Python Kivy。标签没有文本，但是，我将其值分配给了变量

Question

第二个屏幕上有一些按钮。如果其中之一被按下-屏幕将更改，而另一屏幕上将出现一个标签，该标签上应包含按下按钮上的文字。没有错误显示，而标签根本没有任何文本。

Python：

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition
from kivy.lang import Builder
from kivy.uix.button import Button


class ScreenOne(Screen):
    pass

class ScreenTwo(Screen):
    def on_pre_enter(self, *args):
        btn = Button(text = "word is here", on_release =self.pressedFunction)
        self.ids.container.add_widget(btn)

        btn1 = Button(text = "another word is here", on_release    =self.pressedFunction)
        self.ids.container.add_widget(btn1)

    def pressedFunction(self, instance, *args):
        self.manager.current= "three"

        screenThree = ScreenThree()
        text = str(instance.text)

        screenThree.changing_label(text)

class ScreenThree(Screen):
    def changing_label(self, text):
        self.ids.my_label.text = text

class ScreenManagement(ScreenManager):
    pass

presentation = Builder.load_file("example.kv")

class MainApp(App):
    def build(self):
        return presentation

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

基维：

ScreenManagement:
    ScreenOne:
    ScreenTwo:
    ScreenThree:

<ScreenOne>:
    BoxLayout:
        Button:
            text: "press me"
            on_release: app.root.current = "two"

<ScreenTwo>:    
    name: "two"

    BoxLayout:
        id: container


<ScreenThree>:
    name: "three"

    BoxLayout:
        id: labelContainer

        Label:
            text: ""
            id: my_label

我也尝试过在此处打印puttin：

class ScreenThree(Screen):
    def changing_label(self, text):
        self.ids.my_label.text = text
        print(self.ids.my_label.text)

它显示了我期望的正确值，但是标签上仍然没有任何内容。

Answer 1

问题

1。在填充my_label.text

之前切换屏幕

my_label.text的切换是在切换屏幕self.manager.current = "three"后完成的。

2。屏幕三-空白/黑色窗口

ScreenThree 实例化了两次。第一个实例是由ScreenThree:在kv文件中创建的（相当于Python代码中的ScreenThree()）。第二个实例是用Python代码screenThree = ScreenThree()创建的。

my_label.text的填充位于第二个实例/对象中，而不是第一个实例中。因此，ScreenThree是空白/黑色窗口，因为应用程序按照kv文件（即ScreenThree的第一个实例）使用视图。

注意：

如果添加id()功能，它将在屏幕上显示不同的存储位置。

def pressedFunction(self, instance, *args):
    self.manager.current = "three"

    self.debug()

    screenThree = ScreenThree()

    print("screenThree={0}, id(screenThree)={1}".format(screenThree, id(screenThree)))

    self.debug()

    text = str(instance.text)
    screenThree.changing_label(text)

def debug(self):
    print("\ndebug:")
    print("\tself.manager.screen_names=", self.manager.screen_names)
    print("\tself.manager.screens=", self.manager.screens)
    for x in self.manager.screens:
        print("\t\tscreen={0}, id(screen)={1}".format(x, id(x)))

解决方案

kv文件

1. 在id: screen_two下添加ScreenTwo:。这将用于引用ScreenTwo中的类属性/方法。
2. 用app.root.current = "two"替换root.manager.current = "two"，因为默认情况下每个屏幕都具有属性manager

Python代码

1. 添加导入语句from kivy.properties import StringProperty
2. 在ScreenTwo（），text = StringProperty('')类中声明StringProperty，因此不需要传递参数和封装。
3. 在pressedFunction（）方法中，将text = str(instance.text)替换为self.text = str(instance.text)
4. 在切换屏幕之前填充文字
5. 在ScreenThree（）类中，将changing_label()方法重命名为on_pre_enter()方法，然后从参数列表中删除text。
6. 替换self.ids.my_label.text = text' with self.ids.my_label.text = self.manager.ids.screen_two.text`
7. 可选： 减少使用的内存，将return presentation替换为return Builder.load_file("example.kv")，然后删除presentation = Builder.load_file("example.kv")

示例

main.py

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
from kivy.uix.button import Button
from kivy.properties import StringProperty


class ScreenOne(Screen):
    pass


class ScreenTwo(Screen):
    text = StringProperty('')

    def on_pre_enter(self, *args):
        print("\nScreenTwo.on_pre_enter:")

        btn = Button(text = "word is here", on_release =self.pressedFunction)
        self.ids.container.add_widget(btn)

        btn1 = Button(text = "another word is here", on_release =self.pressedFunction)
        self.ids.container.add_widget(btn1)

    def pressedFunction(self, instance, *args):
        self.text = str(instance.text)      # populate before switching screen
        self.manager.current = "three"  # switch screen


class ScreenThree(Screen):
    def on_pre_enter(self, *args):
        self.ids.my_label.text = self.manager.ids.screen_two.text


class ScreenManagement(ScreenManager):
    pass


class MainApp(App):
    def build(self):
        return Builder.load_file("example.kv")


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

example.kv

#:kivy 1.11.0

ScreenManagement:
    ScreenOne:
    ScreenTwo:
        id: screen_two
    ScreenThree:

<ScreenOne>:
    BoxLayout:
        Button:
            text: "press me"
            on_release: root.manager.current = "two"    # every screen has a default property manager

<ScreenTwo>:
    name: "two"

    BoxLayout:
        id: container


<ScreenThree>:
    name: "three"

    BoxLayout:
        id: labelContainer

        Label:
            id: my_label
            text: ""

输出