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Class CObject<Events>

Base class for a collaborative object, containing properties that are themselves Collabs.

See Data Modeling for a guide and examples of how to extend this class.

Extending CObject lets you create a reusable object-oriented unit out of one or more existing Collabs. In particular, you can:

  • Wrap existing Collabs in a domain-specific API.
  • Implement a complex Collab as a composition of simple ones, without the need to send your own messages over the network.

To use CObject:

  1. In your constructor, register any Collab properties using registerCollab. The registrations must be identical across all replicas. We call the registered properties our children.
  2. Implement your methods by calling methods on the Collab properties; operations called this way are collaborative as usual.
  3. You may also register event handlers on your Collab properties that emit your own events in response (in your constructor). For simple CObjects, it may be easier to instead expose Collab properties publicly (so users can add their own event handlers) or have users listen on CRuntime's "Change" event.

A CObject may have non-Collab properties, but they are not automatically collaborative. Typically, such properties will all be readonly. You can also store functional "views" of the Collab properties' states that update in response to events (e.g., a cached length field).

Type Parameters





  • Initializes this Collab with the given InitToken.

    The InitToken must have provided by our parent explicitly for this constructor call.

    Typically, a Collab subclass takes init as its first constructor argument and calls super(init).

    Type Parameters


    Returns CObject<Events>


children: Map<string, Collab<CollabEventsRecord>>

The children (registered Collab properties), keyed by name.

This map should only be read, not mutated. It is exposed to subclasses as a convenience for methods that loop over all children.

name: string

Internal (this/parent) use only.

This Collab's name, which distinguishes it among its siblings in the tree of Collabs.

parent: Parent

Internal (this/parent) use only.

This Collab's parent in the tree of Collabs.

runtime: IRuntime

The ambient IRuntime.

Use this to access utilities like IRuntime.replicaID.


  • canGC(): boolean
  • Internal (parent) use only.

    By default, this method returns true if canGC returns true on every child. Override to change this behavior.

    See Collab.canGC.

    Returns boolean

  • Internal (Collab.send) use only.

    Sends the given message on behalf of child. In general, this parent is then responsible for delivering the given message to Collab.receive on each replica of child, with guarantees set by the runtime.


    Returns void

  • emit<K>(eventName: K, event: Events[K] & CollabEvent, options?: { skipAnyEvent?: boolean }): void
  • Emits an event, which triggers all the registered event handlers.

    See CollabEventsRecord for advice on what events to emit.

    This is a wrapper around EventEmitter.emit that forces events to extend CollabEvent and also emits an "Any" event.

    Type Parameters

    • K extends string | number | symbol


    • eventName: K
    • event: Events[K] & CollabEvent
    • Optional options: { skipAnyEvent?: boolean }
      • Optional skipAnyEvent?: boolean

        Set to true to skip emitting an "Any" event.

    Returns void

  • finalize(): void
  • Internal (parent) use only.

    By default, this methods calls finalize on every child. Override to change this behavior, e.g., to add your own finalization steps (but consider calling super.finalize()).

    Returns void

  • fromID<C>(id: CollabID<C>, startIndex?: number): undefined | C
  • Inverse of idOf.

    Specifically, given a CollabID returned by idOf on some replica of this parent, returns this replica's copy of the original descendant. If that descendant does not exist (e.g., it was deleted or it is not present in this program version), returns undefined.

    Type Parameters


    • id: CollabID<C>
    • Optional startIndex: number

    Returns undefined | C

  • Returns a CollabID for the given strict descendant of this parent.

    The CollabID may be passed to fromID on any replica of this parent (but not other parents) to obtain that replica's copy of descendant.

    Type Parameters


    • descendant: C

    Returns CollabID<C>

  • Internal (parent) use only.

    Called by this Collab's parent to load saved state. See Collab.load.

    A CObject subclass may override this method to load additional state from Collab.save or to perform extra setup - e.g., refreshing functional views that were not automatically updated by children's load events. It is recommended to do so as follows:

    load(savedStateTree: SavedStateTree | null, meta: SavedStateMeta) {
    super.load(savedStateTree, meta);
    // Process your extra saved state from savedStateTree.self.
    const savedState = savedStateTree === null? null: savedStateTree.self!;
    // Perform extra setup as needed.


    Returns void

  • on<K>(eventName: K, handler: ((event: Events[K], caller: CObject<Events>) => void), options?: { once?: boolean }): (() => void)
  • Registers an event handler that is triggered when the event happens.

    Type Parameters

    • K extends string | number | symbol


    • eventName: K

      Name of the event to listen on.

    • handler: ((event: Events[K], caller: CObject<Events>) => void)

      Callback that handles the event.

        • (event: Events[K], caller: CObject<Events>): void
        • Parameters

          • event: Events[K]
          • caller: CObject<Events>

          Returns void

    • Optional options: { once?: boolean }
      • Optional once?: boolean

        If true, the event handler is triggered at most once (the next time the event happens), then unsubscribed.

    Returns (() => void)

    An "off" function that removes the event handler when called.

      • (): void
      • Registers an event handler that is triggered when the event happens.

        Returns void

        An "off" function that removes the event handler when called.

  • receive(messageStack: (string | Uint8Array)[], meta: MessageMeta): void
  • Internal (parent) use only.

    Receives a message sent by send on a local or remote replica of this Collab.

    This method processes the message, changes the local state accordingly, and emits events describing the local changes.

    This method should make assumptions and ensure consistency guarantees appropriate to its use case. For example, CRDTs may assume eventual, exactly-once, causal-order message delivery, and they must ensure strong eventual consistency.


    Returns void

  • registerCollab<C>(name: string, collabCallback: ((init: InitToken) => C)): C
  • Registers a Collab property of this CObject with the given name, making it one of our children.

    Typically, you will call this method during the constructor in the style:

    this.foo = this.registerCollab("foo", (init) => new FooClass(init, constructor args...));

    where readonly foo: FooClass; is a Collab property. See Data Modeling for examples.

    Registrations must be identical across all replicas.

    See also: CRuntime.registerCollab.

    Type Parameters


    • name: string

      A name for this property, unique among this class's registerCollab calls. We recommend using the same name as the property, but you can also use short strings to reduce network usage ("", "0", "1", ...).

    • collabCallback: ((init: InitToken) => C)

      A callback that uses the given InitToken to construct the registered Collab.

    Returns C

    The registered Collab.

  • Internal (parent) use only.

    Returns saved state describing the current state of this CObject. See Collab.save.

    A CObject subclass may override this method to save additional state. It is recommended to do so as follows:

    save() {
    const ans = super.save();
    // Put your extra saved state in ans.self, which is otherwise unused.
    ans.self = <subclass's saved state>;
    return ans;

    Returns SavedStateTree

  • send(messageStack: (string | Uint8Array)[], metaRequests: MetaRequest[]): void
  • Broadcasts a message to other replicas of this Collab. The message will be delivered to all replicas' receive, including locally.

    For convenience, the message may be expressed as a stack of (Uint8Array | string), instead of just a single Uint8Array. This is useful for parents sending messages on behalf of their children; see the implementation of CObject for an example.


    • messageStack: (string | Uint8Array)[]

      The message to send, in the form of a stack of Uint8Arrays. Note that this method may mutate it in-place.

    • metaRequests: MetaRequest[]

      A stack of metadata requests. The runtime will use the union of these when creating the MessageMeta for receive. Note that the stack need not align with messageStack, and this method may mutate it in place.

    Returns void

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