KivyMD中Planlist组件文本不更新及组件通信问题求助

问题原因 #

文本无法更新和列表项点击不生效的核心原因都是实例不对应、属性绑定不符合Kivy规范，具体有三个错误点：

1. Kivy的响应式属性（如StringProperty）必须声明为类属性，你写在Planlist的__init__方法里无法触发绑定更新
2. 你在多处重复创建了Planlist、MainApp的独立实例：
   • Content类的__init__里手动新建了MainApp和Planlist实例
   • MainApp的__init__里手动新建了Planlist实例
   • kv文件中界面渲染的是另一个独立的Planlist实例
     你在提交输入时修改的是MainApp初始化时创建的内存实例，和界面上渲染的实例完全无关，自然看不到更新
3. 列表项on_press中直接调用screen_manager时，该对象不在Planlist的上下文作用域内，找不到对应实例所以点击无效

修改方案 #

1. 修改Python代码 #

from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.metrics import dp
from kivy.properties import StringProperty, ObjectProperty
from kivy.uix.boxlayout import BoxLayout
from kivymd.app import MDApp
from kivymd.uix.button import MDFlatButton
from kivymd.uix.dialog import MDDialog
from kivymd.uix.textfield import MDTextField
from kivymd.uix.list import MDList

class RootWidget(BoxLayout):
    pass

class MyScreenManager(ScreenManager):
    pass

class ScreenPlan1(Screen):
    pass

class Content(BoxLayout):
    # 删掉原有__init__内容，不需要手动新建实例
    pass

class Planlist(MDList):
    # 属性必须声明为类属性才能正常绑定
    text_input_str = StringProperty("plan-unmodifiziert1")
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

class MainApp(MDApp):
    dialog = None
    planlist = ObjectProperty(None)
    def __init__(self, **kwargs):
        self.plan1 = None
        # 删掉原有手动创建root、planlist的代码，避免和kv渲染的实例冲突
        super().__init__(**kwargs)

    def build(self):
        self.theme_cls.theme_style = "Dark"
        self.theme_cls.primary_palette = "BlueGray"
        return Builder.load_file("fit_app.kv")

    def show_confirmation_dialog(self):
        if not self.dialog:
            self.dialog = MDDialog(
                title="Namenseingabe:",
                type="custom",
                content_cls=Content(),
                buttons=[
                    MDFlatButton(
                        text="CANCEL", text_color=self.theme_cls.primary_color,
                        on_release=self.dialog_close
                    ),
                ],
                md_bg_color=(.38, .38, .38, 1)
            )
        self.dialog.open()

    def dialog_close(self, *args):
        self.dialog.dismiss(force=True)

    def on_text_validate(self, widget):
        # 直接修改绑定属性即可自动更新界面
        self.planlist.text_input_str = widget.text
        self.dialog_close()

MainApp().run()

2. 修改KV代码 #

RootWidget:
    orientation: "vertical"

    MDToolbar:
        title: "FitApp"
        md_bg_color: .2,.2,.2,1
        specific_text_color: 1, 1, 1, 1

    MDBottomNavigation:
        MDBottomNavigationItem:
            name: "nav1"
            text: "Dashboard"
            icon: "desktop-mac-dashboard"

            MDLabel:
                text: "Python"
                halign: "center"

        MDBottomNavigationItem:
            name: "nav2"
            text: "Workout"
            icon: "dumbbell"

            MyScreenManager:
                id: screen_manager

                Screen:
                    name: "screen_overview"

                    BoxLayout:
                        orientation: "vertical"
                        MDBoxLayout:
                            size_hint: 1, .2
                            pos_hint: {"top": 1}
                            MDLabel:
                                text: "Neuer Trainingsplan"
                                pos_hint: {"center_x":1, "top":1}
                                text_size: self.size
                                halign: 'left'
                                valign: 'top'
                                padding: "30dp", "30dp"

                            MDIconButton:
                                icon: "plus"
                                pos_hint: {"center_x":1, "center_y":.92}
                                on_release: app.show_confirmation_dialog()

                    # 把界面上的Planlist实例绑定到app的planlist属性
                    Planlist:
                        on_parent: app.planlist = self


                Screen:
                    name: "plan1"

                    MDLabel:
                        text: "Screen"
                        halign: "center"
                    Button:
                        text: "change"
                        on_press:
                            screen_manager.current = 'screen_overview'

        MDBottomNavigationItem:
            name: "nav3"
            text: "History"
            icon: "history"

            MDLabel:
                text: "Cobra"
                halign: "center"
<Content>:
    orientation: "horizontal"
    spacing: "12dp"
    size_hint_y: None
    height: "100dp"

    MDTextField:
        id: plan_name
        hint_text: "Planname"
        # 直接调用全局运行的app实例方法
        on_text_validate: app.on_text_validate(self)

    MDFlatButton:
        text: "OK"
        size: "70dp", "30dp"
        pos_hint: {"right": 1}
        on_release:
            app.on_text_validate(plan_name)

<Planlist>:
    OneLineListItem:
        id: plan1
        # 直接绑定属性自动更新
        text: root.text_input_str
        # 从全局app获取screen_manager实例
        on_press: app.root.ids.screen_manager.current = "plan1"
    OneLineListItem:
        text: "plan-unmodifiziert2"
        on_press: app.root.ids.screen_manager.current = "plan1"
    OneLineListItem:
        text: "plan-unmodifiziert3"
        on_press: app.root.ids.screen_manager.current = "plan1"

修改后文本更新和列表点击跳转的问题都会同时解决。

内容的提问来源于stack exchange，提问作者S4ker900