为什么我无法访问 Screen.ids？

Question

更新：

Ryan P 的回答提供了解决方案。但是，我采用了该解决方案 并对其进行了一些更改，丢弃了所有不存在的数据 正确初始化为 RootWidget 的 on_enter 方法 屏幕。这效果很好。

直到今天，我的 RootWidget 类都是 Widget 的子类，我可以毫无问题地访问它的 id 来获取“grid”的值。然而，我只是改变了 它是Screen的子类，现在它说由于某种原因ID是空的...... Screen确实有一个ID和所有这些，但由于某种原因，它没有注册那个 我在 kv 文件中为 id ''grid'' 分配了一个 GridLayout。谁能告诉我为什么？

回溯：

[INFO   ] [Logger      ] Record log in /home/yerman/.kivy/logs/kivy_14-11-13_201.txt
[INFO   ] Kivy v1.9.0-dev
[INFO   ] [Python      ] v2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2]
[INFO   ] [Factory     ] 172 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_pygame, img_pil, img_gif (img_sdl2, img_ffpyplayer ignored)
[INFO   ] [Window      ] Provider: pygame(['window_egl_rpi'] ignored)
[WARNING] [WinPygame   ] Video: failed (multisamples=2)
[WARNING] [WinPygame   ] trying without antialiasing
[INFO   ] [GL          ] OpenGL version <2.1 Mesa 10.1.3>
[INFO   ] [GL          ] OpenGL vendor <Intel Open Source Technology Center>
[INFO   ] [GL          ] OpenGL renderer <Mesa DRI Intel(R) Ironlake Mobile >
[INFO   ] [GL          ] OpenGL parsed version: 2, 1
[INFO   ] [GL          ] Shading version <1.20>
[INFO   ] [GL          ] Texture max size <8192>
[INFO   ] [GL          ] Texture max units <16>
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
[INFO   ] [Text        ] Provider: pygame(['text_sdl2'] ignored)
{}                     #<<< note the emtpy ids I printed out
 Traceback (most recent call last):
   File "main.py", line 169, in <module>
     MineSweeperApp().run()
   File "/usr/lib/python2.7/dist-packages/kivy/app.py", line 799, in run
     root = self.build()
   File "main.py", line 163, in build
     return Manager()
   File "/usr/lib/python2.7/dist-packages/kivy/uix/screenmanager.py", line 844, in __init__
     super(ScreenManager, self).__init__(**kwargs)
   File "/usr/lib/python2.7/dist-packages/kivy/uix/floatlayout.py", line 66, in __init__
     super(FloatLayout, self).__init__(**kwargs)
   File "/usr/lib/python2.7/dist-packages/kivy/uix/layout.py", line 66, in __init__
     super(Layout, self).__init__(**kwargs)
   File "/usr/lib/python2.7/dist-packages/kivy/uix/widget.py", line 269, in __init__
     Builder.apply(self)
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1837, in apply
     self._apply_rule(widget, rule, rule)
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1942, in _apply_rule
     child = cls(__no_builder=True)
   File "main.py", line 43, in __init__
     self.grid = self.ids["grid"]
 KeyError: 'grid'

kv 文件：

#:kivy 1.8.0

<RootWidget>:
    GridLayout:
        id: grid
        size: root.size
        cols: root.sides

<Blank>:
    background_color: 1, 1, 1, 1
    background_disabled_down: "kivy_white_bg.png"
    on_press: self.parent.parent.sweep(self)

<Mine>:
    background_color: 1, 1, 1, 1
    background_disabled_down: "kivy_white_bg.png"
    on_press: self.parent.parent.sweep(self)

<TryAgain>:
    anchor_x: 'center'
    anchor_y: 'center'
    BoxLayout:
        size: root.size
        orientation: 'vertical'
        padding_bottom: '20dp'

        Label:
            font_size: '20dp'
            text: root.text

        BoxLayout:
            size_hint: 1, .3
            spacing: 10
            padding: 10
            Button:
                size_hint: .4, 1
                font_size: '20dp'
                text: "yes"
                on_press: app.stop(); app.run()
            Button:
                size_hint: .4, 1
                font_size: '20dp'
                text: "no"
                on_press: root.quit()

<Menu>:
    GridLayout:
        rows: 2
        Button:
            text: "8x8"
            on_press: root.manager.current = 'game_screen'
        Button:
            text: "16x16"
            on_press: root.manager.current = 'game_screen'
        Button:
            text: "30x16"
            on_press: root.manager.current = 'game_screen'
        Button:
            text: "custom"
            on_press: root.manager.current = 'game_screen'

<Manager>:
    id: _manager
    menu: menu
    game: game
    current: menu_screen

    Menu:
        id: menu
        manager: _manager
        name: 'menu_screen'

    RootWidget:
        id: game
        manager: _manager
        name: 'game_screen'

ma​​in.py:

#!/usr/bin/env python

from random import sample
import sys
import kivy
kivy.require('1.8.0')
from kivy.app import App
from kivy.core.window import Window
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.properties import NumericProperty, ListProperty, StringProperty, ObjectProperty
from kivy.uix.gridlayout import GridLayout
from kivy.uix.modalview import ModalView
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.clock import Clock

class Blank(Button):

    index = ListProperty([0, 0])
    count = NumericProperty(0)

    def __init__(self, **kwargs):
        super(Blank, self).__init__(**kwargs)


class Mine(Button):

    index = ListProperty([0, 0])
    count = NumericProperty(0) # not really necessary

    def __init__(self, **kwargs):
        super(Mine, self).__init__(**kwargs)


class RootWidget(Screen):

    sides = NumericProperty(10)
    mine_count = NumericProperty(20)

    def __init__(self, **kwargs):
        super(RootWidget, self).__init__(**kwargs)
        self.grid = self.ids["grid"]

        # generate random mine indices
        mines = sample(xrange(self.sides**2), self.mine_count)

        x, y = -1, 0
        for i in xrange(self.sides**2):
            if x == self.sides - 1:
                x = 0
                y += 1
            else:
                x += 1

            if i not in mines:
                b = Blank(index=[x, y])

            else:
                b = Mine(index=[x, y])
            self.grid.add_widget(b)

        # record mine, blank and safe blank indices
        self.all_btns = [c.index for c in self.grid.children]
        self.mines = [c.index for c in self.grid.children if isinstance(c, Mine)]
        self.blanks = [c.index for c in self.grid.children if isinstance(c, Blank)]
        # a safe blank has no adjacent mines
        self.safe_blanks = [c.index for c in self.grid.children if self.is_safe(c)]

        # give each btn an 'adjacent mines count'
        for x, y in self.all_btns:
            btn = self.get_child_by_index([x, y])
            for index in self.field(x, y):
                if index in self.mines:
                    btn.count += 1

    def field(self, x, y):
        """ the minefield surrounding a btn """
        field = [[x-1, y], [x+1, y], [x, y+1], [x, y-1],
            [x+1, y+1], [x-1, y-1], [x+1, y-1], [x-1, y+1]]

        get = self.get_child_by_index        
        return [i for i in field if i in self.all_btns and get(i).disabled == False]

    def sweep(self, instance):
        instance.disabled = True

        if instance.index in self.mines: 
            print "Boom!"              # It's a mine! You lose
            instance.text = "Boom!"
            self.game_over() 

        pressed = sum(1 for c in self.grid.children if c.disabled == True)
        print pressed
        if self.sides**2 - pressed == self.mine_count:
            self.game_over(win=True)

        if instance.count > 0:
            instance.text = str(instance.count)
            instance.disabled = True
            return
        else:
            x, y = instance.index

            for index in self.field(x, y):
                if index not in self.mines:
                    blank = self.get_child_by_index(index)
                    blank.disable = True
                    if blank.count > 0:
                        blank.text = str(blank.count)
                    self.sweep(blank)

    def is_safe(self, btn):
        x, y = btn.index
        for index in self.field(x, y):
            if index in self.mines:
                return False
        return True

    def get_child_by_index(self, index):
        for child in self.grid.children:
            if child.index == index:
                return child

    def game_over(self, q=False, win=False):
        if q == True:
            sys.exit()

        if win == True:
            result = "Win"
        elif win == False:
            result = "lost"

        view = TryAgain(
            size_hint = (None, None),
            width = self.width/2,
            height = self.height/2,
            center = self.center,
            text = "You {}! Try Again?".format(result))
        view.open()

class TryAgain(ModalView):

    text = StringProperty('')

    def quit(self):
        sys.exit()

class Menu(Screen):
    pass


class Manager(ScreenManager):

    menu = ObjectProperty(None)
    game = ObjectProperty(None)


class MineSweeperApp(App):

    def build(self):
        return Manager()



if __name__ == "__main__":

    MineSweeperApp().run()

Answer 1

在原始Widget 完成实例化之前，不会应用kv 规则。在这种情况下，您的 Manager 小部件是初始小部件 - 它反过来创建其他小部件，包括 RootWidget。这意味着在您的RootWidget.__init__ 中，ids 尚未填充！它们将在 Manager 完成实例化后立即出现 - 所以最好的方法是延迟其余的初始化，如下所示：

class RootWidget(Screen):
    def __init__(self, **kwargs):
        super(RootWidget, self).__init__(**kwargs)
        Clock.schedule_once(self._finish_init)

    def _finish_init(self, dt):
        self.grid = self.ids.grid
        # etc

Answer 2

Since Kivy 1.11.0，你可以使用on_kv_post事件：

在与小部件关联的所有 kv 规则以及任何这些规则中的所有其他小部件都应用了所有 kv 规则后触发。 base_widget 是最基础的小部件，其实例化触发了 kv 规则（即从 Python 实例化的小部件，例如 MyWidget()）。

class RootWidget(Screen):
    def on_kv_post(self, base_widget):
        # self.ids will be populated here