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.. currentmodule:: echo
Quick Start
-----------
There are two syntaxes for creating callback properties. The first option is to
create a CallbackProperty instance at the class level::
class Foo(object):
bar = CallbackProperty(5) # initial value is 5
baz = CallbackProperty() # intial value is None
These behave like normal instance attributes. Altnernatively, you can use the
decorator syntax (all lowercase and with an underscore) to implement custom
getting and setting logic::
class Foo(object):
@callback_property
def bar(self):
... getter function ....
@bar.setter
def bar(self, value):
... setter function ...
To attach a function to a callback property, use the :func:`add_callback`
function::
def callback(new_value):
print('new value is %g' % new_value)
f = Foo()
add_callback(f, 'bar', callback)
# The following will print 'new value is 10'
f.bar = 10
.. note:: When calling :func:`add_callback`, the callback property is
referred to by name, as a string.
You can also write callbacks that receive both the old and new value of the
property, by setting ``echo_old=True`` in :func:`add_callback`::
def callback(old, new):
print('changed from %s to %s' % (old, new))
f = Foo()
add_callback(f, 'bar', callback, echo_old=True)
# The following will print 'changed from 5 to 10'
f.bar = 10
Delaying, ignoring, and removing callbacks
------------------------------------------
Callback functions can be removed with :func:`remove_callback`::
remove_callback(f, 'bar', callback)
The :func:`ignore_callback` and :func:`delay_callback` context managers
temporarily disable callbacks from being called::
with delay_callback(f, 'bar'):
f.bar = 10
f.bar = 20
f.bar = 30
Inside the context manager, none of the callbacks for ``f.bar`` will be called
-- this is useful in situations where you might be making temporary,
incremental changes to a callback property. The difference between
:func:`delay_callback` and :func:`ignore_callback` is whether any
callbacks will be invoked at the end of the context block. Callbacks are
never triggered inside :func:`ignore_callback`, where as they are triggered
a single time inside :func:`delay_callback` if the final state has changed.
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