# 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 `BaseComponent` or `Component` or by using the `handler()` decorator.

     1#!/usr/bin/env python
     2
     3from circuits import BaseComponent, Debugger, handler
     4
     5
     6class MyComponent(BaseComponent):
     7    def __init__(self):
     8        super().__init__()
     9
    10        Debugger().register(self)
    11
    12    @handler('started', channel='*')
    13    def system_started(self, component):
    14        print('Start event detected')
    15
    16
    17MyComponent().run()

`Download 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 `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 `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 `handler()`.

## Implicit Event Handlers

To make things easier for the developer when creating many event handlers and thus save on some typing, the `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 `handler()` decorator.

Example:

    #!/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()`.