从Kivy Python中的类属性出价

Question

I have multiple classes happily communicating thanks to x being an object of class foo:

class foo(EventDispatcher):
   def variable_input(self, *args):
      self.a='This is an observed value being changed'

   def __init__(self):
      self.a=StringProperty('')

now I create an object X being an instance of class foo

X=foo()

I have a Screen as a part of a ScreenManager containing a Label with id vystup

class Hlavne(Screen):
   pass

What I am trying to do is to write function on_propname with Xa as a propname. How do I point to value belonging to instance of other class.

I have read:

Warning

Be careful with 'on_'. If you are creating such a callback on a property you are inheriting, you must not forget to call the superclass function too.

But as its common with kivy's documentation it does not tell you how.

Answer 1

Kivy Properties are declared at class level, not in an instance. (bad)

def __init__(self):
   self.a=StringProperty('')

Binding to a property and using an on_property event are two different things.

from kivy.clock import Clock
from kivy.base import runTouchApp
from kivy.uix.widget import Widget
from kivy.properties import NumericProperty


class Test(Widget):
    hop = NumericProperty(0)

    def __init__(self, **kwargs):
        super(Test, self).__init__(**kwargs)
        Clock.schedule_interval(
            lambda *dt: setattr(self, 'hop', self.hop + 1), 0.1
        )

        # bind to property
        self.bind(hop=self.bind_hop)

    def on_hop(self, instance, value):
        # no event, no super() necessary
        # unless inherrited from "Test"
        print('on_hop:', instance, value)

    def bind_hop(self, instance, value):
        print('bind_hop:', instance, value)

runTouchApp(Test())

You can check for an instance in each of the function with isinstance because of the instance argument passed into each event/bind of a Kivy property mentioned here .