..
   This file is part of Logtalk <https://logtalk.org/>  
   Copyright 1998-2020 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.
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       http://www.apache.org/licenses/LICENSE-2.0

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.. index:: single: reflection
.. _reflection_reflection:

Reflection
==========

Logtalk provides support for both *structural* and *behavioral* reflection.
Structural reflection supports computations over an application structure
while behavioral reflection computations over what an application does while
running. The structural and behavioral reflection APIs are used by all the
:doc:`developer tools <../devtools/index>`, which are regular applications.

.. index:: single: structural reflection
.. _reflection_structural:

Structural reflection
---------------------

Structural reflection allows querying the properties of objects, categories,
protocols, and predicates. This API provides two views on the structure of
an application: a *transparent-box view* and a *black-box view*, described
next.

.. index:: single: transparent-box view

Transparent-box view
~~~~~~~~~~~~~~~~~~~~

The transparent-box view provides a structural view of the contents and
properties of entities, predicates, and source files akin to accessing
the corresponding source code.

For entities, built-in predicates are provided for
:ref:`enumerating entities <enumerating_entity_predicates>`,
:ref:`enumerating entity properties <enumerating_entity_property_predicates>`
(including entity declared, defined, called, and updated predicates),
and :ref:`enumerating entity relations <entity_relation_predicates>`.
For a detailed description of the supported entity properties, see the sections
on :ref:`object properties <objects_properties>`,
:ref:`protocol properties <protocols_properties>`, and
:ref:`category properties <categories_properties>`.
For examples of querying entity relations, see the sections
on :ref:`object relations <objects_relationships>`,
:ref:`protocol relations <protocols_relationships>`, and
:ref:`category relations <categories_relationships>`.

.. note::

   Some entity and predicate properties are only available when the source
   files are compiled with the :ref:`source_data <flag_source_data>` flag
   turned on.

The :ref:`logtalk <apis:logtalk/0>` built-in object provides predicates for
querying loaded source files and their properties. 

.. index:: single: black-box view

Black-box view
~~~~~~~~~~~~~~

The black-box view provides a view that respects entity encapsulation and
thus only allow querying about predicates and operators that are within
scope of the entity calling the reflection methods.

Built-in methods are provided for querying the :ref:`predicates that are
declared and can be called or used as messages <methods_current_predicate_1>`
and for querying the :ref:`predicate properties <methods_predicate_property_2>`.
It is also possible to enumerate :ref:`entity operators <methods_current_op_3>`.
See the sections on :ref:`finding declared predicates <predicates_finding>` and
on :ref:`predicate properties <predicates_properties>` for more details.

.. index:: single: behavioral reflection
.. _reflection_behavioral:

Behavioral reflection
---------------------

Behavioral reflection provides insight on what an application does when running.
Specifically, by observing and acting on the messages being exchanged between
objects. See the section on :ref:`event-driven programming <events_events>`
for details. There is also a :doc:`../libraries/dependents` library that
provides an implementation of Smalltalk dependents mechanism.

For use in debugging tools, there is also a small reflection API providing
:ref:`trace and debug event predicates <debugging_events>` provided by the
:ref:`logtalk <apis:logtalk/0>` built-in object.
