如果小部件在 kv 文件中使用，kivy 崩溃，如果在 Python 文件中使用则成功

Question

我遇到了以下问题：

示例（mainok.py、testok.kv）启动成功，

$ cat mainok.py
from kivy.app import App
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout
class BuddyList(BoxLayout):
    list_view = ObjectProperty()
    def __init__(self, **kwargs):
        super(BuddyList, self).__init__(**kwargs)
        assert(self.list_view is not None)
        self.list_view.adapter.data = ['v1', 'v2']
    def roster_converter(self, index, txt):
        result = {
            "text": "%s %d" % (txt, index),
            'size_hint_y': None,
            'height': 25
        }
        return result
class TestForm(BoxLayout):
    text_input = ObjectProperty()

    def __init__(self, **kwargs):
        super(TestForm, self).__init__(**kwargs)
        self.buddy_list = BuddyList()
        self.add_widget(self.buddy_list)
class TestokApp(App):
    pass
if __name__ == '__main__':
    TestokApp().run()

$ cat testok.kv
#:import la kivy.adapters.listadapter
#:import ku kivy.uix.listview
TestForm:
<BuddyList>:
   list_view: list_view
    ListView:
        id: list_view
        adapter:
            la.ListAdapter(data=[], cls=ku.ListItemButton,
            args_converter=root.roster_converter)
<TestForm>:
    orientation: 'vertical'
    BoxLayout:
        Label:
            text: 'the label'

示例（mainko.py、testko.kv）崩溃：

$ cat mainko.py
from kivy.app import App
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout
class BuddyList(BoxLayout):
    list_view = ObjectProperty()
    def __init__(self, **kwargs):
        super(BuddyList, self).__init__(**kwargs)
        assert(self.list_view is not None)
        self.list_view.adapter.data = ['v1', 'v2']
    def roster_converter(self, index, txt):
        result = {
            "text": "%s %d" % (txt, index),
            'size_hint_y': None,
            'height': 25
        }
        return result
class TestForm(BoxLayout):
    text_input = ObjectProperty()
    def __init__(self, **kwargs):
        super(TestForm, self).__init__(**kwargs)
class TestkoApp(App):
    pass
if __name__ == '__main__':
    TestkoApp().run()

$ cat testko.kv
#:import la kivy.adapters.listadapter
#:import ku kivy.uix.listview
TestForm:
<BuddyList>:
    list_view: list_view
    ListView:
        id: list_view
        adapter:
            la.ListAdapter(data=[], cls=ku.ListItemButton,
            args_converter=root.roster_converter)
<TestForm>:
    orientation: 'vertical'
    BoxLayout:
        Label:
            text: 'the label'
    BuddyList:

崩溃并出现以下错误

  assert(self.list_view is not None)
     AssertionError

两者的区别是：

$ diff -u mainko.py ../ok/mainok.py
--- mainko.py       2013-10-23 14:11:26.976723764 +0200
+++ ../ok/mainok.py 2013-10-23 14:12:34.976841090 +0200
@@ -26,10 +26,13 @@

 def __init__(self, **kwargs):
     super(TestForm, self).__init__(**kwargs)
+        self.buddy_list = BuddyList()
+        self.add_widget(self.buddy_list)

-class TestkoApp(App):
+
+class TestokApp(App):
     pass

 if __name__ == '__main__':
-    TestkoApp().run()
+    TestokApp().run()

$ diff -u testko.kv ../ok/testok.kv
--- testko.kv       2013-10-23 14:10:11.948299722 +0200
+++ ../ok/testok.kv 2013-10-23 14:16:51.352688453 +0200
@@ -16,5 +16,4 @@
 BoxLayout:
     Label:
         text: 'the label'
-    BuddyList:

知道为什么第一个成功，第二个失败吗？

谢谢，

Answer 1

看起来问题是__init__ 方法实际上并没有完全解析和设置 kv 指令，尽管我不确定细节 - 我猜它在事件循环中安排了一些东西。当您尝试检查 self.list_view 时，您会得到 None，因为尚未设置。

如果您使用时钟，您可以安排在到目前为止安排的所有其他事情之后（以及在完全执行 kv 指令之后）但在下一帧之前执行您的 post_init 内容。这是对你的 mainko.py 做这样的事情的修改。它似乎对我有用。

from kivy.app import App 
from kivy.properties import ObjectProperty 
from kivy.uix.boxlayout import BoxLayout 
from kivy.clock import Clock 

class BuddyList(BoxLayout): 
    list_view = ObjectProperty() 

    def __init__(self, **kwargs): 
        super(BuddyList, self).__init__(**kwargs) 
        Clock.schedule_once(self.post_init, 0) 

    def post_init(self, *args): 
        assert self.list_view is not None 
        self.list_view.adapter.data = ['v1', 'v2'] 

    def roster_converter(self, index, txt): 
        result = { 
            "text": "%s %d" % (txt, index), 
            'size_hint_y': None, 
            'height': 25 
        } 
        return result 

class TestForm(BoxLayout): 
    text_input = ObjectProperty() 
    def __init__(self, **kwargs): 
        super(TestForm, self).__init__(**kwargs) 


class TestkoApp(App): 
    pass 


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