Move content from the todo tutorial to the extdev/index

2 files changed, 119 insertions, 110 deletions

diff --git a/doc/development/tutorials/todo.rst b/doc/development/tutorials/todo.rst
index ec5081dc5..e68a39342 100644
--- a/doc/development/tutorials/todo.rst
+++ b/doc/development/tutorials/todo.rst
@@ -1,5 +1,3 @@
-.. _exttuto-todo:
-
 Developing a "TODO" extension
 =============================
 
@@ -12,112 +10,12 @@ include todo entries in the documentation, and to collect these in a central
 place.  (A similar "todo" extension is distributed with Sphinx.)
 
 
-Important objects
------------------
-
-There are several key objects whose API you will use while writing an
-extension.  These are:
-
-**Application**
-   The application object (usually called ``app``) is an instance of
-   :class:`.Sphinx`.  It controls most high-level functionality, such as the
-   setup of extensions, event dispatching and producing output (logging).
-
-   If you have the environment object, the application is available as
-   ``env.app``.
-
-**Environment**
-   The build environment object (usually called ``env``) is an instance of
-   :class:`.BuildEnvironment`.  It is responsible for parsing the source
-   documents, stores all metadata about the document collection and is
-   serialized to disk after each build.
-
-   Its API provides methods to do with access to metadata, resolving references,
-   etc.  It can also be used by extensions to cache information that should
-   persist for incremental rebuilds.
-
-   If you have the application or builder object, the environment is available
-   as ``app.env`` or ``builder.env``.
-
-**Builder**
-   The builder object (usually called ``builder``) is an instance of a specific
-   subclass of :class:`.Builder`.  Each builder class knows how to convert the
-   parsed documents into an output format, or otherwise process them (e.g. check
-   external links).
-
-   If you have the application object, the builder is available as
-   ``app.builder``.
-
-**Config**
-   The config object (usually called ``config``) provides the values of
-   configuration values set in :file:`conf.py` as attributes.  It is an instance
-   of :class:`.Config`.
-
-   The config is available as ``app.config`` or ``env.config``.
-
-
-Build Phases
-------------
-
-One thing that is vital in order to understand extension mechanisms is the way
-in which a Sphinx project is built: this works in several phases.
-
-**Phase 0: Initialization**
-
-   In this phase, almost nothing of interest to us happens.  The source
-   directory is searched for source files, and extensions are initialized.
-   Should a stored build environment exist, it is loaded, otherwise a new one is
-   created.
-
-**Phase 1: Reading**
-
-   In Phase 1, all source files (and on subsequent builds, those that are new or
-   changed) are read and parsed.  This is the phase where directives and roles
-   are encountered by docutils, and the corresponding code is executed.  The
-   output of this phase is a *doctree* for each source file; that is a tree of
-   docutils nodes.  For document elements that aren't fully known until all
-   existing files are read, temporary nodes are created.
-
-   There are nodes provided by docutils, which are documented `in the docutils
-   documentation <http://docutils.sourceforge.net/docs/ref/doctree.html>`__.
-   Additional nodes are provided by Sphinx and :ref:`documented here <nodes>`.
-
-   During reading, the build environment is updated with all meta- and cross
-   reference data of the read documents, such as labels, the names of headings,
-   described Python objects and index entries.  This will later be used to
-   replace the temporary nodes.
-
-   The parsed doctrees are stored on the disk, because it is not possible to
-   hold all of them in memory.
-
-**Phase 2: Consistency checks**
-
-   Some checking is done to ensure no surprises in the built documents.
-
-**Phase 3: Resolving**
-
-   Now that the metadata and cross-reference data of all existing documents is
-   known, all temporary nodes are replaced by nodes that can be converted into
-   output using components called tranform.  For example, links are created for
-   object references that exist, and simple literal nodes are created for those
-   that don't.
-
-**Phase 4: Writing**
-
-   This phase converts the resolved doctrees to the desired output format, such
-   as HTML or LaTeX.  This happens via a so-called docutils writer that visits
-   the individual nodes of each doctree and produces some output in the process.
-
-.. note::
-
-   Some builders deviate from this general build plan, for example, the builder
-   that checks external links does not need anything more than the parsed
-   doctrees and therefore does not have phases 2--4.
-
-
 Extension Design
 ----------------
 
+.. note:: To understand the design this extension, refer to
+   :ref:`important-objects` and :ref:`build-phases`.
+
 We want the extension to add the following to Sphinx:
 
 * A "todo" directive, containing some content that is marked with "TODO", and
@@ -174,12 +72,13 @@ the individual calls do is the following:
 
   If the third argument was ``'html'``, HTML documents would be full rebuild if the
   config value changed its value.  This is needed for config values that
-  influence reading (build phase 1).
+  influence reading (build :ref:`phase 1 <build-phases>`).
 
 * :meth:`~Sphinx.add_node` adds a new *node class* to the build system.  It also
   can specify visitor functions for each supported output format.  These visitor
-  functions are needed when the new nodes stay until phase 4 -- since the
-  ``todolist`` node is always replaced in phase 3, it doesn't need any.
+  functions are needed when the new nodes stay until :ref:`phase 4 <build-phases>`
+  -- since the ``todolist`` node is always replaced in :ref:`phase 3 <build-phases>`,
+  it doesn't need any.
 
   We need to create the two node classes ``todo`` and ``todolist`` later.
 
@@ -276,7 +175,7 @@ The ``todo`` directive function looks like this::
            return [targetnode, todo_node]
 
 Several important things are covered here. First, as you can see, you can refer
-to the build environment instance using ``self.state.document.settings.env``.
+to the :ref:`build environment instance <important-objects>` using ``self.state.document.settings.env``.
 
 Then, to act as a link target (from the todolist), the todo directive needs to
 return a target node in addition to the todo node.  The target ID (in HTML, this
@@ -340,7 +239,8 @@ Here we clear out all todos whose docname matches the given one from the
 added again during parsing.
 
 The other handler belongs to the :event:`doctree-resolved` event.  This event is
-emitted at the end of phase 3 and allows custom resolving to be done::
+emitted at the end of :ref:`phase 3 <build-phases>` and allows custom resolving
+to be done::
 
    def process_todo_nodes(app, doctree, fromdocname):
        if not app.config.todo_include_todos:
diff --git a/doc/extdev/index.rst b/doc/extdev/index.rst
index 809ea1220..5e0292b2a 100644
--- a/doc/extdev/index.rst
+++ b/doc/extdev/index.rst
@@ -52,6 +52,115 @@ Note that it is still necessary to register the builder using
 
 .. _entry points: https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins
 
+.. _important-objects:
+
+Important objects
+-----------------
+
+There are several key objects whose API you will use while writing an
+extension. These are:
+
+**Application**
+   The application object (usually called ``app``) is an instance of
+   :class:`.Sphinx`.  It controls most high-level functionality, such as the
+   setup of extensions, event dispatching and producing output (logging).
+
+   If you have the environment object, the application is available as
+   ``env.app``.
+
+**Environment**
+   The build environment object (usually called ``env``) is an instance of
+   :class:`.BuildEnvironment`.  It is responsible for parsing the source
+   documents, stores all metadata about the document collection and is
+   serialized to disk after each build.
+
+   Its API provides methods to do with access to metadata, resolving references,
+   etc.  It can also be used by extensions to cache information that should
+   persist for incremental rebuilds.
+
+   If you have the application or builder object, the environment is available
+   as ``app.env`` or ``builder.env``.
+
+**Builder**
+   The builder object (usually called ``builder``) is an instance of a specific
+   subclass of :class:`.Builder`.  Each builder class knows how to convert the
+   parsed documents into an output format, or otherwise process them (e.g. check
+   external links).
+
+   If you have the application object, the builder is available as
+   ``app.builder``.
+
+**Config**
+   The config object (usually called ``config``) provides the values of
+   configuration values set in :file:`conf.py` as attributes.  It is an instance
+   of :class:`.Config`.
+
+   The config is available as ``app.config`` or ``env.config``.
+
+To see an example of use of these objects, refer to :doc:`../development/tutorials/index`.
+
+.. _build-phases:
+
+Build Phases
+------------
+
+One thing that is vital in order to understand extension mechanisms is the way
+in which a Sphinx project is built: this works in several phases.
+
+**Phase 0: Initialization**
+
+   In this phase, almost nothing of interest to us happens.  The source
+   directory is searched for source files, and extensions are initialized.
+   Should a stored build environment exist, it is loaded, otherwise a new one is
+   created.
+
+**Phase 1: Reading**
+
+   In Phase 1, all source files (and on subsequent builds, those that are new or
+   changed) are read and parsed.  This is the phase where directives and roles
+   are encountered by docutils, and the corresponding code is executed.  The
+   output of this phase is a *doctree* for each source file; that is a tree of
+   docutils nodes.  For document elements that aren't fully known until all
+   existing files are read, temporary nodes are created.
+
+   There are nodes provided by docutils, which are documented `in the docutils
+   documentation <http://docutils.sourceforge.net/docs/ref/doctree.html>`__.
+   Additional nodes are provided by Sphinx and :ref:`documented here <nodes>`.
+
+   During reading, the build environment is updated with all meta- and cross
+   reference data of the read documents, such as labels, the names of headings,
+   described Python objects and index entries.  This will later be used to
+   replace the temporary nodes.
+
+   The parsed doctrees are stored on the disk, because it is not possible to
+   hold all of them in memory.
+
+**Phase 2: Consistency checks**
+
+   Some checking is done to ensure no surprises in the built documents.
+
+**Phase 3: Resolving**
+
+   Now that the metadata and cross-reference data of all existing documents is
+   known, all temporary nodes are replaced by nodes that can be converted into
+   output using components called tranform.  For example, links are created for
+   object references that exist, and simple literal nodes are created for those
+   that don't.
+
+**Phase 4: Writing**
+
+   This phase converts the resolved doctrees to the desired output format, such
+   as HTML or LaTeX.  This happens via a so-called docutils writer that visits
+   the individual nodes of each doctree and produces some output in the process.
+
+.. note::
+
+   Some builders deviate from this general build plan, for example, the builder
+   that checks external links does not need anything more than the parsed
+   doctrees and therefore does not have phases 2--4.
+
+To see an example of application, refer to :doc:`../development/tutorials/todo`.
+
 .. _ext-metadata:
 
 Extension metadata