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|
Values
======
.. module:: circuits.core.values
The :mod:`~circuits.core` :class:`~Value`
class is an internal part of circuits'
`Futures and Promises <http://en.wikipedia.org/wiki/Futures_and_promises>`_
used to fulfill promises of the return value of
an event handler and any associated chains of
events and event handlers.
Basically when you fire an event ``foo()``
such as:
.. code-block:: python
x = self.fire(foo())
``x`` here is an instance of the
:class:`~Value` class which will
contain the value returned by the
event handler for ``foo`` in
the ``.value`` property.
.. note:: There is also :meth:`~Value.getValue`
which can be used to also retrieve the
underlying value held in the instance
of the :class:`~Value` class but you
should not need to use this as the
``.value`` property takes care of this
for you.
The only other API you may need in your application
is the :py:attr:`~Value.notify` which can be used
to trigger a ``value_changed`` event when the
underlying :class:`~Value` of the event handler has
changed. In this way you can do something asynchronously
with the event handler's return value no matter when
it finishes.
Example Code:
.. code-block:: python
:linenos:
#!/usr/bin/python -i
from circuits import handler, Event, Component, Debugger
class hello(Event):
"hello Event"
class test(Event):
"test Event"
class App(Component):
def hello(self):
return "Hello World!"
def test(self):
return self.fire(hello())
@handler("hello_value_changed")
def _on_hello_value_changed(self, value):
print("hello's return value was: {}".format(value))
app = App()
Debugger().register(app)
Example Session:
.. code-block:: python
:linenos:
$ python -i ../app.py
>>> x = app.fire(test())
>>> x.notify = True
>>> app.tick()
<registered[*] (<Debugger/* 27798:MainThread (queued=0) [S]>, <App/* 27798:MainThread (queued=1) [S]> )>
<test[*] ( )>
>>> app.tick()
<hello[*] ( )>
>>> app.tick()
<test_value_changed[<App/* 27798:MainThread (queued=0) [S]>] (<Value ('Hello World!') result: True errors: False for <test[*] ( )> )>
>>> app.tick()
>>> x
<Value ('Hello World!') result: True errors: False for <test[*] ( )>
>>> x.value
'Hello World!'
>>>
The :py:attr:`Value.notify` attribute can also be
set to the name of an event which should be used
to fire the ``value_changed`` event to.
If the form ``x.notify = True`` used then the event that
gets fired is a concatenation of the original event
and the ``value_changed`` event. e.g: ``foo_value_changed``.
.. note:: This is a bit advanced and should only be used
by experienced users of the circuits framework.
If you simply want basic synchronization of
event handlers it's recommended that you try
the :meth:`circuits.Component.call`
and :meth:`circuits.Component.wait`
synchronization primitives first.
|
