EventSource extension type

The EventSource interface is web content's interface to server-sent events.

An EventSource instance opens a persistent connection to an HTTP server, which sends events in text/event-stream format. The connection remains open until closed by calling EventSource.close.

Once the connection is opened, incoming messages from the server are delivered to your code in the form of events. If there is an event field in the incoming message, the triggered event is the same as the event field value. If no event field is present, then a generic EventSource.message_event event is fired.

Unlike WebSockets, server-sent events are unidirectional; that is, data messages are delivered in one direction, from the server to the client (such as a user's web browser). That makes them an excellent choice when there's no need to send data from the client to the server in message form. For example, EventSource is a useful approach for handling things like social media status updates, news feeds, or delivering data into a client-side storage mechanism like IndexedDB or web storage.

Warning: When not used over HTTP/2, SSE suffers from a limitation to the maximum number of open connections, which can be specially painful when opening various tabs as the limit is per browser and set to a very low number (6). The issue has been marked as "Won't fix" in Chrome and Firefox. This limit is per browser + domain, so that means that you can open 6 SSE connections across all of the tabs to www.example1.com and another 6 SSE connections to www.example2.com. (from Stackoverflow). When using HTTP/2, the maximum number of simultaneous HTTP streams is negotiated between the server and the client (defaults to 100).

API documentation sourced from MDN Web Docs.

on

JSObject

Implemented types

Available extensions

Constructors

EventSource(String url, [EventSourceInit eventSourceInitDict]): factory

Properties

hashCode → int: The hash code for this object.
no setterinherited
onerror ↔ EventHandler?: getter/setter pair
onmessage ↔ EventHandler?: getter/setter pair
onopen ↔ EventHandler?: getter/setter pair
readyState → int: The readyState read-only property of the EventSource interface returns a number representing the state of the connection.
no setter
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited
url → String: The url read-only property of the EventSource interface returns a string representing the URL of the source.
no setter
withCredentials → bool: The withCredentials read-only property of the EventSource interface returns a boolean value indicating whether the EventSource object was instantiated with CORS credentials set.
no setter

Methods

addEventListener(String type, EventListener? callback, [JSAny options]) → void: The addEventListener() method of the EventTarget interface sets up a function that will be called whenever the specified event is delivered to the target.
inherited
close() → void: The close() method of the EventSource interface closes the connection, if one is made, and sets the EventSource.readyState attribute to 2 (closed).
dispatchEvent(Event event) → bool: The dispatchEvent() method of the EventTarget sends an Event to the object, (synchronously) invoking the affected event listeners in the appropriate order. The normal event processing rules (including the capturing and optional bubbling phase) also apply to events dispatched manually with dispatchEvent().
inherited
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
removeEventListener(String type, EventListener? callback, [JSAny options]) → void: The removeEventListener() method of the EventTarget interface removes an event listener previously registered with EventTarget.addEventListener from the target. The event listener to be removed is identified using a combination of the event type, the event listener function itself, and various optional options that may affect the matching process; see Matching event listeners for removal.
inherited
toString() → String: A string representation of this object.
inherited

Operators

operator ==(Object other) → bool: The equality operator.
inherited

Constants

CLOSED → const int
CONNECTING → const int
OPEN → const int