Handlers
========


Explicit Event Handlers
-----------------------


Event Handlers are methods of components that are invoked when a matching
event is dispatched. These can be declared explicitly on a
:class:`~circuits.core.components.BaseComponent` or
:class:`~circuits.core.components.Component` or by using the
:func:`~circuits.core.handlers.handler` decorator.

.. literalinclude:: examples/handler_annotation.py
   :language: python
   :linenos:

:download:`Download handler_annotation.py <examples/handler_annotation.py>`

The handler decorator on line 14 turned the method ``system_started`` into an
event handler for the event ``started``.

When defining explicit event handlers in this way, it's convention to
use the following pattern::

    @handler("foo")
    def print_foobar(self, ...):
        print("FooBar!")

This makes reading code clear and concise and obvious to the reader
that the method is not part of the class's public API
(*leading underscore as per Python convention*) and that it is invoked
for events of type ``SomeEvent``.

The optional keyword argument "``channel``" can be used to attach the
handler to a different channel than the component's channel
(*as specified by the component's channel attribute*).

Handler methods must be declared with arguments and keyword arguments that
match the arguments passed to the event upon its creation. Looking at the
API for :class:`~circuits.core.events.started` you'll find that the
component that has been started is passed as an argument to its constructor.
Therefore, our handler method must declare one argument (*Line 14*).

The :func:`~circuits.core.handlers.handler` decorator accepts other keyword arguments that
influence the behavior of the event handler and its invocation. Details can
be found in the API description of :func:`~circuits.core.handlers.handler`.


Implicit Event Handlers
-----------------------


To make things easier for the developer when creating many event handlers
and thus save on some typing, the :class:`~circuits.core.components.Component`
can be used and subclassed instead which provides an implicit mechanism for
creating event handlers.

Basically every method in the component is automatically and implicitly
marked as an event handler with ``@handler(<name>)`` where ``<name>`` is
the name of each method applied.

The only exceptions are:

- Methods that start with an underscore ``_``.
- Methods already marked explicitly with the :func:`~circuits.core.handlers.handler` decorator.

Example:

.. code:: python

    #!/usr/bin/env python


    from circuits import handler, Component, Event


    class hello(Event):
        """hello Event"""


    class App(Component):

        def _say(self, message):
            """Print the given message

            This is a private method as denoted via the prefixed underscore.
            This will not be turned into an event handler.
            """

            print(message)

        def started(self, manager):
            self._say("App Started!")
            self.fire(hello())
            raise SystemExit(0)

        @handler("hello")
        def print_hello(self):
            """hello Event Handlers

            Print "Hello World!" when the ``hello`` Event is received.

            As this is already decorated with the ``@handler``
            decorator, it will be left as it is and won't get
            touched by the implicit event handler creation
            mechanisms.
            """

            print("Hello World!")

        @handler(False)
        def test(self, *args, **kwargs):
            """A simple test method that does nothing

            This will not be turned into an event handlers
            because of the ``False`` argument passed to the
            ``@handler`` decorator. This only makes sense
            when subclassing ``Component`` and you want to
            have fine grained control over what methods
            are not turned into event handlers.
            """

            pass

    App().run()


.. note::
   You can specify that a method will not be marked as an event handler by passing ``False`` as the
   first argument to ``@handler()``.
