使用KivyMD MDChip组件报KeyError: 'pos_hint'错误如何解决

使用KivyMD的MDChip组件开发时，运行代码抛出KeyError: 'pos_hint'错误，测试代码如下：

from kivy.lang import Builder
from kivymd.app import MDApp

class MyApp(MDApp):

    def build(self):

        return Builder.load_string(KV)

KV = '''

MDBoxLayout:
    pos_hint: {'center_y': 1, 'center_x':1}  
    
    MDChip:
        text: 'Google'
        icon: 'Google'
        check: True
'''

MyApp().run()

完整错误追踪栈：

Traceback (most recent call last):
   File "c:\Users\Kelvin Loh\Documents\kivyMD\chip_1.py", line 18, in <module>
     MyApp().run()
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\app.py", line 949, in run
     self._run_prepare()
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\app.py", line 919, in _run_prepare
     root = self.build()
   File "c:\Users\Kelvin Loh\Documents\kivyMD\chip_1.py", line 6, in build
     return Builder.load_string(KV)
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\lang\builder.py", line 408, in load_string
     self._apply_rule(
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\lang\builder.py", line 661, in _apply_rule
     child.apply_class_lang_rules(
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\uix\widget.py", line 463, in apply_class_lang_rules
     Builder.apply(
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\lang\builder.py", line 541, in apply
     self._apply_rule(
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\lang\builder.py", line 663, in _apply_rule
     self._apply_rule(
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\lang\builder.py", line 660, in _apply_rule
     widget.add_widget(child)
   File "C:\Users\Kelvin Loh\Documents\kivyMD\kivy_venv\lib\site-packages\kivy\uix\floatlayout.py", line 135, in add_widget
     widget.bind(
   File "kivy\_event.pyx", line 429, in kivy._event.EventDispatcher.bind
 KeyError: 'pos_hint'

• 根布局直接使用MDBoxLayout时，MDChip在开启check: True属性后会动态插入勾选标记组件，插入过程中会触发FloatLayout类布局的pos_hint绑定校验逻辑，MDBoxLayout不属于FloatLayout系布局，无法完成属性绑定直接抛错。
• 代码中传入的icon值'Google'不符合KivyMD图标命名规范，Material图标名均为全小写格式，该问题不会触发本次的KeyError，但会导致图标加载失败。

将根布局替换为FloatLayout系的MDScreen（KivyMD推荐作为页面根容器），同时修正图标名，给MDChip显式添加必要的布局属性即可，修复后可运行代码如下：

from kivy.lang import Builder
from kivymd.app import MDApp

class MyApp(MDApp):
    def build(self):
        return Builder.load_string(KV)

KV = '''
MDScreen:
    MDBoxLayout:
        pos_hint: {'center_y': 1, 'center_x': 1}
        adaptive_size: True
        spacing: "12dp"
        padding: "12dp"

        MDChip:
            text: 'Google'
            icon: 'google'
            check: True
            pos_hint: {"center_x": .5, "center_y": .5}
'''

MyApp().run()

如果需要保留MDBoxLayout作为最外层容器，只需要在MDBoxLayout外层包裹一层MDScreen即可，不需要改动原有MDBoxLayout的内部布局逻辑。

内容的提问来源于stack exchange，提问作者Loh Boon How