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+.. _create-plugin:
+
+Creating a Slixmpp Plugin
+===========================
+
+One of the goals of Slixmpp is to provide support for every draft or final
+XMPP extension (`XEP <http://xmpp.org/extensions/>`_). To do this, Slixmpp has a
+plugin mechanism for adding the functionalities required by each XEP. But even
+though plugins were made to quickly implement and prototype the official XMPP
+extensions, there is no reason you can't create your own plugin to implement
+your own custom XMPP-based protocol.
+
+This guide will help walk you through the steps to
+implement a rudimentary version of `XEP-0077 In-band
+Registration <http://xmpp.org/extensions/xep-0077.html>`_. In-band registration
+was implemented in example 14-6 (page 223) of `XMPP: The Definitive
+Guide <http://oreilly.com/catalog/9780596521271>`_ because there was no Slixmpp
+plugin for XEP-0077 at the time of writing. We will partially fix that issue
+here by turning the example implementation from *XMPP: The Definitive Guide*
+into a plugin. Again, note that this will not a complete implementation, and a
+different, more robust, official plugin for XEP-0077 may be added to Slixmpp
+in the future.
+
+.. note::
+
+    The example plugin created in this guide is for the server side of the
+    registration process only. It will **NOT** be able to register new accounts
+    on an XMPP server.
+
+First Steps
+-----------
+Every plugin inherits from the class :mod:`BasePlugin <slixmpp.plugins.base.BasePlugin`,
+and must include a ``plugin_init`` method. While the
+plugins distributed with Slixmpp must be placed in the plugins directory
+``slixmpp/plugins`` to be loaded, custom plugins may be loaded from any
+module. To do so, use the following form when registering the plugin:
+
+.. code-block:: python
+
+    self.register_plugin('myplugin', module=mod_containing_my_plugin)
+
+The plugin name must be the same as the plugin's class name.
+
+Now, we can open our favorite text editors and create ``xep_0077.py`` in
+``Slixmpp/slixmpp/plugins``. We want to do some basic house-keeping and
+declare the name and description of the XEP we are implementing. If you
+are creating your own custom plugin, you don't need to include the ``xep``
+attribute.
+
+.. code-block:: python
+
+    """
+    Creating a Slixmpp Plugin
+
+    This is a minimal implementation of XEP-0077 to serve
+    as a tutorial for creating Slixmpp plugins.
+    """
+
+    from slixmpp.plugins.base import BasePlugin
+
+    class xep_0077(BasePlugin):
+        """
+        XEP-0077 In-Band Registration
+        """
+
+        def plugin_init(self):
+            self.description = "In-Band Registration"
+            self.xep = "0077"
+
+Now that we have a basic plugin, we need to edit
+``slixmpp/plugins/__init__.py`` to include our new plugin by adding
+``'xep_0077'`` to the ``__all__`` declaration.
+
+Interacting with Other Plugins
+------------------------------
+
+In-band registration is a feature that should be advertised through `Service
+Discovery <http://xmpp.org/extensions/xep-0030.html>`_. To do that, we tell the
+``xep_0030`` plugin to add the ``"jabber:iq:register"`` feature. We put this
+call in a method named ``post_init`` which will be called once the plugin has
+been loaded; by doing so we advertise that we can do registrations only after we
+finish activating the plugin.
+
+The ``post_init`` method needs to call ``BasePlugin.post_init(self)``
+which will mark that ``post_init`` has been called for the plugin. Once the
+Slixmpp object begins processing, ``post_init`` will be called on any plugins
+that have not already run ``post_init``. This allows you to register plugins and
+their dependencies without needing to worry about the order in which you do so.
+
+**Note:** by adding this call we have introduced a dependency on the XEP-0030
+plugin. Be sure to register ``'xep_0030'`` as well as ``'xep_0077'``. Slixmpp
+does not automatically load plugin dependencies for you.
+
+.. code-block:: python
+
+    def post_init(self):
+        BasePlugin.post_init(self)
+        self.xmpp['xep_0030'].add_feature("jabber:iq:register")
+
+Creating Custom Stanza Objects
+------------------------------
+
+Now, the IQ stanzas needed to implement our version of XEP-0077 are not very
+complex, and we could just interact with the XML objects directly just like
+in the *XMPP: The Definitive Guide* example. However, creating custom stanza
+objects is good practice.
+
+We will create a new ``Registration`` stanza. Following the *XMPP: The
+Definitive Guide* example, we will add support for a username and password
+field. We also need two flags: ``registered`` and ``remove``. The ``registered``
+flag is sent when an already registered user attempts to register, along with
+their registration data. The ``remove`` flag is a request to unregister a user's
+account.
+
+Adding additional `fields specified in
+XEP-0077 <http://xmpp.org/extensions/xep-0077.html#registrar-formtypes-register>`_
+will not be difficult and is left as an exercise for the reader.
+
+Our ``Registration`` class needs to start with a few descriptions of its
+behaviour:
+
+* ``namespace``
+    The namespace our stanza object lives in. In this case,
+    ``"jabber:iq:register"``.
+
+* ``name``
+    The name of the root XML element. In this case, the ``query`` element.
+
+* ``plugin_attrib``
+    The name to access this type of stanza. In particular, given a
+    registration stanza, the ``Registration`` object can be found using:
+    ``iq_object['register']``.
+
+* ``interfaces``
+    A list of dictionary-like keys that can be used with the stanza object.
+    When using ``"key"``, if there exists a method of the form ``getKey``,
+    ``setKey``, or``delKey`` (depending on context) then the result of calling
+    that method will be returned. Otherwise, the value of the attribute ``key``
+    of the main stanza element is returned if one exists.
+
+    **Note:** The accessor methods currently use title case, and not camel case.
+    Thus if you need to access an item named ``"methodName"`` you will need to
+    use ``getMethodname``. This naming convention might change to full camel
+    case in a future version of Slixmpp.
+
+* ``sub_interfaces``
+    A subset of ``interfaces``, but these keys map to the text of any
+    subelements that are direct children of the main stanza element. Thus,
+    referencing ``iq_object['register']['username']`` will either execute
+    ``getUsername`` or return the value in the ``username`` element of the
+    query.
+
+    If you need to access an element, say ``elem``, that is not a direct child
+    of the main stanza element, you will need to add ``getElem``, ``setElem``,
+    and ``delElem``. See the note above about naming conventions.
+
+.. code-block:: python
+
+    from slixmpp.xmlstream import ElementBase, ET, JID, register_stanza_plugin
+    from slixmpp import Iq
+
+    class Registration(ElementBase):
+        namespace = 'jabber:iq:register'
+        name = 'query'
+        plugin_attrib = 'register'
+        interfaces = {'username', 'password', 'registered', 'remove'}
+        sub_interfaces = interfaces
+
+        def getRegistered(self):
+            present = self.xml.find('{%s}registered' % self.namespace)
+            return present is not None
+
+        def getRemove(self):
+            present = self.xml.find('{%s}remove' % self.namespace)
+            return present is not None
+
+        def setRegistered(self, registered):
+            if registered:
+                self.addField('registered')
+            else:
+                del self['registered']
+
+        def setRemove(self, remove):
+            if remove:
+                self.addField('remove')
+            else:
+                del self['remove']
+
+        def addField(self, name):
+            itemXML = ET.Element('{%s}%s' % (self.namespace, name))
+            self.xml.append(itemXML)
+
+Setting a ``sub_interface`` attribute to ``""`` will remove that subelement.
+Since we want to include empty registration fields in our form, we need the
+``addField`` method to add the empty elements.
+
+Since the ``registered`` and ``remove`` elements are just flags, we need to add
+custom logic to enforce the binary behavior.
+
+Extracting Stanzas from the XML Stream
+--------------------------------------
+
+Now that we have a custom stanza object, we need to be able to detect when we
+receive one. To do this, we register a stream handler that will pattern match
+stanzas off of the XML stream against our stanza object's element name and
+namespace. To do so, we need to create a ``Callback`` object which contains
+an XML fragment that can identify our stanza type. We can add this handler
+registration to our ``plugin_init`` method.
+
+Also, we need to associate our ``Registration`` class with IQ stanzas;
+that requires the use of the ``register_stanza_plugin`` function (in
+``slixmpp.xmlstream.stanzabase``) which takes the class of a parent stanza
+type followed by the substanza type. In our case, the parent stanza is an IQ
+stanza, and the substanza is our registration query.
+
+The ``__handleRegistration`` method referenced in the callback will be our
+handler function to process registration requests.
+
+.. code-block:: python
+
+    def plugin_init(self):
+        self.description = "In-Band Registration"
+        self.xep = "0077"
+
+        self.xmpp.register_handler(
+          Callback('In-Band Registration',
+            MatchXPath('{%s}iq/{jabber:iq:register}query' % self.xmpp.default_ns),
+            self.__handleRegistration))
+        register_stanza_plugin(Iq, Registration)
+
+Handling Incoming Stanzas and Triggering Events
+-----------------------------------------------
+There are six situations that we need to handle to finish our implementation of
+XEP-0077.
+
+**Registration Form Request from a New User:**
+
+    .. code-block:: xml
+
+        <iq type="result">
+         <query xmlns="jabber:iq:register">
+          <username />
+          <password />
+         </query>
+        </iq>
+
+**Registration Form Request from an Existing User:**
+
+    .. code-block:: xml
+
+        <iq type="result">
+         <query xmlns="jabber:iq:register">
+          <registered />
+          <username>Foo</username>
+          <password>hunter2</password>
+         </query>
+        </iq>
+
+**Unregister Account:**
+
+    .. code-block:: xml
+
+        <iq type="result">
+         <query xmlns="jabber:iq:register" />
+        </iq>
+
+**Incomplete Registration:**
+
+    .. code-block:: xml
+
+        <iq type="error">
+          <query xmlns="jabber:iq:register">
+            <username>Foo</username>
+          </query>
+         <error code="406" type="modify">
+          <not-acceptable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
+         </error>
+        </iq>
+
+**Conflicting Registrations:**
+
+    .. code-block:: xml
+
+        <iq type="error">
+         <query xmlns="jabber:iq:register">
+          <username>Foo</username>
+          <password>hunter2</password>
+         </query>
+         <error code="409" type="cancel">
+          <conflict xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
+         </error>
+        </iq>
+
+**Successful Registration:**
+
+    .. code-block:: xml
+
+        <iq type="result">
+         <query xmlns="jabber:iq:register" />
+        </iq>
+
+Cases 1 and 2: Registration Requests
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Responding to registration requests depends on if the requesting user already
+has an account. If there is an account, the response should include the
+``registered`` flag and the user's current registration information. Otherwise,
+we just send the fields for our registration form.
+
+We will handle both cases by creating a ``sendRegistrationForm`` method that
+will create either an empty of full form depending on if we provide it with
+user data. Since we need to know which form fields to include (especially if we
+add support for the other fields specified in XEP-0077), we will also create a
+method ``setForm`` which will take the names of the fields we wish to include.
+
+.. code-block:: python
+
+    def plugin_init(self):
+        self.description = "In-Band Registration"
+        self.xep = "0077"
+        self.form_fields = ('username', 'password')
+        ... remainder of plugin_init
+
+    ...
+
+    def __handleRegistration(self, iq):
+        if iq['type'] == 'get':
+            # Registration form requested
+            userData = self.backend[iq['from'].bare]
+            self.sendRegistrationForm(iq, userData)
+
+    def setForm(self, *fields):
+        self.form_fields = fields
+
+    def sendRegistrationForm(self, iq, userData=None):
+        reg = iq['register']
+        if userData is None:
+            userData = {}
+        else:
+            reg['registered'] = True
+
+        for field in self.form_fields:
+            data = userData.get(field, '')
+            if data:
+                # Add field with existing data
+                reg[field] = data
+            else:
+                # Add a blank field
+                reg.addField(field)
+
+        iq.reply().set_payload(reg.xml)
+        iq.send()
+
+Note how we are able to access our ``Registration`` stanza object with
+``iq['register']``.
+
+A User Backend
+++++++++++++++
+You might have noticed the reference to ``self.backend``, which is an object
+that abstracts away storing and retrieving user information. Since it is not
+much more than a dictionary, we will leave the implementation details to the
+final, full source code example.
+
+Case 3: Unregister an Account
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The next simplest case to consider is responding to a request to remove
+an account. If we receive a ``remove`` flag, we instruct the backend to
+remove the user's account. Since your application may need to know about
+when users are registered or unregistered, we trigger an event using
+``self.xmpp.event('unregister_user', iq)``. See the component examples below for
+how to respond to that event.
+
+.. code-block:: python
+
+     def __handleRegistration(self, iq):
+        if iq['type'] == 'get':
+            # Registration form requested
+            userData = self.backend[iq['from'].bare]
+            self.sendRegistrationForm(iq, userData)
+        elif iq['type'] == 'set':
+            # Remove an account
+            if iq['register']['remove']:
+                self.backend.unregister(iq['from'].bare)
+                self.xmpp.event('unregistered_user', iq)
+                iq.reply().send()
+                return
+
+Case 4: Incomplete Registration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For the next case we need to check the user's registration to ensure it has all
+of the fields we wanted. The simple option that we will use is to loop over the
+field names and check each one; however, this means that all fields we send to
+the user are required. Adding optional fields is left to the reader.
+
+Since we have received an incomplete form, we need to send an error message back
+to the user. We have to send a few different types of errors, so we will also
+create a ``_sendError`` method that will add the appropriate ``error`` element
+to the IQ reply.
+
+.. code-block:: python
+
+    def __handleRegistration(self, iq):
+        if iq['type'] == 'get':
+            # Registration form requested
+            userData = self.backend[iq['from'].bare]
+            self.sendRegistrationForm(iq, userData)
+        elif iq['type'] == 'set':
+            if iq['register']['remove']:
+                # Remove an account
+                self.backend.unregister(iq['from'].bare)
+                self.xmpp.event('unregistered_user', iq)
+                iq.reply().send()
+                return
+
+            for field in self.form_fields:
+                if not iq['register'][field]:
+                    # Incomplete Registration
+                    self._sendError(iq, '406', 'modify', 'not-acceptable'
+                                    "Please fill in all fields.")
+                    return
+
+    ...
+
+    def _sendError(self, iq, code, error_type, name, text=''):
+        iq.reply().set_payload(iq['register'].xml)
+        iq.error()
+        iq['error']['code'] = code
+        iq['error']['type'] = error_type
+        iq['error']['condition'] = name
+        iq['error']['text'] = text
+        iq.send()
+
+Cases 5 and 6: Conflicting and Successful Registration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We are down to the final decision on if we have a successful registration. We
+send the user's data to the backend with the ``self.backend.register`` method.
+If it returns ``True``, then registration has been successful. Otherwise,
+there has been a conflict with usernames and registration has failed. Like
+with unregistering an account, we trigger an event indicating that a user has
+been registered by using ``self.xmpp.event('registered_user', iq)``. See the
+component examples below for how to respond to this event.
+
+.. code-block:: python
+
+    def __handleRegistration(self, iq):
+        if iq['type'] == 'get':
+            # Registration form requested
+            userData = self.backend[iq['from'].bare]
+            self.sendRegistrationForm(iq, userData)
+        elif iq['type'] == 'set':
+            if iq['register']['remove']:
+                # Remove an account
+                self.backend.unregister(iq['from'].bare)
+                self.xmpp.event('unregistered_user', iq)
+                iq.reply().send()
+                return
+
+            for field in self.form_fields:
+                if not iq['register'][field]:
+                    # Incomplete Registration
+                    self._sendError(iq, '406', 'modify', 'not-acceptable',
+                                    "Please fill in all fields.")
+                    return
+
+            if self.backend.register(iq['from'].bare, iq['register']):
+                # Successful registration
+                self.xmpp.event('registered_user', iq)
+                iq.reply().set_payload(iq['register'].xml)
+                iq.send()
+            else:
+                # Conflicting registration
+                self._sendError(iq, '409', 'cancel', 'conflict',
+                                "That username is already taken.")
+
+Example Component Using the XEP-0077 Plugin
+-------------------------------------------
+Alright, the moment we've been working towards - actually using our plugin to
+simplify our other applications. Here is a basic component that simply manages
+user registrations and sends the user a welcoming message when they register,
+and a farewell message when they delete their account.
+
+Note that we have to register the ``'xep_0030'`` plugin first,
+and that we specified the form fields we wish to use with
+``self.xmpp.plugin['xep_0077'].setForm('username', 'password')``.
+
+.. code-block:: python
+
+    import slixmpp.componentxmpp
+
+    class Example(slixmpp.componentxmpp.ComponentXMPP):
+
+        def __init__(self, jid, password):
+            slixmpp.componentxmpp.ComponentXMPP.__init__(self, jid, password, 'localhost', 8888)
+
+            self.register_plugin('xep_0030')
+            self.register_plugin('xep_0077')
+            self.plugin['xep_0077'].setForm('username', 'password')
+
+            self.add_event_handler("registered_user", self.reg)
+            self.add_event_handler("unregistered_user", self.unreg)
+
+        def reg(self, iq):
+            msg = "Welcome! %s" % iq['register']['username']
+            self.send_message(iq['from'], msg, mfrom=self.fulljid)
+
+        def unreg(self, iq):
+            msg = "Bye! %s" % iq['register']['username']
+            self.send_message(iq['from'], msg, mfrom=self.fulljid)
+
+**Congratulations!** We now have a basic, functioning implementation of
+XEP-0077.
+
+Complete Source Code for XEP-0077 Plugin
+----------------------------------------
+Here is a copy of a more complete implementation of the plugin we created, but
+with some additional registration fields implemented.
+
+.. code-block:: python
+
+    """
+    Creating a Slixmpp Plugin
+
+    This is a minimal implementation of XEP-0077 to serve
+    as a tutorial for creating Slixmpp plugins.
+    """
+
+    from slixmpp.plugins.base import BasePlugin
+    from slixmpp.xmlstream.handler.callback import Callback
+    from slixmpp.xmlstream.matcher.xpath import MatchXPath
+    from slixmpp.xmlstream import ElementBase, ET, JID, register_stanza_plugin
+    from slixmpp import Iq
+    import copy
+
+
+    class Registration(ElementBase):
+        namespace = 'jabber:iq:register'
+        name = 'query'
+        plugin_attrib = 'register'
+        interfaces = {'username', 'password', 'email', 'nick', 'name',
+                      'first', 'last', 'address', 'city', 'state', 'zip',
+                      'phone', 'url', 'date', 'misc', 'text', 'key',
+                      'registered', 'remove', 'instructions'}
+        sub_interfaces = interfaces
+
+        def getRegistered(self):
+            present = self.xml.find('{%s}registered' % self.namespace)
+            return present is not None
+
+        def getRemove(self):
+            present = self.xml.find('{%s}remove' % self.namespace)
+            return present is not None
+
+        def setRegistered(self, registered):
+            if registered:
+                self.addField('registered')
+            else:
+                del self['registered']
+
+        def setRemove(self, remove):
+            if remove:
+                self.addField('remove')
+            else:
+                del self['remove']
+
+        def addField(self, name):
+            itemXML = ET.Element('{%s}%s' % (self.namespace, name))
+            self.xml.append(itemXML)
+
+
+    class UserStore(object):
+        def __init__(self):
+            self.users = {}
+
+        def __getitem__(self, jid):
+            return self.users.get(jid, None)
+
+        def register(self, jid, registration):
+            username = registration['username']
+
+            def filter_usernames(user):
+                return user != jid and self.users[user]['username'] == username
+
+            conflicts = filter(filter_usernames, self.users.keys())
+            if conflicts:
+                return False
+
+            self.users[jid] = registration
+            return True
+
+        def unregister(self, jid):
+            del self.users[jid]
+
+    class xep_0077(BasePlugin):
+        """
+        XEP-0077 In-Band Registration
+        """
+
+        def plugin_init(self):
+            self.description = "In-Band Registration"
+            self.xep = "0077"
+            self.form_fields = ('username', 'password')
+            self.form_instructions = ""
+            self.backend = UserStore()
+
+            self.xmpp.register_handler(
+                Callback('In-Band Registration',
+                         MatchXPath('{%s}iq/{jabber:iq:register}query' % self.xmpp.default_ns),
+                         self.__handleRegistration))
+            register_stanza_plugin(Iq, Registration)
+
+        def post_init(self):
+            BasePlugin.post_init(self)
+            self.xmpp['xep_0030'].add_feature("jabber:iq:register")
+
+        def __handleRegistration(self, iq):
+            if iq['type'] == 'get':
+                # Registration form requested
+                userData = self.backend[iq['from'].bare]
+                self.sendRegistrationForm(iq, userData)
+            elif iq['type'] == 'set':
+                if iq['register']['remove']:
+                    # Remove an account
+                    self.backend.unregister(iq['from'].bare)
+                    self.xmpp.event('unregistered_user', iq)
+                    iq.reply().send()
+                    return
+
+                for field in self.form_fields:
+                    if not iq['register'][field]:
+                        # Incomplete Registration
+                        self._sendError(iq, '406', 'modify', 'not-acceptable',
+                                        "Please fill in all fields.")
+                        return
+
+                if self.backend.register(iq['from'].bare, iq['register']):
+                    # Successful registration
+                    self.xmpp.event('registered_user', iq)
+                    reply = iq.reply()
+                    reply.set_payload(iq['register'].xml)
+                    reply.send()
+                else:
+                    # Conflicting registration
+                    self._sendError(iq, '409', 'cancel', 'conflict',
+                                    "That username is already taken.")
+
+        def setForm(self, *fields):
+            self.form_fields = fields
+
+        def setInstructions(self, instructions):
+            self.form_instructions = instructions
+
+        def sendRegistrationForm(self, iq, userData=None):
+            reg = iq['register']
+            if userData is None:
+                userData = {}
+            else:
+                reg['registered'] = True
+
+            if self.form_instructions:
+                reg['instructions'] = self.form_instructions
+
+            for field in self.form_fields:
+                data = userData.get(field, '')
+                if data:
+                    # Add field with existing data
+                    reg[field] = data
+                else:
+                    # Add a blank field
+                    reg.addField(field)
+
+            reply = iq.reply()
+            reply.set_payload(reg.xml)
+            reply.send()
+
+        def _sendError(self, iq, code, error_type, name, text=''):
+            reply = iq.reply()
+            reply.set_payload(iq['register'].xml)
+            reply.error()
+            reply['error']['code'] = code
+            reply['error']['type'] = error_type
+            reply['error']['condition'] = name
+            reply['error']['text'] = text
+            reply.send()