Why ‘on_<propname>’ in kivy is not called? - properties

Question

in the kivy document (1.9.0-dev), it says
Observe using ‘on_<propname>’ If you created the class yourself, you
can use the ‘on_<propname>’ callback:
class MyClass(EventDispatcher):
a = NumericProperty(1)
def on_a(self, instance, value):
print(’My property a changed to’, value)
My code is
class MyClass(EventDispatcher):
a = StringProperty('')
def __init__(self, **kwargs):
...
self.bind(a=self.on_a) <--- if I remove this
def on_a(self, instance, value):
print(’My property a changed to’, value)
This works. But if I remove self.bind(a=self.on_a)
Then on_a function is not called. I thought if I put on_ as a function name then I do not need to do bind(). Do I miss something?
=================================================
Ps. I simplified my code below. It is a full run-able code.
course_view.py:
from kivy.app import App
from kivy.properties import StringProperty
from kivy.event import EventDispatcher
from kivy.uix.listview import ListView, ListItemButton
from kivy.adapters.dictadapter import DictAdapter
from kivy.uix.boxlayout import BoxLayout
from kivy.factory import Factory
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition
from kivy.properties import ObjectProperty
from kivy.properties import StringProperty
class ChangeTest(App):
pass
class StartScreen(Screen):
pass
def load_view(self):
self.course_view_object = CourseCnDetailListView(view_box = self.ids.view_box)
self.clear_widgets()
self.add_widget(self.course_view_object.master_item_list)
class CourseCnDetailListView(EventDispatcher):
course_code = StringProperty('course_code_str')
def __init__(self, **kwargs):
self.view_box = kwargs.get('view_box', None)
self.course_data= {"1": {"course_code": "it123"},
"2": {"course_code": "it456"}
}
list_item_args_converter = \
lambda row_index, rec: {'text': rec["course_code"],
'size_hint_y': None,
'height': 25}
dict_adapter = DictAdapter(sorted_keys=sorted(self.course_data.keys()),
data=self.course_data,
args_converter=list_item_args_converter,
selection_mode='single',
allow_empty_selection=False,
cls=ListItemButton)
self.master_item_list = ListView(adapter=dict_adapter,
size_hint=(.3, 1.0))
dict_adapter.bind(on_selection_change=self.course_changed)
#self.bind(course_code=self.on_course_code) <-- un-comment this will work
def on_course_code(self, instance, value):
print "on_course_code: update string value:", value
def redraw(self, *args):
pass
def course_changed(self, list_adapter, *args):
if len(list_adapter.selection) != 0:
selection = list_adapter.selection[0]
if type(selection) is str:
self.course_code = selection
else:
self.course_code = selection.text
self.redraw()
ChangeTest().run()
Factory.register('StartScreen', cls=StartScreen)
ChangeTest.kv
#: kivy 1.9
#: import ScreenManager kivy.uix.screenmanager.ScreenManager
#: import Screen kivy.uix.screenmanager.ScreenManager
ScreenManager:
id: screen_manager
StartScreen:
id: start_screen
name: 'StartScreen'
manager: screen_manager
<StartScreen>:
BoxLayout:
id: view_box
Button:
text: "load view"
on_release: root.load_view()

Answer 1

Thank you zeeMonkeez.
Yes. That is exactly the problem. After I add the constructor super(CourseCnDetailListView, self).__init__(**kwargs). It works perfectly.
It was accidentally removed when I changed the structure. Thank you very much.
Also it is good to know that it is the default constructor of EventDispatcher make <on_propname> work.