Python & Kivy：从另一个屏幕获取值以设置按钮 on_release 值

Question

好的，所以我的目标是，当您按下按钮让我们说“anotherScreen”时，我想被发送到“Screen 0”。在这里，根据从 SQLite 数据库中获取的 0 或 1 值，屏幕上的按钮“btn”会将您发送到 screen1（如果值为 0）或如果值不是 0 (1) 则发送到 screen2。然而，这会产生一些错误。

这里有一些代码可以更好地解释它：

pythonfile.py

import sql_server_file as app_users
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.app import App

class screen0(Screen):
    def __init__(self):
        self.sql_cursor = app_users.AppUsers()
        self.user_id = self.manager.get_screen("anotherScreen").user_id   # <---- This gives 
                                                                      # me an NoneType Error

        self.value = self.sql_cursor.fetch_ids100_done_status(self.user_id)
        if(self.value == 0):
            self.ids["btn"].on_release = "app.root.current = 'screen1'" # <---- This is 
                                          # syntactically wrong, however, how can I do this?

        else:
            self.ids["btn"].on_release = "app.root.current = 'screen2'" # <-- See above

class screen1(Screen):
    pass
class screen2(screen):
    pass
class anotherScreen(Screen):
    def __init__(self):
        self.user_id = 0

class WindowManager(ScreenManager):
    pass

class ReturnClass(App):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(anotherScreen())
        sm.add_widget(screen0())
        sm.add_widget(screen1())
        sm.add_widget(screen2())
        return sm

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

所以，当输入“anotherScreen”之后的屏幕时，我需要它“自动”运行。

出现错误：

AttributeError: 'NoneType' object has no attribute 'get_screen'

kivy 文件可能如下所示：

kivyfile.kv

WindowManager:
    anotherScreen:
    screen0:
    screen1:
    screen2:

<anotherScreen>:
    Button:
        text: "get to screen0"
        on_release:
            app.root.current = "screen0"

<screen0>:
    Button:
        text: "Get to either screen1 or screen2 depending on value in sqlite-database"

<screen1>:
    Label:
        text: "Screen1"

<screen2>:
    Label:
        text: "Screen2"

Answer 1

screen0 的__init__() 方法在分配给ScreenManager 之前被调用，所以self.manager 是None。这会导致您看到的错误。解决方法是将该代码移至分配ScreenManager 后调用的不同方法。一个不错的选择是在进入屏幕时自动调用的on_enter() 方法。

那么screen0类可以是：

class screen0(Screen):
    # no __init__() method needed

    def goto_screen1(self):   # go to screen1
        self.manager.current = 'screen1'

    def goto_screen2(self):   # go to screen2
        self.manager.current = 'screen2'

    def on_enter(self, *args)::   # the new method
        self.sql_cursor = app_users.AppUsers()
        self.user_id = self.manager.get_screen("anotherScreen").user_id

        self.value = self.sql_cursor.fetch_ids100_done_status(self.user_id)
        if(self.value == 0):
            self.ids["btn"].on_release = self.goto_screen1

        else:
            self.ids["btn"].on_release = self.goto_screen2

请注意，Screen 类应该分配有 name。

另外，kv 中的行：

WindowManager:
    anotherScreen:
    screen0:
    screen1:
    screen2:

是多余的（因为您在 build() 方法中构建了相同的小部件树），并且可以删除。