Channels

The Channels object, accessed from the realtime library client constructor, is used to create and destroy Channel objects. It exposes the following public methods:

Channel get(String channelName, ChannelOptions channelOptions)

Creates a new Channel object if none for the channel exists, or returns the existing channel object.

release(String channelName)

Releases a Channel object, deleting it and enabling it to be garbage collected. It also removes any listeners associated with the channel.

The Channel object, created via the Channels object, is used to interact with a specific channel.

The Channel object exposes the following public properties:

The current ChannelState of this Channel. See the supported channel states for more information.

When a channel failure occurs this attribute is populated.

The type is ErrorInfo.

The name String unique to this channel.

Provides access to the Presence object for this channel which can be used to access members present on the channel, or participate in presence.

Provides access to the PushChannel object for this channel which can be used to access members present on the channel, or participate in presence.

There are two overloaded versions of this method:

publish(String name, Object data, callback(ErrorInfo err))

Publish a single message on this channel based on a given event name and payload. A callback may optionally be passed in to this call to be notified of success or failure of the operation. When publish is called with this client library, it won’t attempt to implicitly attach to the channel, so long as transient publishing is available in the library. Otherwise, the client will implicitly attach.

publish(Object[] messages, callback(ErrorInfo err))

Publish several messages on this channel. A callback may optionally be passed in to this call to be notified of success or failure of the operation. If the channel is initialized (i.e. no attempt to attach has yet been made for this channel), then calling publish will implicitly attach the channel.

The entire messages array is published atomically. This means that:

• Either they will all be successfully published or none of them will
• The max message size limit applies to the total size of all messages in the array
• The publish will only count as a single message for the purpose of per-channel rate limit
• If you are using client-specified message IDs, they must conform to certain restrictions

Parameters

name

event name for the published messageType: String

message

A message object to publishType: Message

messages

An array of message objects to publishType: Message []

There are overloaded versions of this method:

subscribe(listener(Message))

Subscribe to messages on this channel. The caller supplies a listener function, which is called each time one or more messages arrives on the channel.

subscribe(String name, listener(Message))

Subscribe to messages with a given event name on this channel. The caller supplies a listener function, which is called each time one or more matching messages arrives on the channel.

Parameters

name

The event name to subscribe toType: String

Considerations

• If the channel is initialized (i.e. no attempt to attach has yet been made for this channel), then calling subscribe will implicitly attach the channel. However, regardless of the implicit attach outcome, the listener will still be registered
• Calling subscribe with an event name is a convenient way to subscribe only to messages matching that event name. But this is purely a clientside filter, for convenience. It has no effect on what messages are actually sent by the server to the client. All clients attached to a given channel receive every message sent on that channel (that their capabilities allow them to receive); the channel is the unit of distribution.
• If subscribe is called more than once with the same listener, then duplicates will be registered. For example, if you subscribe twice with the same listener and a message is later received, the same listener will be invoked twice
• The registered listener remains active on the channel regardless of the current channel state. For example, if you call subscribe when a channel is attached and it later becomes detached or even failed, when the channel is reattached and a message is received, the listeners originally registered will still be invoked. Listeners are only removed when calling unsubscribe or when a channel is released using the Realtime.channels.release(name) method
• If an exception is thrown in the subscribe listener and bubbles up to the event emitter, it will be caught and logged at error level, so as not to affect other listeners for the same event

Parameters

name

The event name to unsubscribe fromType: String

history(Object options, callback(ErrorInfo err, PaginatedResult<Message> resultPage))

Gets a paginated set of historical messages for this channel. If the channel is configured to persist messages to disk, then message history will typically be available for 24 – 72 hours. If not, messages are only retained in memory by the Ably service for two minutes.

Parameters

options

an optional object containing the query parameters, as specified in the message history API documentationType: Object

Further details of the supported options params, see message history API documentation.

attach(callback(ErrorInfo err))

Attach to this channel ensuring the channel is created in the Ably system and all messages published on the channel will be received by any channel listeners registered using subscribe(). Any resulting channel state change will be emitted to any listeners registered using the on or once methods.

As a convenience, attach() will be called implicitly if subscribe() for the Channel is called, or enter() or subscribe() is called on the Presence for this Channel.

Parameters

detach(callback(ErrorInfo err))

Detach from this channel. Any resulting channel state change will be emitted to any listeners registered using the on or once methods.

Please note: Once all clients globally have detached from the channel, the channel will be released in the Ably service within two minutes.

Parameters

There are two overloaded versions of this method:

on(String event, listener(ChannelStateChange stateChange))

Register the given listener for the specified ChannelEvent on the Channel.

The listener is passed a ChannelStateChange object that contains the current state, previous state, a boolean indicating whether the channel was resumed, and an optional reason for the state change.

on(listener(ChannelStateChange stateChange))

Register the given listener for all ChannelEvents on the Channel.

The listener is passed a ChannelStateChange object that contains the current state, previous state, the event, a boolean indicating whether the channel was resumed, and an optional reason for the state change.

If an exception is thrown in the listener and bubbles up to the event emitter, it will be caught and logged at error level, so as not to affect other listeners for the same event

Parameters

There are two overloaded versions of this method:

once(String event, listener(ChannelStateChange stateChange))

Register the given listener for a single occurrence of the specified ChannelEvent on the Channel. Once the listener has been called, it is removed as a registered listener and will not be called again.

The listener is passed a ChannelStateChange object that contains the current state, previous state, the event, a boolean indicating whether the channel was resumed, and an optional reason for the event change.

once(listener(ChannelStateChange stateChange))

Register the given listener for a single occurrence of any ChannelEvent on the Channel. Once the listener has been called, it is removed as a registered listener and will not be called again.

The listener is passed a ChannelStateChange object that contains the current state, previous state, the event, a boolean indicating whether the channel was resumed, and an optional reason for the event change.

If an exception is thrown in the listener and bubbles up to the event emitter, it will be caught and logged at error level, so as not to affect other listeners for the same event

Parameters

There are two overloaded versions of this method:

off(String event, listener)

Remove the given listener for the ChannelEvent.

off(listener)

Remove the given listener for all ChannelEvents.

Parameters

A ChannelStateChange is a type encapsulating state change information emitted by the Channel object. See Channel#on to register a listener for one or more events.

Attributes

current

the new current stateType: ChannelState

previous

the previous state. (for the update event, this will be equal to the current state)Type: ChannelState

event

the event that triggered this state changeType: ChannelEvent

reason

an ErrorInfo containing any information relating to the transitionType: ErrorInfo

resumed

a boolean indicated whether message continuity on this channel is preserved, see Nonfatal channel errors for more info.Type: Boolean

A Message represents an individual message that is sent to or received from Ably.

Properties

name

Event name, if providedType: String

data

The presence update payload, if providedType:

extras

Metadata and/or ancillary payloads, if provided. The only currently valid payloads for extras are the push, ref and privileged objects.Type:

id

Unique ID assigned by Ably to this message. Can optionally be assigned by the client as part of idempotent publishingType: String

clientId

The client ID of the publisher of this messageType: String

connectionId

The connection ID of the publisher of this messageType: String

timestamp

Timestamp when the message was received by the Ably service, as milliseconds since the epochType: Integer

encoding

This will typically be empty as all messages received from Ably are automatically decoded client-side using this value. However, if the message encoding cannot be processed, this attribute will contain the remaining transformations not applied to the data payloadType: String

Message.fromEncoded(Object encodedMsg, ChannelOptions channelOptions?) → Message

A static factory method to create a Message from a deserialized Message-like object encoded using Ably’s wire protocol.

Parameters

encodedMsg

a Message-like deserialized object.Type: Object

channelOptions

an optional ChannelOptions. If you have an encrypted channel, use this to allow the library can decrypt the data.Type: Object

Returns

A Message object

Message.fromEncodedArray(Object[] encodedMsgs, ChannelOptions channelOptions?) → Message[]

A static factory method to create an array of Messages from an array of deserialized Message-like object encoded using Ably’s wire protocol.

Parameters

encodedMsgs

an array of Message-like deserialized objects.Type: Array

channelOptions

an optional ChannelOptions. If you have an encrypted channel, use this to allow the library can decrypt the data.Type: Object

Returns

An Array of Message objects

Channel options are used for configuring encryption.

Properties

A PaginatedResult is a type that represents a page of results for all message and presence history, stats and REST presence requests. The response from a Ably REST API paginated query is accompanied by metadata that indicates the relative queries available to the PaginatedResult object.

Properties

items

contains the current page of results (for example an Array of Message or PresenceMessage objects for a channel history request)Type: Array <Message, Presence, Stats>

Methods

first(callback(ErrorInfo err, PaginatedResult resultPage))

Returns a new PaginatedResult for the first page of results.

Boolean hasNext()

Returns true if there are more pages available by calling next and returns false if this page is the last page available.

Boolean isLast()

Returns true if this page is the last page and returns false if there are more pages available by calling next available.

next(callback(ErrorInfo err, PaginatedResult resultPage))

Returns a new PaginatedResult loaded with the next page of results. If there are no further pages, then null is returned.

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channel.history(function(err, paginatedResult) {
  console.log('Page 0 item 0:' + paginatedResult.items[0].data);
  paginatedResult.next(function(err, nextPage) {
    console.log('Page 1 item 1: ' + nextPage.items[1].data);
    console.log('Last page?: ' + nextPage.isLast());
  });
});

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