..
   This file is part of Logtalk <https://logtalk.org/>  
   Copyright 1998-2019 Paulo Moura <pmoura@logtalk.org>

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
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.. index:: pair: define_events/5; Built-in predicate
.. _predicates_define_events_5:

define_events/5
===============

Description
-----------

::

   define_events(Event, Object, Message, Sender, Monitor)

Defines a new set of events. The two types of events are represented by
the atoms ``before`` and ``after``. When the predicate is called with
the first argument unbound, both types of events are defined. The object
``Monitor`` must define the event handler methods required by the
``Event`` argument.

Modes and number of proofs
--------------------------

::

   define_events(@term, @term, @term, @term, +object_identifier) - one

Errors
------

| Event is neither a variable nor a valid event identifier:
|     ``type_error(event, Event)``
| Object is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Object)``
| Message is neither a variable nor a callable term:
|     ``type_error(callable, Message)``
| Sender is neither a variable nor a valid object identifier:
|     ``type_error(object_identifier, Sender)``
| Monitor is a variable:
|     ``instantiation_error``
| Monitor is neither a variable nor a valid object identifier:
|     ``existence_error(object_identifier, Monitor)``
| Monitor does not define the required ``before/3`` method:
|     ``existence_error(procedure, before/3)``
| Monitor does not define the required ``after/3`` method:
|     ``existence_error(procedure, after/3)``

Examples
--------

::

   % define "debugger" as a monitor for member/2 messages
   % sent to the "list" object:
   | ?- define_events(_, list, member(_, _), _ , debugger).

.. seealso::

   :ref:`predicates_abolish_events_5`,
   :ref:`predicates_current_event_5`,
   :ref:`methods_before_3`,
   :ref:`methods_after_3`
