为什么我必须单击两次才能滚动？kivy、python中的可滚动标签

我有一个问题：我需要写一个应用程序，在那里我会为每个问题显示正确的答案。我和kivy写了一些代码，但有一件事我很挣扎。我创建了一个页面。有一个显示答案的按钮，但按下一次后，我只能看到部分答案，无法滚动。但是，当我第二次按下按钮时，一切都很好。你能告诉我为什么吗？怎么修理？我想看到所有的答案后，按下一次按钮，并能够滚动

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.uix.scrollview import ScrollView
from kivy.uix.boxlayout import BoxLayout
from kivy.core.window import Window
from kivy.config import Config
Config.set('graphics', 'resizable', True)
import os
import sys

class MyApp(App):
    def build(self):
        self.screen_manager = ScreenManager()

        self.answers = Answers()
        screen = Screen(name = "Answers")
        screen.add_widget(self.answers)
        self.screen_manager.add_widget(screen)

        return self.screen_manager

class Answers(GridLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.rows = 3

        self.label = Label(text = "Answers: ", font_size = 40)
        self.add_widget(self.label)

        self.button = Button(text="Show answers")
        self.button.bind(on_press=self.showanswers)
        self.add_widget(self.button)

        self.scroll = ScrollableLabel(height = Window.size[1]*0.75, size_hint_y = None)
        self.add_widget(self.scroll)


    def showanswers(self, instance):
        f = open("text.txt", "r")
        lines = f.readlines()

        ScrollableLabel.update(self.scroll, lines)
        myapp.screen_manager.current = "Answers"


class ScrollableLabel(ScrollView):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.layout = GridLayout(cols = 1,  size_hint_y = None)
        self.add_widget(self.layout)

        self.lines = Label(size_hint_x = 1, size_hint_y = None, text_size = (self.width, None))

        self.scroll_to_point = Label()
        self.scroll_to_point.bind(texture_size=self.scroll_to_point.setter('size'))

        self.layout.add_widget(self.lines)
        self.layout.add_widget(self.scroll_to_point)


    def update(self, lines):
        self.lines.text = '\n'

        for i in range(len(lines)):
            self.lines.text += '\n ' +str(i+1) + ". " + lines[i]

        self.layout.height = self.lines.texture_size[1]
        self.lines.height = self.lines.texture_size[1]
        self.lines.text_size = (self.lines.width*0.75, None)

        self.scroll_to(self.scroll_to_point)

f = open("text.txt", 'a+')
for i in range(30):
    f.write("Important text \n")
f.close()

myapp = MyApp()
myapp.run()

---

我认为您的小部件高度没有正确更新，要纠正这一点，需要进行一些绑定。由于“kv”语言自动执行绑定，因此我提供了一个使用“kv”的答案：

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.uix.scrollview import ScrollView
from kivy.core.window import Window
from kivy.config import Config
Config.set('graphics', 'resizable', True)

class MyApp(App):
    def build(self):
        self.screen_manager = ScreenManager()

        self.answers = Answers()
        screen = Screen(name = "Answers")
        screen.add_widget(self.answers)
        self.screen_manager.add_widget(screen)

        return self.screen_manager

class Answers(GridLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.rows = 3

        self.label = Label(text = "Answers: ", font_size = 40)
        self.add_widget(self.label)

        self.button = Button(text="Show answers")
        self.button.bind(on_press=self.showanswers)
        self.add_widget(self.button)

        self.scroll = ScrollableLabel(height = Window.size[1]*0.75, size_hint_y = None)
        self.add_widget(self.scroll)


    def showanswers(self, instance):
        f = open("text.txt", "r")
        lines = f.readlines()

        self.scroll.update(lines)
        myapp.screen_manager.current = "Answers"


class ScrollableLabel(ScrollView):

    def update(self, lines):
        self.ids.lines.text = '\n'
        for i in range(len(lines)):
            self.ids.lines.text += '\n ' +str(i+1) + ". " + lines[i]

Builder.load_string('''
<ScrollableLabel>:
    size_hint_y: None
    GridLayout:
        cols: 1
        size_hint_y: None
        height: self.minimum_height  # adjust height to handle the Label
        Label:
            id: lines
            size_hint_y: None
            height: self.texture_size[1]  # set height based on text
''')

f = open("text.txt", 'a+')
for i in range(30):
    f.write("Important text \n")
f.close()

myapp = MyApp()
myapp.run()

致：

ScrollableLabel.update(self.scroll, lines)

self.scroll.update(lines)