This specification defines an API to enable Web content to access presentation displays and use them for presenting Web content.
Since publication as Candidate Recommendation on 14 July
2016, the Working Group updated most algorithms in the spec to fix
issues identified through testing and implementation feedback.
Interfaces defined in this document did not change, except
PresentationConnectionClosedReason and
PresentationConnectionClosedEvent, which were renamed to
improve consistency with the rest of the Web platform. The API was also
restricted to secure contexts. See the list of changes for details.
No feature has been identified as being at risk.
The Second Screen Working Group will refine the test suite for the Presentation API during the Candidate Recommendation period and update the preliminary implementation report. For this specification to advance to Proposed Recommendation, two independent, interoperable implementations of each feature must be demonstrated, as detailed in the Candidate Recommendation exit criteria section.
The Presentation API aims to make presentation displays such as projectors, attached monitors, and network-connected TVs available to the Web. It takes into account displays that are attached using wired (HDMI, DVI, or similar) and wireless technologies (Miracast, Chromecast, DLNA, AirPlay, or similar).
Devices with limited screen size lack the ability to show Web content to a larger audience: a group of colleagues in a conference room, or friends and family at home, for example. Web content shown on a larger presentation display has greater perceived quality, legibility, and impact.
At its core, the Presentation API enables a controller page to show a presentation page on a presentation display and exchange messages with it. How the presentation page is transmitted to the display and how messages are exchanged between it and the controller page are left to the implementation; this allows the use of a wide variety of display technologies.
For example, if the presentation display is connected by HDMI or Miracast, which only allow audio and video to be transmitted, the user agent (UA) hosting the controller will also render the presentation. It then uses the operating system to send the resulting graphical and audio output to the presentation display. We refer to this situation as the 1-UA mode implementation of the Presentation API. The only requirements are that the user agent is able to send graphics and audio from rendering the presentation to the presentation display, and exchange messages internally between the controller and presentation pages.
If the presentation display is able to render HTML natively and communicate with the controller via a network, the user agent hosting the controller does not need to render the presentation. Instead, the user agent acts as a proxy that requests the presentation display to load and render the presentation page itself. Message exchange is done over a network connection between the user agent and the presentation display. We refer to this situation as the 2-UA mode implementation of the Presentation API.
The Presentation API is intended to be used with user agents that attach to presentation displays in 1-UA mode, 2-UA mode, and possibly other means not listed above. To improve interoperability between user agents and presentation displays, standardization of network communication between browsers and displays is being considered in the Second Screen Community Group.
Use cases and requirements are captured in a separate Presentation API Use Cases and Requirements document.
Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and terminate these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)
This specification describes the conformance criteria for two classes of user agents.
Web browsers that conform to the specifications of a
controlling user agent must be able to start and control
presentations by providing a controlling browsing context
as described in this specification. This context implements the
Presentation,
PresentationAvailability,
PresentationConnection,
PresentationConnectionAvailableEvent,
PresentationConnectionCloseEvent, and
PresentationRequest interfaces.
Web browsers that conform to the specifications of a receiving
user agent must be able to render presentations by providing
a receiving browsing context as described in this
specification. This context implements the
Presentation,
PresentationConnection,
PresentationConnectionAvailableEvent,
PresentationConnectionCloseEvent,
PresentationConnectionList, and
PresentationReceiver interfaces.
One user agent may act both as a controlling user agent and as a receiving user agent, if it provides both browsing contexts and implements all of their required interfaces. This can happen when the same user agent is able to host the controlling browsing context and the receiving browsing context for a presentation, as in the 1-UA mode implementation of the API.
Conformance requirements phrased against a user agent apply either to a controlling user agent, a receiving user agent or to both classes, depending on the context.
The following terms are defined in [[!HTML51]]:
EventHandler
Navigator
The term JavaScript realm is defined in [[!ECMASCRIPT]].
The terms EventTarget,
Event,
EventInit
are defined in [[!DOM]].
The term MessageEvent
is defined in [[!WEBMESSAGING]].
This document provides interface definitions using the Web IDL standard [[!WEBIDL-2]].
The terms throw, Promise, ArrayBuffer, ArrayBufferView, and the following exception names are defined in [[!WEBIDL-2]]:
InvalidAccessError
NotFoundError
NotSupportedError
OperationError
SecurityError
SyntaxError
NotAllowedError
The terms resolving a Promise and rejecting a Promise are used as explained in [[!PROMGUIDE]].
The term URL is defined in the WHATWG URL standard [[!URL]].
The term Blob
is defined in the File API specification [[!FILEAPI]].
The header Accept-Language is defined in HTTP/1.1 [[!rfc7231]].
HTTP authentication is defined in HTTP/1.1: Authentication [[!rfc7235]].
The term RTCDataChannel
is defined in the WebRTC API specification [[WEBRTC]].
The term cookie store is defined in RFC 6265 [[!COOKIES]].
The term UUID is defined in RFC 4122 [[!rfc4122]].
The terms permission descriptor type and permission state are defined in [[!PERMISSIONS]].
The term database is defined in [[!INDEXEDDB]].
The terms local storage areas and session storage areas are defined in [[!WEBSTORAGE]].
The term a priori authenticated URL is defined in [[!MIXED-CONTENT]].
The terms service worker, a list of registered service worker registrations, caches, window client and worker client are defined in [[!SERVICE-WORKERS]].
The term DIAL is defined in [[DIAL]].
This section shows example codes that highlight the usage of main
features of the Presentation API. In these examples,
controller.html implements the controller and
presentation.html implements the presentation. Both pages
are served from the domain https://example.org
(https://example.org/controller.html and
https://example.org/presentation.html). These examples
assume that the controlling page is managing one presentation at a
time. Please refer to the comments in the code examples for further
details.
<!-- controller.html -->
<button id="presentBtn" style="display: none;">Present</button>
<script>
// The Present button is visible if at least one presentation display is available
var presentBtn = document.getElementById("presentBtn");
// It is also possible to use relative presentation URL e.g. "presentation.html"
var presUrls = ["https://example.com/presentation.html",
"https://example.net/alternate.html"];
// show or hide present button depending on display availability
var handleAvailabilityChange = function(available) {
presentBtn.style.display = available ? "inline" : "none";
};
// Promise is resolved as soon as the presentation display availability is
// known.
var request = new PresentationRequest(presUrls);
request.getAvailability().then(function(availability) {
// availability.value may be kept up-to-date by the controlling UA as long
// as the availability object is alive. It is advised for the Web developers
// to discard the object as soon as it's not needed.
handleAvailabilityChange(availability.value);
availability.onchange = function() { handleAvailabilityChange(this.value); };
}).catch(function() {
// Availability monitoring is not supported by the platform, so discovery of
// presentation displays will happen only after request.start() is called.
// Pretend the devices are available for simplicity; or, one could implement
// a third state for the button.
handleAvailabilityChange(true);
});
</script>
<!-- controller.html -->
<script>
presentBtn.onclick = function () {
// Start new presentation.
request.start()
// The connection to the presentation will be passed to setConnection on
// success.
.then(setConnection);
// Otherwise, the user canceled the selection dialog or no screens were
// found.
};
</script>
<!-- controller.html -->
<button id="reconnectBtn" style="display: none;">Reconnect</button>
<script>
var reconnect = function () {
// read presId from localStorage if exists
var presId = localStorage["presId"];
// presId is mandatory when reconnecting to a presentation.
if (!!presId) {
request.reconnect(presId)
// The new connection to the presentation will be passed to
// setConnection on success.
.then(setConnection);
// No connection found for presUrl and presId, or an error occurred.
}
};
// On navigation of the controller, reconnect automatically.
document.addEventListener("DOMContentLoaded", reconnect);
// Or allow manual reconnection.
const reconnectBtn = document.querySelector("#reconnectBtn");
reconnectBtn.onclick = reconnect;
</script>
<!-- controller.html -->
<!-- Setting presentation.defaultRequest allows the page to specify the
PresentationRequest to use when the controlling UA initiates a
presentation. -->
<script>
navigator.presentation.defaultRequest = new PresentationRequest(presUrls);
navigator.presentation.defaultRequest.onconnectionavailable = function(evt) {
setConnection(evt.connection);
};
</script>
<!-- controller.html -->
<button id="disconnectBtn" style="display: none;">Disconnect</button>
<button id="stopBtn" style="display: none;">Stop</button>
<script>
let connection;
// The Disconnect and Stop buttons are visible if there is a connected presentation
const stopBtn = document.querySelector("#stopBtn");
const disconnectBtn = document.querySelector("#disconnectBtn");
stopBtn.onclick = _ => {
connection && connection.terminate();
};
disconnectBtn.onclick = _ => {
connection && connection.close();
};
function setConnection(newConnection) {
// Disconnect from existing presentation, if not attempting to reconnect
if (connection && connection != newConnection && connection.state != 'closed') {
connection.onclosed = undefined;
connection.close();
}
// Set the new connection and save the presentation ID
connection = newConnection;
localStorage["presId"] = connection.id;
function showConnectedUI() {
// Allow the user to disconnect from or terminate the presentation
stopBtn.style.display = "inline";
disconnectBtn.style.display = "inline";
reconnectBtn.style.display = "none";
}
function showDisconnectedUI() {
disconnectBtn.style.display = "none";
stopBtn.style.display = "none";
reconnectBtn.style.display = localStorage["presId"] ? "inline" : "none";
}
// Monitor the connection state
connection.onconnect = _ => {
showConnectedUI();
// Register message handler
connection.onmessage = message => {
console.log(`Received message: ${message.data}`);
};
// Send initial message to presentation page
connection.send("Say hello");
};
connection.onclose = _ => {
connection = null;
showDisconnectedUI();
};
connection.onterminate = _ => {
// Remove presId from localStorage if exists
delete localStorage["presId"];
connection = null;
showDisconnectedUI();
};
};
</script>
<!-- presentation.html -->
<script>
var addConnection = function(connection) {
this.onmessage = function (message) {
if (message.data == "say hello")
this.send("hello");
};
};
navigator.presentation.receiver.connectionList.then(function (list) {
list.connections.map(function (connection) {
addConnection(connection);
});
list.onconnectionavailable = function (evt) {
addConnection(evt.connection);
};
});
</script>
<!-- controller.html -->
<script>
connection.send("{string: '你好，世界!', lang: 'zh-CN'}");
connection.send("{string: 'こんにちは、世界!', lang: 'ja'}");
connection.send("{string: '안녕하세요, 세계!', lang: 'ko'}");
connection.send("{string: 'Hello, world!', lang: 'en-US'}");
</script>
<!-- presentation.html -->
<script>
connection.onmessage = function (message) {
var messageObj = JSON.parse(message.data);
var spanElt = document.createElement("SPAN");
spanElt.lang = messageObj.lang;
spanElt.textContent = messageObj.string;
document.appendChild(spanElt);
};
</script>
A presentation display refers to a graphical and/or audio output device available to the user agent via an implementation specific connection technology.
A presentation connection is an object relating a controlling browsing context to its receiving browsing context and enables two-way-messaging between them. Each presentation connection has a presentation connection state, a unique presentation identifier to distinguish it from other presentations, and a presentation URL that is a URL used to create or reconnect to the presentation. A valid presentation identifier consists of alphanumeric ASCII characters only and is at least 16 characters long.
Some presentation displays may only be able to display a subset of Web content because of functional, security or hardware limitations. Examples are set-top boxes, smart TVs, or networked speakers capable of rendering only audio. We say that such a display is an available presentation display for a presentation URL if the controlling user agent can reasonably guarantee that presentation of the URL on that display will succeed.
A
controlling browsing context (or controller
for short) is a browsing context that has connected to a
presentation by calling
start or reconnect, or received a presentation
connection via a connectionavailable event. In algorithms
for PresentationRequest, the controlling
browsing context is the responsible browsing context whose
JavaScript realm was used to construct the
PresentationRequest.
The receiving browsing context (or presentation for short) is the browsing context responsible for rendering to a presentation display. A receiving browsing context can reside in the same user agent as the controlling browsing context or a different one. A receiving browsing context is created by following the steps to create a receiving browsing context.
In a procedure, the destination browsing context is the receiving browsing context when the procedure is initiated at the controlling browsing context, or the controlling browsing context if it is initiated at the receiving browsing context.
The set of controlled presentations, initially empty, contains the presentation connections created by the controlling browsing contexts for the controlling user agent (or a specific user profile within that user agent). The set of controlled presentations is represented by a list of PresentationConnection objects that represent the underlying presentation connections. Several PresentationConnection objects may share the same presentation URL and presentation identifier in that set, but there can be only one PresentationConnection with a specific presentation URL and presentation identifier for a given controlling browsing context.
The set of presentation controllers, initially empty, contains the presentation connections created by a receiving browsing context for the receiving user agent. The set of presentation controllers is represented by a list of PresentationConnection objects that represent the underlying presentation connections. All presentation connections in this set share the same presentation URL and presentation identifier.
In a receiving browsing context, the presentation
controllers monitor, initially set to null,
exposes the current set of presentation controllers to the
receiving application. The presentation controllers monitor is
represented by a PresentationConnectionList.
In a receiving browsing context, the presentation
controllers promise, which is initially set to
null, provides the presentation controllers
monitor once the initial presentation connection is
established. The presentation controllers promise is
represented by a Promise that resolves with the
presentation controllers monitor.
In a controlling browsing context, the default
presentation request, which is initially set to
null, represents the request to use when the user wishes
to initiate a presentation connection from the browser chrome.
The task source for the tasks mentioned in this specification is the presentation task source.
Unless otherwise specified, the JavaScript realm for script objects constructed by algorithm steps is the current realm.
partial interface Navigator {
[SecureContext, SameObject] readonly attribute Presentation presentation;
};
[SecureContext]
interface Presentation {
};
The presentation attribute is
used to retrieve an instance of the Presentation
interface. It MUST return the Presentation
instance.
Controlling user agents MUST implement the following partial interface:
partial interface Presentation {
attribute PresentationRequest? defaultRequest;
};
The defaultRequest
attribute MUST return the default presentation request if
any, null otherwise. On setting, the default
presentation request MUST be set to the new value.
The controlling user agent SHOULD initiate presentation using the default presentation request only when the user has expressed an intention to do so via a user gesture, for example by clicking a button in the browser chrome.
To initiate presentation using the default presentation request, the controlling user agent MUST follow the steps to start a presentation from a default presentation request.
Support for initiating a presentation using the default presentation request is OPTIONAL.
Receiving user agents MUST implement the following partial interface:
partial interface Presentation {
readonly attribute PresentationReceiver? receiver;
};
The receiver attribute
MUST return the PresentationReceiver instance
associated with the receiving browsing context and created
by the receiving user agent when the receiving browsing
context is created. In any other
browsing context (including nested browsing contexts of the
receiving browsing context) it MUST return
null.
Web developers can use navigator.presentation.receiver to detect when a document is loaded as a presentation.
[SecureContext,
Constructor(USVString url),
Constructor(sequence<USVString> urls)]
interface PresentationRequest : EventTarget {
Promise<PresentationConnection> start();
Promise<PresentationConnection> reconnect(USVString presentationId);
Promise<PresentationAvailability> getAvailability();
attribute EventHandler onconnectionavailable;
};
A PresentationRequest object is associated with a
request to initiate or reconnect to a presentation made by a
controlling browsing context. The
PresentationRequest object MUST be implemented in
a controlling browsing context provided by a controlling
user agent.
When a PresentationRequest is constructed, the
given urls MUST be used as the list of presentation request URLs which are
each a possible presentation URL for the
PresentationRequest instance.
When the PresentationRequest constructor is called, the controlling user agent MUST run these steps:
When the start
method is called, the user agent MUST run the following
steps to select a presentation display.
When the user expresses an intent to start presentation of a document on a presentation display using the browser chrome (via a dedicated button, user gesture, or other signal), that user agent MUST run the following steps to start a presentation from a default presentation request. If no default presentation request is set on the document, these steps MUST not be run.
null value
of navigator.presentation.defaultRequest
set on W
When the user agent is to start a presentation connection, it MUST run the following steps:
http
or https schemes; behavior for other schemes is not
defined by this specification.
When the reconnect method is called, the user agent MUST run the following steps to reconnect to a presentation:
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the PresentationRequest interface:
| Event handler | Event handler event type |
|---|---|
onconnectionavailable
|
connectionavailable
|
[SecureContext]
interface PresentationAvailability : EventTarget {
readonly attribute boolean value;
attribute EventHandler onchange;
};
A PresentationAvailability object exposes the
presentation display availability for a presentation request.
The presentation display availability for a
PresentationRequest stores whether there is currently any
available presentation display for at least one of the
presentation request URLs of the request.
The presentation display availability for a presentation request is eligible for garbage collection when no ECMASCript code can observe the PresentationAvailability object.
If the controlling user agent can monitor the list of
available presentation displays in the background (without a
pending request to start), the
PresentationAvailability object MUST be
implemented in a controlling browsing context.
The value attribute MUST return the last value it was set to. The value is initialized and updated by the monitor the list of available presentation displays algorithm.
The onchange attribute is
an event handler whose corresponding event handler event
type is change.
The user agent MUST keep track of the set of presentation availability objects created by the getAvailability method. The set of presentation availability objects is represented as a set of tuples (A, availabilityUrls), initially empty, where:
The user agent MUST keep a list of available presentation displays. The list of available presentation displays is represented by a list of tuples (availabilityUrl, display). An entry in this list means that display is currently an available presentation display for availabilityUrl. This list of presentation displays may be used for starting new presentations, and is populated based on an implementation specific discovery mechanism. It is set to the most recent result of the algorithm to monitor the list of available presentation displays.
While the set of presentation availability objects is not empty, the user agent MAY monitor the list of available presentation displays continuously, so that pages can use the value property of a PresentationAvailability object to offer presentation only when there are available displays. However, the user agent may not support continuous availability monitoring in the background; for example, because of platform or power consumption restrictions. In this case the Promise returned by getAvailability is rejected, and the algorithm to monitor the list of available presentation displays will only run as part of the select a presentation display algorithm.
When the set of presentation availability objects is empty (that is, there are no availabilityUrls being monitored), user agents SHOULD NOT monitor the list of available presentation displays to satisfy the power saving non-functional requirement. To further save power, the user agent MAY also keep track of whether a page holding a PresentationAvailability object is in the foreground. Using this information, implementation specific discovery of presentation displays can be resumed or suspended.
When the getAvailability method is called, the user agent MUST run the following steps:
null, then:
If the set of presentation availability objects is non-empty, or there is a pending request to select a presentation display, the user agent MUST monitor the list of available presentation displays by running the following steps:
null, then
run the following substeps:
false.
true.
When a presentation display availability object is eligible for garbage collection, the user agent SHOULD run the following steps:
[SecureContext,
Constructor(DOMString type, PresentationConnectionAvailableEventInit eventInitDict)]
interface PresentationConnectionAvailableEvent : Event {
[SameObject] readonly attribute PresentationConnection connection;
};
dictionary PresentationConnectionAvailableEventInit : EventInit {
required PresentationConnection connection;
};
A controlling user agent fires a trusted event
named connectionavailable on a
PresentationRequest when a connection associated with the
object is created. It is fired at the PresentationRequest
instance, using the PresentationConnectionAvailableEvent
interface, with the connection attribute set
to the PresentationConnection object that was
created. The event is fired for each connection that is created for
the controller, either by the controller calling
start or reconnect, or by the controlling user
agent creating a connection on the controller's behalf via
defaultRequest.
A receiving user agent fires a trusted event
named connectionavailable
on a PresentationReceiver when an incoming connection is
created. It is fired at the presentation controllers
monitor, using the PresentationConnectionAvailableEvent
interface, with the connection attribute set
to the PresentationConnection object that was
created. The event is fired for all connections that are created
when monitoring incoming presentation connections.
The connection attribute MUST return the value it was set to when the PresentationConnection object was created.
When the PresentationConnectionAvailableEvent constructor is called, the user agent MUST construct a new PresentationConnectionAvailableEvent object with its connection attribute set to the connection member of the PresentationConnectionAvailableEventInit object passed to the constructor.
Each presentation connection is represented by a PresentationConnection object. Both the controlling user agent and receiving user agent MUST implement PresentationConnection.
enum PresentationConnectionState { "connecting", "connected", "closed", "terminated" };
enum BinaryType { "blob", "arraybuffer" };
[SecureContext]
interface PresentationConnection : EventTarget {
readonly attribute USVString id;
readonly attribute USVString url;
readonly attribute PresentationConnectionState state;
void close();
void terminate();
attribute EventHandler onconnect;
attribute EventHandler onclose;
attribute EventHandler onterminate;
// Communication
attribute BinaryType binaryType;
attribute EventHandler onmessage;
void send (DOMString message);
void send (Blob data);
void send (ArrayBuffer data);
void send (ArrayBufferView data);
};
The id attribute specifies the
presentation connection's presentation identifier.
The url attribute specifies the
presentation connection's presentation URL.
The state attribute represents the
presentation connection's current state. It can take one of
the values of PresentationConnectionState depending on
the connection state:
When the close method is called on a PresentationConnection S, the user agent MUST start closing the presentation connection S with closed as closeReason and an empty message as closeMessage.
When the terminate method is called on a PresentationConnection S in a controlling browsing context, the user agent MUST run the algorithm to terminate a presentation in a controlling browsing context using S.
When the terminate method is called on a PresentationConnection S in a receiving browsing context, the user agent MUST run the algorithm to terminate a presentation in a receiving browsing context using S.
The binaryType attribute can take one of the values of BinaryType. When a PresentationConnection object is created, its binaryType attribute MUST be set to the string "arraybuffer". On getting, it MUST return the last value it was set to. On setting, the user agent MUST set the attribute to the new value.
arraybuffer". This attribute has no effect on data
sent in a string form.
When the send method is called on a PresentationConnection S, the user agent MUST run the algorithm to send a message through S.
When a PresentationConnection object S is discarded (because the document owning it is navigating or is closed) while the presentation connection state of S is connecting or connected, the user agent MUST start closing the presentation connection S with wentaway as closeReason and an empty closeMessage.
If the user agent receives a signal from the destination browsing context that a PresentationConnection S is to be closed, it MUST close the presentation connection S with closed or wentaway as closeReason and an empty closeMessage.
When the user agent is to establish a presentation connection using a presentation connection, it MUST run the following steps:
DOMString and binary payloads in a reliable
and in-order fashion as described in the Send a Message and
Receive a Message steps below.
RTCDataChannel in reliable mode.
Let presentation message data be the payload data to be
transmitted between two browsing contexts. Let presentation
message type be the type of that data, one of
text or binary.
When the user agent is to send a message through a presentation connection, it MUST run the following steps:
InvalidStateError exception.
binary if messageOrData is of type
ArrayBuffer, ArrayBufferView, or Blob. Let
messageType be text if
messageOrData is of type DOMString.
To assist applications in recovery from an error sending a message through a presentation connection, the user agent should include details of which attempt failed in closeMessage, along with a human readable string explaining the failure reason. Example renditions of closeMessage:
Unable to send text message (network_error):
"hello" for DOMString messages, where
"hello" is the first 256 characters of the failed
message.
Unable to send binary message (invalid_message)
for ArrayBuffer, ArrayBufferView and Blob
messages.
When the user agent has received a transmission from the remote side consisting of presentation message data and presentation message type, it MUST run the following steps to receive a message through a PresentationConnection:
text, then
initialize event's data attribute to
messageData with type DOMString.
binary, and
binaryType attribute is set to "blob", then initialize event's
data attribute to a new Blob object with
messageData as its raw data.
binary, and
binaryType attribute is set to "arraybuffer", then initialize
event's data attribute to a new
ArrayBuffer object whose contents are
messageData.
If the user agent encounters an unrecoverable error while receiving a message through presentationConnection, it MUST abruptly close the presentation connection presentationConnection with error as closeReason. It SHOULD use a human readable description of the error encountered as closeMessage.
enum PresentationConnectionCloseReason { "error", "closed", "wentaway" };
[SecureContext,
Constructor(DOMString type, PresentationConnectionCloseEventInit eventInitDict)]
interface PresentationConnectionCloseEvent : Event {
readonly attribute PresentationConnectionCloseReason reason;
readonly attribute DOMString message;
};
dictionary PresentationConnectionCloseEventInit : EventInit {
required PresentationConnectionCloseReason reason;
DOMString message = "";
};
A PresentationConnectionCloseEvent is fired when a presentation connection enters a closed state. The reason attribute provides the reason why the connection was closed. It can take one of the values of PresentationConnectionCloseReason:
When the reason attribute is error, the user agent SHOULD set the message attribute to a human readable description of how the communication channel encountered an error.
When the PresentationConnectionCloseEvent constructor is called, the user agent MUST construct a new PresentationConnectionCloseEvent object, with its reason attribute set to the reason member of the PresentationConnectionCloseEventInit object passed to the constructor, and its message attribute set to the message member of this PresentationConnectionCloseEventInit object if set, to an empty string otherwise.
When the user agent is to start closing a presentation connection, it MUST do the following:
When the user agent is to close a presentation connection, it MUST do the following:
When a controlling user agent is to terminate a presentation in a controlling browsing context using connection, it MUST run the following steps:
When any of the following occur, the receiving user agent MUST terminate a presentation in a receiving browsing context:
This could happen by an explicit user action, or as a policy of the user agent. For example, the receiving user agent could be configured to terminate presentations whose PresentationConnection objects are all closed for 30 minutes.
When a receiving user agent is to terminate a presentation in a receiving browsing context, it MUST run the following steps:
Only one termination confirmation needs to be sent per controlling user agent.
When a receiving user agent is to send a termination confirmation for a presentation P, and that confirmation was received by a controlling user agent, the controlling user agent MUST run the following steps:
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the PresentationConnection interface:
| Event handler | Event handler event type |
|---|---|
onmessage
|
message
|
onconnect
|
connect
|
onclose
|
close
|
onterminate
|
terminate
|
[SecureContext]
interface PresentationReceiver {
readonly attribute Promise<PresentationConnectionList> connectionList;
};
The PresentationReceiver interface allows a receiving browsing context to access the controlling browsing contexts and communicate with them. The PresentationReceiver interface MUST be implemented in a receiving browsing context provided by a receiving user agent.
On getting, the connectionList attribute MUST return the result of running the following steps:
null, return the presentation controllers promise
and abort all remaining steps.
null, resolve the presentation controllers
promise with the presentation controllers monitor.
When the user agent is to create a receiving browsing context, it MUST run the following steps:
"denied".
All nested browsing contexts created by the presented document, i.e. that have the receiving browsing context as their top-level browsing context, MUST also have restrictions 2-4 above. All of these browsing contexts MUST also share the same browsing state (storage) for features 5-10 listed above.
Window clients and worker clients associated with the receiving browsing context and its list of descendant browsing contexts must not be exposed to service workers associated with each other.
When the receiving browsing context is terminated, any service workers associated with it and the browsing contexts in its list of descendant browsing contexts MUST be unregistered and terminated. Any browsing state associated with the receiving browsing context and the browsing contexts in its list of descendant browsing contexts, including session history, the cookie store, any HTTP authentication state, the application cache, any databases, the session storage areas, the local storage areas, the list of registered service worker registrations and the caches MUST be discarded and not used for any other browsing context.
This algorithm is intended to create a well defined environment to allow interoperable behavior for 1-UA and 2-UA presentations, and to minimize the amount of state remaining on a presentation display used for a 2-UA presentation.
The receiving user agent SHOULD fetch resources in a receiving browsing context with an HTTP Accept-Language header that reflects the language preferences of the controlling user agent (i.e., with the same Accept-Language that the controlling user agent would have sent). This will help the receiving user agent render the presentation with fonts and locale-specific attributes that reflect the user's preferences.
Given the operating context of the presentation display, some Web APIs will not work by design (for example, by requiring user input) or will be obsolete (for example, by attempting window management); the receiving user agent should be aware of this. Furthermore, any modal user interface will need to be handled carefully. The sandboxed modals flag is set on the receiving browsing context to prevent most of these operations.
[SecureContext]
interface PresentationConnectionList : EventTarget {
readonly attribute FrozenArray<PresentationConnection> connections;
attribute EventHandler onconnectionavailable;
};
The connections attribute MUST return the non-terminated set of presentation connections in the set of presentation controllers.
When the receiving user agent is to start monitoring incoming presentation connections in a receiving browsing context from controlling browsing contexts, it MUST listen to and accept incoming connection requests from a controlling browsing context using an implementation specific mechanism. When a new connection request is received from a controlling browsing context, the receiving user agent MUST run the following steps:
null, run the following steps in parallel.
null, resolve the presentation
controllers promise with the presentation controllers
monitor.
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the PresentationConnectionList interface:
| Event handler | Event handler event type |
|---|---|
onconnectionavailable
|
connectionavailable
|
The change event fired on the PresentationAvailability object reveals one bit of information about the presence or absence of a presentation display, often discovered through the browser's local area network. This could be used in conjunction with other information for fingerprinting the user. However, this information is also dependent on the user's local network context, so the risk is minimized.
The API enables monitoring the list of available presentation displays. How the user agent determines the compatibility and availability of a presentation display with a given URL is an implementation detail. If a controlling user agent matches a presentation request URL to a DIAL application to determine its availability, this feature can be used to probe information about which DIAL applications the user has installed on the presentation display without user consent.
A presentation is allowed to be accessed across origins; the presentation URL and presentation identifier used to create the presentation are the only information needed to reconnect to a presentation from any origin in the controlling user agent. In other words, a presentation is not tied to a particular opening origin.
This design allows controlling contexts from different origins to connect to a shared presentation resource. The security of the presentation identifier prevents arbitrary origins from connecting to an existing presentation.
This specification also allows a receiving user agent to publish information about its set of controlled presentations, and a controlling user agent to reconnect to presentations started from other devices. This is possible when the controlling browsing context obtains the presentation URL and presentation identifier of a running presentation from the user, local storage, or a server, and then connects to the presentation via reconnect.
This specification makes no guarantee as to the identity of any party connecting to a presentation. Once connected, the presentation may wish to further verify the identity of the connecting party through application-specific means. For example, the presentation could challenge the controller to provide a token via send that the presentation uses to verify identity and authorization.
When the user is asked permission to use a presentation display during the steps to select a presentation display, the controlling user agent should make it clear what origin is requesting presentation and what origin will be presented.
Display of the origin requesting presentation will help the user understand what content is making the request, especially when the request is initiated from a nested browsing context. For example, embedded content may try to convince the user to click to trigger a request to start an unwanted presentation.
The sandboxed top-level navigation browsing context flag is set on the receiving browsing context to enforce that the top-level origin of the presentation remains the same during the lifetime of the presentation.
When a user starts a presentation, the user will begin with exclusive control of the presentation. However, the Presentation API allows additional devices (likely belonging to distinct users) to connect and thereby control the presentation as well. When a second device connects to a presentation, it is recommended that all connected controlling user agents notify their users via the browser chrome that the original user has lost exclusive access, and there are now multiple controllers for the presentation.
In addition, it may be the case that the receiving user agent is capable of receiving user input, as well as acting as a presentation display. In this case, the receiving user agent should notify its user via browser chrome when a receiving browsing context is under the control of a remote party (i.e., it has one or more connected controllers).
The presentation API abstracts away what "local" means for displays, meaning that it exposes network-accessible displays as though they were directly attached to the user's device. The Presentation API requires user permission for a page to access any display to mitigate issues that could arise, such as showing unwanted content on a display viewable by others.
The presentation URL and presentation identifier can be used to connect to a presentation from another browsing context. They can be intercepted if an attacker can inject content into the controlling page.
The content displayed on the presentation is different from the controller. In particular, if the user is logged in in both contexts, then logs out of the controlling browsing context, she will not be automatically logged out from the receiving browsing context. Applications that use authentication should pay extra care when communicating between devices.
The set of presentations known to the user agent should be cleared when the user requests to "clear browsing data."
When in private browsing mode ("incognito"), the initial set of controlled presentations in that browsing session must be empty. Any presentation connections added to it must be discarded when the session terminates.
This spec will not mandate communication protocols between the controlling browsing context and the receiving browsing context, but it should set some guarantees of message confidentiality and authenticity between corresponding presentation connections.
Thanks to Addison Phillips, Anne Van Kesteren, Anssi Kostiainen, Anton Vayvod, Chris Needham, Christine Runnegar, Daniel Davis, Domenic Denicola, Erik Wilde, François Daoust, 闵洪波 (Hongbo Min), Hongki CHA, Hubert Sablonnière, Hyojin Song, Hyun June Kim, Jean-Claude Dufourd, Joanmarie Diggs, Jonas Sicking, Louay Bassbouss, Mark Watson, Martin Dürst, Matt Hammond, Mike West, Mounir Lamouri, Nick Doty, Oleg Beletski, Philip Jägenstedt, Richard Ishida, Shih-Chiang Chien, Takeshi Kanai, Tobie Langel, Tomoyuki Shimizu, Travis Leithead, and Wayne Carr for help with editing, reviews and feedback to this draft.
AirPlay, HDMI, Chromecast, DLNA and Miracast are registered trademarks of Apple Inc., HDMI Licensing LLC., Google Inc., the Digital Living Network Alliance, and the Wi-Fi Alliance, respectively. They are only cited as background information and their use is not required to implement the specification.
For this specification to be advanced to Proposed Recommendation, there must be, for each of the conformance classes it defines (controlling user agent and receiving user agent), at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. Additionally, implementations of the controlling user agent conformance class must include at least one implementation of the 1-UA mode, and one implementation of the 2-UA mode. 2-UA mode implementations may only support non http/https presentation URLs. Implementations of the receiving user agent conformance class may not include implementations of the 2-UA mode.
The API was recently restricted to secure contexts. Deprecation of the API in non secure contexts in early implementations takes time. The group may request transition to Proposed Recommendation with implementations that still expose the API in non secure contexts, provided there exists a timeline to restrict these implementations in the future.
For the purposes of these criteria, we define the following terms:
This section lists changes made to the spec since it was first published as Candidate Recommendation in July 2016, with links to related issues on the group's issue tracker.