zulip/docs/subsystems/widgets.md

Widgets (experimental)

[Note: this document is currently intended to be a roadmap/design document. It may be converted over time to permanent documentation.]

Overview

During 2018 we built out a "widget system" in Zulip. It includes these features:

• /ping
• /day (and /night, /light, /dark)
• /poll (and /todo) (BETA)
• zform-enabled messages for the trivia_quiz bot (BETA)

The beta features are only turned on for chat.zulip.org as of this writing.

There's a strong overlap between widgets and slash commands, and many widgets are launched by slash commands. A few exceptions are worth noting. If you type "/me shrugs" in the compose box, it's just a message that gets slightly customized rendering. And if you type "/settings", it's just a shortcut to open the settings popup. Neither of these are really "widgets," per se.

Another exception, in the opposite direction, is our trivia_quiz bot. It does not involve slash commands. Instead it sends "extra_data" in messages to invoke zforms (which enable button-based UIs in the messages).

Here are some code entities used in the above features:

• SubMessage database table
• /json/zcommand API endpoint
• /json/submessage API endpoint
• static/js/zcommand.js
• static/js/submessage.js
• static/js/poll_widget.js
• static/js/widgetize.js
• static/js/zform.js
• static/templates/widgets/
• zerver/lib/widget.py
• zerver/lib/zcommand.py
• zerver/views/submessage.py

Simple slash commands

We support a few very simple slash commands that are intended for single users to do simple tasks:

• Ping the server
• Toggle day/night mode

Data flow

These commands have client-side support in zcommands.js. They send commands to the server using the /json/command endpoint.

In the case of "/ping", the server code in zcommand.py basically just acks the client. The client then computes the round trip time and shows a little message above the compose box that the user can see and then dismiss.

For commands like "/day" and "/night", the server does a little bit of logic to toggle the user's night mode setting, and this is largely done inside zcommand.py. The server sends a very basic response, and then the client actually changes the display colors. The client also shows the user a little message above the compose box instructing them how to reverse the change.

It's a bit of a stretch to label "/ping" and "/day" as widgets. In some ways they're just compose-box shortcuts for doing UI tasks. The commands share the new "zcommand" namespace in the code, and both have some common UI for talking to users.

(It's possible that we don't really need a general /json/zcommand endpoint for these, and we may decide later to just use custom API endpoints for each command. There's some logic in having a central API for these, though, since they are typically things that only UI-based clients will invoke, and they may share validation code.)

Availability

The above commands are available for all Zulip servers that use 1.9 or above. You must use the web app client to get the features; other clients will send the messages without any translation (e.g. "/day" will just be a message that says "/day" if you use the mobile app).

Poll, todo lists, and games

The most interactive widgets that we built during 2018 are for polls, todo lists, and games. You launch widgets by sending one of the following messages:

• /poll
• /todo

The web app client provides the "widget experience" by default. Other clients just show raw messages like "/poll", and should be adding support for widgets soon.

Our customers have long requested a poll/survey widget. See this issue. There are workaround ways to do polls using things like emoji reactions, but our poll widget provides a more interactive experience.

Data flow

The poll widget uses the "submessage" architecture. We'll use the poll widget as a concrete example.

The SubMessage table, as the name indicates, allows you to associate multiple submessages to any given Message row. When a message gets sent, there's a hook inside of widget.py that will detect slash commands like "/poll". If a message needs to be widgetized, an initial SubMessage row will be created with an appropriate msg_type (and persisted to the database). This data will also be included in the normal Zulip message event payload. Clients can choose to ignore the submessage-related data, in which case they'll gracefully degrade to seeing "/poll". Of course, the web app client actually recognizes the appropriate widgets.

The web app client will next collect poll options and votes from users. The web app client has code in submessage.js that dispatches events to widgetize.js, which in turns sends events to individual widgets. The widgets know how to render themselves and set up click/input handlers to collect data. They can then post back to /json/submessage to attach more data to the message (and the details are encapsulated with a callback). The server will continue to persist SubMessage rows in the database. These rows are encoded as JSON, and the schema of the messages is driven by the individual widgets. Most of the logic is in the client; things are fairly opaque to the server at this point.

The "submessage" architecture is generic. Our todo list widget uses the same architecture as "poll".

If a client joins Zulip after a message has accumulated several submessage events, it will see all of those events the first time it sees the parent message. Clients need to know how to build/rebuild their state as each submessage comes in. They also need to tolerate misformatted data, ideally just dropping data on the floor. If a widget throws an exception, it's caught before the rest of the message feed is affected.

As far as rendering is concerned, each widget module is given a parent elem when its activate function is called. This is just a <div> inside of the parent message in the message pane. The widget has access to jQuery and template.render, and the developer can create new templates in static/templates/widgets/.

A good way to learn the system is to read the code in static/js/poll_widget.js. It is worth noting that writing a new widget requires only minor backend changes in the current architecture. This could change in the future, but for now a frontend developer mostly needs to know JS, CSS, and HTML.

It may be useful to think of widgets in terms of a bunch of clients exchanging peer-to-peer messages. The server's only real role is to decide who gets delivered which submessages. It's a lot like a "subchat" system.

Backward compatibility

Our "submessage" widgets are still evolving, and we want to have a plan for allowing future progress without breaking old messages.

Widget developers can revise code to improve a widget's visual polish without too much concern for breaking how old messages get widgetized. They will need to be more cautious if they change the actual data structures passed around in the submessage payloads.

For significant schema changes, it would be worthwhile to add some kind of versioning scheme inside of SubMessages, either at the DB level or more at the JSON level within fields. This has yet to be designed. One thing to consider is that most widgets are somewhat ephemeral in nature, so it's not the end of the world if upgrades cause some older messages to be obsolete, as long as the code degrades gracefully.

Mission critical widgets should have a deprecation strategy. For example, you could add optional features for one version bump and then only make them mandatory for the next version, as long as you don't radically change the data model. And if you're truly making radical changes, you can always write a Django migration for the SubMessage data.

Adding widgets

Right now we don't have a plugin model for the above widgets; they are served up by the core Zulip server implementation. Of course, anybody who wishes to build their own widget has the option of forking the server code and self-hosting, but we want to encourage folks to submit widget code to our codebase in PRs. If we get to a critical mass of contributed widgets, we will want to explore a more dynamic mechanism for "plugging in" code from outside sources, but that is not in our immediate roadmap.

This is sort of a segue to the next section of this document. Suppose you want to write your own custom bot, and you want to allow users to click buttons to respond to options, but you don't want to have to modify the Zulip server codebase to turn on those features. This is where our "zform" architecture comes to the rescue.

zform (trivia quiz bot)

This section will describe our "zform" architecture.

For context, imagine a naive triva bot. The trivia bot sends a question with the answers labeled as A, B, C, and D. Folks who want to answer the bot send back an answer have to send an actual Zulip message with something like @trivia_bot answer A to Q01, which is kind of tedious to type. Wouldn't it be nice if the bot could serve up some kind of buttons with canned replies, so that the user just hits a button?

That is where zforms come in. Zulip's trivia bot sends the Zulip server a JSON representation of a form it wants rendered, and then the client renders a generic "zform" with buttons corresponding to short_name fields inside a choices list inside of the JSON payload.

Here is what an example payload looks like:

{
    "extra_data": {
        "type": "choices",
        "heading": "05: What color is a blueberry?",
        "choices": [
            {
                "type": "multiple_choice",
                "reply": "answer 05 A",
                "long_name": "red",
                "short_name": "A"
            },
            {
                "type": "multiple_choice",
                "reply": "answer 05 B",
                "long_name": "blue",
                "short_name": "B"
            },
            {
                "type": "multiple_choice",
                "reply": "answer 05 C",
                "long_name": "yellow",
                "short_name": "C"
            },
            {
                "type": "multiple_choice",
                "reply": "answer 05 D",
                "long_name": "orange",
                "short_name": "D"
            }
        ]
    },
    "widget_type": "zform"
}

When users click on the buttons, generic click handlers automatically simulate a client reply using a field called reply (in choices) as the content of the message reply. Then the bot sees the reply and grades the answer using ordinary chat-bot coding.

The beautiful thing is that any third party developer can enhance bots that are similar to the trivia_quiz bot without touching any Zulip code, because zforms are completely generic.

Data flow

We can walk through the steps from the bot generating the zform to the client rendering it.

First, here is the code that produces the JSON.

def format_quiz_for_widget(quiz_id: str, quiz: Dict[str, Any]) -> str:
    widget_type = 'zform'
    question = quiz['question']
    answers = quiz['answers']

    heading = quiz_id + ': ' + question

    def get_choice(letter: str) -> Dict[str, str]:
        answer = answers[letter]
        reply = 'answer ' + quiz_id + ' ' + letter

        return dict(
            type='multiple_choice',
            short_name=letter,
            long_name=answer,
            reply=reply,
        )

    choices = [get_choice(letter) for letter in 'ABCD']

    extra_data = dict(
        type='choices',
        heading=heading,
        choices=choices,
    )

    widget_content = dict(
        widget_type=widget_type,
        extra_data=extra_data,
    )
    payload = json.dumps(widget_content)
    return payload

The above code processes data that is specific to a trivia quiz, but it follows a generic schema.

The bot sends the JSON payload to the server using the send_reply callback.

The bot framework looks for the optional widget_content parameter in send_reply and includes that in the message payload it sends to the server.

The server validates the schema of widget_content using check_widget_content.

Then code inside of zerver/lib/widget.py builds a single SubMessage row to contain the zform payload, and the server also sends this payload to all clients who are recipients of the parent message.

When the message gets to the client, the codepath for zform is actually quite similar to what happens with a more customized widget like poll. (In fact, zform is a sibling of poll and zform just has a somewhat more generic job to do.) In static/js/widgetize.js you will see where this code converges, with snippets like this:

widgets.poll = poll_widget;
widgets.todo = todo_widget;
widgets.zform = zform;

The code in static/js/zform.js renders the form (not shown here) and then sets up a click handler like below:

    elem.find('button').on('click', function (e) {
        e.stopPropagation();

        // Grab our index from the markup.
        var idx = $(e.target).attr('data-idx');

        // Use the index from the markup to dereference our
        // data structure.
        var reply_content = data.choices[idx].reply;

        transmit.reply_message({
            message: opts.message,
            content: reply_content,
        });
    });

And then we are basically done!

Slash commands

This document is more about "widget" behavior than "slash command" interfaces, but there is indeed a lot of overlap between the two concepts.

Typeahead for slash commands is implemented via the slash_commands object in static/js/composebox_typeahead.js.