）：

from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import Screen
import sys

screen_helper = """
ScreenManager:
    MenuScreen:
        name: 'menu'
    SecondScreen:
        name: 'second'

<MenuScreen>:
    MDToolbar:
        title: "Menu Screen"
        pos_hint: {"top": 1}
        anchor_title: "center"
        md_bg_color: (0, 0, 0, 1)
    Image:
        source: "Logo_h_black.png"
        pos_hint: {"center_x": 0.5, "center_y": 0.75}
        size_hint_x: (0.25)
        size_hint_y: (0.25)
    MDTextField:
        id: input_username
        hint_text: "Username"
        size_hint: (0.5, 0.1)
        pos_hint: {"center_x": 0.5, "center_y": 0.55}
        font_size: 20
        mode: "rectangle"
    MDTextField:
        id: input_password
        hint_text: "Password"
        size_hint: (0.5, 0.1)
        pos_hint: {"center_x": 0.5, "center_y": 0.45}
        font_size: 20
        mode: "rectangle"
    MDFillRoundFlatButton:
        text: "LOG IN"
        font_size: 17
        pos_hint: {"center_x": 0.5, "center_y": 0.25}
        on_press: app.loginFunction()
    
<SecondScreen>:
    MDLabel:
        text: 'Profile'
        halign: 'center'
    MDRectangleFlatButton:
        text: 'Back'
        pos_hint: {'center_x':0.5,'center_y':0.1}
        on_press: root.manager.current = 'menu'
        

"""

class MenuScreen(Screen):
    pass

class SecondScreen(Screen):
    pass


class myApp(MDApp):
    sm = None
    def on_start():
        self.sm = self.root

    def loginFunction(self, args=None):                       
        if args is None:
           args = sys.argv

        mainScreenInstance = self.sm.get_screen('menu')

        username = mainScreenInstance.ids["input_username"].text
        password = mainScreenInstance.ids["input_password"].text

        print (username)
        print (password)

        # if self.is_login_correct(username, password):
        #     self.sm.current('second') 

    def build(self):
        return Builder.load_string(screen_helper) 

myApp().run()

就个人而言，为了保持一致性，我会将所有与登录相关的功能移至 MenuScreen 类。