zulip/docs/new-feature-tutorial.md

# Writing a new application feature

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 Jinja2
templates located in `templates/zerver`. 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
`frontend_tests/casper_tests/`. The unit tests use Node's `assert`
module are located in `frontend_tests/node_tests/`. For more
information on writing and running tests see the [testing
documentation](testing.html).

## 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 existing `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 existing 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.