zulip/docs/new-feature-tutorial.rst

====================
New Feature Tutorial
====================

The changes needed to add a new feature will vary, of course, but this document
provides a general outline of what you may need to do, as well as an example of
the specific steps needed to add a new feature: adding a new option to the 
application that is dynamically synced through the data system in real-time to
all browsers the user may have open.

General Process
===============

Adding a field to the database
------------------------------

**Update the model:** The server accesses the underlying database in `zerver/
models.py`. Add a new field in the appropriate class.

**Create and run the migration:** To create and apply a migration, run: ::

./manage.py makemigrations
./manage.py migrate

**Test your changes:** Once you've run the migration, restart memcached on your 
development server (``/etc/init.d/memcached restart``) and then restart 
``run-dev.py`` to avoid interacting with cached objects.

Backend changes
---------------

**Database interaction:** Add any necessary code for updating and interacting
with the database in ``zerver/lib/actions.py``. It should update the database and 
send an event announcing the change.

**Application state:** Modify the ``fetch_initial_state_data`` and ``apply_events`` 
functions in ``zerver/lib/actions.py`` to update the state based on the event you 
just created.

**Backend implementation:** Make any other modifications to the backend required for 
your change.

**Testing:** At the very least, add a test of your event data flowing through 
the system in ``test_events.py``.


Frontend changes
----------------

**JavaScript:** Zulip's JavaScript is located in the directory ``static/js/``. 
The exact files you may need to change depend on your feature. If you've added a 
new event that is sent to clients, be sure to add a handler for it to
``static/js/server_events.js``.

**CSS:** The primary CSS file is ``static/styles/zulip.css``. If your new 
feature requires UI changes, you may need to add additional CSS to this file.

**Templates:** The initial page structure is rendered via Django templates 
located in ``template/server``. For JavaScript, Zulip uses Handlebars templates located in
``static/templates``. Templates are precompiled as part of the build/deploy
process.

**Testing:** There are two types of frontend tests: node-based unit tests and 
blackbox end-to-end tests. The blackbox tests are run in a headless browser 
using Casper.js and are located in ``zerver/tests/frontend/tests/``. The unit
tests use Node's ``assert`` module are located in ``zerver/tests/frontend/node/``.
For more information on writing and running tests see the :doc:`testing 
documentation <testing>`.

Example Feature
===============

This example describes the process of adding a new setting to Zulip:
a flag that restricts inviting new users to admins only (the default behavior
is that any user can invite other users). It is based on an actual Zulip feature,
and you can review `the original commit in the Zulip git repo <https://github.com/zulip/zulip/commit/5b7f3466baee565b8e5099bcbd3e1ccdbdb0a408>`_.
(Note that Zulip has since been upgraded from Django 1.6 to 1.8, so the migration
format has changed.)

First, update the database and model to store the new setting. Add a 
new boolean field, ``realm_invite_by_admins_only``, to the Realm model in
``zerver/models.py``.

Then create a Django migration that adds a new field, ``invite_by_admins_only``,
to the ``zerver_realm`` table.

In ``zerver/lib/actions.py``, create a new function named 
``do_set_realm_invite_by_admins_only``. This function will update the database
and trigger an event to notify clients when this setting changes. In this case 
there was an exisiting ``realm|update`` event type which was used for setting 
similar flags on the Realm model, so it was possible to add a new property to 
that event rather than creating a new one. The property name matches the 
database field to make it easy to understand what it indicates.

The second argument to ``send_event`` is the list of users whose browser 
sessions should be notified. Depending on the setting, this can be a single user
(if the setting is a personal one, like time display format), only members in a
particular stream or all active users in a realm. ::

  # zerver/lib/actions.py

  def do_set_realm_invite_by_admins_only(realm, invite_by_admins_only):
    realm.invite_by_admins_only = invite_by_admins_only
    realm.save(update_fields=['invite_by_admins_only'])
    event = dict(
      type="realm",
      op="update",
      property='invite_by_admins_only',
      value=invite_by_admins_only,
    )
    send_event(event, active_user_ids(realm))
    return {}

You then need to add code that will handle the event and update the application
state. In ``zerver/lib/actions.py`` update the ``fetch_initial_state`` and
``apply_events`` functions. ::

  def fetch_initial_state_data(user_profile, event_types, queue_id):
    # ...
    state['realm_invite_by_admins_only'] = user_profile.realm.invite_by_admins_only`

In this case you don't need to change ``apply_events`` because there is already
code that will correctly handle the realm update event type: ::

  def apply_events(state, events, user_profile):
    for event in events:
      # ...
      elif event['type'] == 'realm':
         field = 'realm_' + event['property']
         state[field] = event['value']

You then need to add a view for clients to access that will call the newly-added
``actions.py`` code to update the database. This example feature adds a new
parameter that should be sent to clients when the application loads and be
accessible via JavaScript, and there is already a view that does this for
related flags: ``update_realm``. So in this case, we can add out code to the
exisiting view instead of creating a new one. ::

  # zerver/views/__init__.py

  def home(request):
    # ...
    page_params = dict(
      # ...
      realm_invite_by_admins_only = register_ret['realm_invite_by_admins_only'],
      # ...
    )

Since this feature also adds a checkbox to the admin page, and adds a new
property the Realm model that can be modified from there, you also need to make
changes to the ``update_realm`` function in the same file: ::

  # zerver/views/__init__.py

  def update_realm(request, user_profile,
    name=REQ(validator=check_string, default=None),
    restricted_to_domain=REQ(validator=check_bool, default=None),
    invite_by_admins_only=REQ(validator=check_bool,default=None)):

    # ...

    if invite_by_admins_only is not None and
      realm.invite_by_admins_only != invite_by_admins_only:
        do_set_realm_invite_by_admins_only(realm, invite_by_admins_only)
        data['invite_by_admins_only'] = invite_by_admins_only

Then make the required front end changes: in this case a checkbox needs to be
added to the admin page (and its value added to the data sent back to server
when a realm is updated) and the change event needs to be handled on the client.

To add the checkbox to the admin page, modify the relevant template,
``static/templates/admin_tab.handlebars`` (omitted here since it is relatively
straightforward). Then add code to handle changes to the new form control in
``static/js/admin.js``. ::

  var url = "/json/realm";
  var new_invite_by_admins_only =
    $("#id_realm_invite_by_admins_only").prop("checked");
  data[invite_by_admins_only] = JSON.stringify(new_invite_by_admins_only);

  channel.patch({
    url: url,
    data: data,
    success: function (data) {
      # ...
      if (data.invite_by_admins_only) {
        ui.report_success("New users must be invited by an admin!", invite_by_admins_only_status);
      } else {
        ui.report_success("Any user may now invite new users!", invite_by_admins_only_status);
      }
      # ...
    }
  });

Finally, update ``server_events.js`` to handle related events coming from the
server. ::

  # static/js/server_events.js

  function get_events_success(events) {
    # ...
    var dispatch_event = function dispatch_event(event) {
        switch (event.type) {
        # ...
        case 'realm':
          if (event.op === 'update' && event.property === 'invite_by_admins_only') {
            page_params.realm_invite_by_admins_only = event.value;
        }
    }
  }

Any code needed to update the UI should be placed in ``dispatch_event`` callback
(rather than the ``channel.patch``) function. This ensures the appropriate code
will run even if the changes are made in another browser window. In this example
most of the changes are on the backend, so no UI updates are required.