Tutorial: 💻 Multiuser Chat

Copyright © 2020-2021 Croquet Corporation

This is an example of how to keep track of different users within the same session. It's a simple chat application that maintains a list of all currently connected users. New users are assigned a random nickname.

See the Pen Chat by Croquet (@croquet) on CodePen.

Try it out!

The first thing to do is click or scan the QR code above. This will launch a new Codepen instance of this session. Typing a message in either window will post the text to the shared chat screen under a randomly assigned nickname. Other people who are reading this documentation right now can also post messages to the same conversation, so you might find yourself talking to another Croquet developer!

There are five things we will learn here:

  1. How to use "view-join" and "view-exit" events to track connections.
  2. How to use the viewId to store view-specific information inside the model.
  3. How to directly read from the model without breaking synchronization.
  4. How to use future() messages and model.now() to schedule actions based on timeouts.
  5. How to use modelOnly() to prevent accidentally writing to the model.

Simple Chat Model

Our Croquet application uses a single Model subclass named ChatModel. The model can be thought of as doing four things: it maintains a mapping between the active views and their nicknames; it maintains the history of chat entries; it listens for chat-post or chat-reset events coming from views; and it unilaterally clears the chat after a period of inactivity.

A "newPost" event is sent by a view when its user enters a chat message. The event is reflected to the model of all users in the session. Each user's model adds the post to its chat history, and informs its local view to update its display.

A "reset" event is sent by a view when its user types the special string "/reset", or when a newly joining view finds that it's alone, and with a chat history that it played no part in.

"view-join" and "view-exit" are system-generated events. They don't originate inside your application, but come from the Teatime system itself. When a new user joins a session, a "view-join" event is sent to the model of everyone in the session (including the user who just joined). Similarly, whenever a user leaves, a "view-exit" event is sent - though in this case the user who just left will not get the event, because they are already gone!


this.views = new Map();

views holds a list of nicknames indexed by the users' unique view IDs.

It would be possible to store the views in a standard JavaScript object ({view1: "nick1"; view2: "nick2", ...}), but a Map has the additional property that when serialized and deserialized it is guaranteed to maintain the order of its items. This particular app never needs to iterate over this list, but to ensure that key-value collections held in models behave identically across users - whenever each user happened to join the session - we recommend using Maps instead of plain objects. (For views, use whatever you like. Views aren't obliged to behave identically.)

this.participants = 0;

participants is the number of currently active views.

this.history = [];

history is an array of items each holding a chat-post HTML string and the view ID of the user who posted it.

this.subscribe(this.sessionId, "view-join", this.viewJoin);
this.subscribe(this.sessionId, "view-exit", this.viewExit);

These subscriptions handle users entering or leaving. In both cases the scope is set to this.sessionId, which is the default scope for all system-generated events. The data passed to both events is the joining or exiting view's unique viewId.

If a user's view leaves due to becoming inactive, then later re-enters (for example, if it is running on a phone that is put to sleep, then re-awakened), the viewId on re-entry will be the same as before. On the other hand, if a user joins the session from multiple browser tabs even on the same device, the viewId for each tab will be different.

Inside your application you can use viewID as a unique identifier for each participant in a session. You can store data about individual participants using viewID as the key. Or you can use viewID as the scope of an event to specify who sent it, or limit who receives it.

this.subscribe("input", "newPost", this.newPost);

This subscription handles new chat posts. It's given the scope "input" as a way to remind us where the event is coming from. (It also means we could use "newPost" as a different event somewhere else in our application without the two events being confused with each other.)


  viewJoin(viewId) {
    const existing = this.views.get(viewId);
    if (!existing) {
      const nickname = this.randomName();
      this.views.set(viewId, nickname);
    this.publish("viewInfo", "refresh");

When a user joins the session, the model checks whether this viewId has already been seen. If not, it generates a new random nickname and stores it in the view list using the viewID as the access key. It then increments the participants count and publishes an event that will trigger the view to refresh its display. (In this case we're using "viewInfo" as the scope; even a generic event name like "refresh" is safe to use, as long as the app assigns different scopes for any other uses of "refresh".)


  viewExit(viewId) {
    this.publish("viewInfo", "refresh");

When a user leaves the session, the model decrements the participants count, removes the user's entry in the view list, and publishes the same refresh event.


  newPost(post) {
    const postingView = post.viewId;
    const nickname = this.views.get(postingView);
    const chatLine = `<b>${nickname}:</b> ${this.escape(post.text)}`;
    this.addToHistory({ viewId: postingView, html: chatLine });
    this.lastPostTime = this.now();

    if (this.history.length > 100) this.history.shift();
    this.publish("history", "refresh");

The data supplied with a "newPost" event includes the sender's viewId. When the model receives a new post, it uses this ID to look up the user's nickname. It then builds an HTML chat line that includes the nickname and the message, and invokes addToHistory() to append the line to the chat history along with the view ID, for later lookup.

addToHistory publishes an event to the view informing it that the history has changed (noting, as explained above, that the "refresh" event here uses the "history" scope, so there is no conflict with "viewInfo" events). If there are more than 100 entries in the history, it discards the oldest entry to prevent the array from growing too large.

newPost then records this.now() in the model's lastPostTime property. This is the simulation time (which runs independently of wall-clock time; see the tutorial on Simulation Time and Future Sends) at which this chat post arrived. It then schedules a "future send" to itself, the model object: specifically, at a simulation time that is exactly this.inactivity_timeout_ms milliseconds after the current simulation time, the method this.resetIfInactive() will be invoked.

this.inactivity_timeout_ms was set in init(), to a value corresponding to 20 minutes.


  resetIfInactive() {
    if (this.lastPostTime !== this.now() - this.inactivity_timeout_ms) return;

    this.resetHistory("due to inactivity");

The aim of this method is to reset the chat when no-one has posted for a long time. It will be invoked, through the "future send" mechanism, a set interval - this.inactivity_timeout_ms simulation milliseconds - after each post. To go ahead with the reset, it must confirm that the whole interval since the post that triggered it has indeed passed without any later post arriving. To check that, we look at the current simulation time (this.now()) and see if it is exactly inactivity_timeout_ms after the value currently in lastPostTime. If it is, clearly no later posts did arrive, so the reset should go ahead.

Instead of the original post time, or a more recent one, lastPostTime can also be null indicating that the history has been reset (see below). In that case, too, there is no need to do anything here.

Note that the handling of future sends, like that of messages from views, is guaranteed to be identical for every user's model instance. Every instance will make the same decision about whether to reset or not.


  resetHistory(reason) {
    this.history = [{ html: `<i>chat reset ${reason}</i>` }];
    this.lastPostTime = null;
    this.publish("history", "refresh");

This is called in three circumstances:

  1. If ChatModel.resetIfInactive() is invoked and finds that inactivity_timeout_ms simulation milliseconds have passed since the last post.
  2. If a user types the text /reset as a chat line.
  3. If a user's ChatView constructor (see below) detects that they are the only user in the session, and that the existing chat contains no items that they posted.

For the latter two cases, the invocation is the result of the model's subscription to the "reset" event, that the affected view will have published:

  this.subscribe("input", "reset", this.resetHistory);

The view methods that publish this event are described below.

After replacing the contents of the history array with a single entry displaying the reason for the reset, this method resets lastPostTime to null. This ensures that any outstanding future-send invocations of resetIfInactive() will see that they have nothing to do.


randomName() {
  const names = ["Acorn" ..."Zucchini"];
  return names[Math.floor(Math.random() * names.length)];

When a new user joins, their nickname is picked at a random from an array. Note that even though a separate instance of this code is running locally for each user, each of the instances will "randomly" pick the same name. This is because - as also seen in the Simple Animation tutorial - calls to Math.random() from inside the model are deterministic. They will generate exactly the same sequence of random numbers in every instance of the model, ensuring they all stay in synch.


  this.model = model;

We store a reference to the model so that we can use it later to pull data directly.

(Note: This reference is only to the root model that was created during Croquet.Session.join. If your root model contains sub-models that you need to read from, you should store references to them inside the root model.)

  sendButton.onclick = () => this.send();

This is the event handler for the HTML "Send" button. It is called when the user clicks the button (or, thanks to our HTML setup, when the user presses Enter in the input field).

  this.subscribe("history", "refresh", this.refreshHistory);
  this.subscribe("viewInfo", "refresh", this.refreshViewInfo);

We subscribe to two different refresh events from the model. One is sent when the model has updated the chat history (either adding a message or performing a reset), and the other is sent when someone joins or exits the session.


When the view starts we pull the current history from the model and post it to the screen. We do this because when a user joins an existing chat session, there may already be a history of previous chat messages to be shown; it makes no sense to wait until someone adds a new post.

We do the same for refreshing the view info. Croquet guarantees that the model will have processed this view's own view-join event before the view is constructed. This means that a ChatView will never see a viewInfo event generated in response to its own joining, but it can assume during its own initialization that the ChatModel already has an entry for it.

This is a general rule in Croquet development: a newly constructed view should set itself up completely from the model state at the time of construction. After that, it should rely on its subscriptions to hear about updates in the model and to respond appropriately.

  if (model.participants === 1 &&
    !model.history.find(item => item.viewId === this.viewId)) {
    this.publish("input", "reset", "for new participants");

The final task for the constructor is to check for the condition of the view being alone in the session, with no messages contributed by itself in the chat history. In that case, it sends an event to reset the history.

The necessary information is held by the model, and the view is allowed to directly read from the model at any time. Here it checks the number of participants, and iterates through the history in search of any items provided by itself.

The view must NEVER directly write to the model! Because Croquet exposes the model to the view for read access, it is possible to author a Croquet application where the view directly writes to the model. However, doing so will break synchronization and prevent the application from functioning properly.

If your view needs to change some information that is held by the model, it must do so by publishing an event that the model subscribes to. This will ensure that the change is mirrored by the reflector and executed identically by all instances of the model.


send() {
  const text = textIn.value;
  textIn.value = "";
  if (text === "/reset") {
    this.publish("input", "reset", "at user request");
  } else {
    this.publish("input", "newPost", {viewId: this.viewId, text});

When the user presses the send button, we examine the text contents of the input field. If it's the special value "/reset", we send a "reset" event to the model; for any other string (including an empty one) we send a "newPost" event. Only the latter event needs to include the viewId of the sending view, so that the model can tag the post with the appropriate user nickname.

Any class that inherits from View has this.viewId as a member. It contains the unique viewId that was assigned to this user when they joined the session. We can use this.viewId whenever we want to tell the model that a particular user has done something.


  refreshViewInfo() {
    nickname.innerHTML = "<b>Nickname:</b> " + this.model.views.get(this.viewId);
    viewCount.innerHTML = "<b>Total Views:</b> " + this.model.participants;

Again the view reaches directly into the model, in this case to get its own nickname and the total number of views currently connected. In the view's constructor we stored a pointer to the model just for this purpose.


  refreshHistory() {
    textOut.innerHTML = "<b>Welcome to Croquet Chat!</b><br><br>" +
      this.model.history.map(item => item.html).join("<br>");
    textOut.scrollTop = Math.max(10000, textOut.scrollHeight);

refreshHistory() is similar to refreshViewInfo() in that it reads the chat history directly from the model and posts it to the screen. It also makes sure that the chat window scrolls to the bottom when a new message arrives.

this.model.history.map(item => item.html) is a JavaScript array operation that extracts all the HTML strings from the history. The join("<br>") then concatenates them into a single output string, inserting a line break between each.

Model Get/Set Routines & modelOnly()

One way to guard against accidentally writing to the model is to create explicit Get and Set methods for reading and writing. For example:

class MyModel extends Croquet.Model {

  init() {

  getData() {
    return this.data;

  setData(newData) {
    this.data = newData;


Model.modelOnly() is a utility function that throws an error if called outside normal model execution. You can use it inside a model Set method to make sure it doesn't accidentally get called by the view.