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<!DOCTYPE html>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title>
Presentation API
</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c-common" async
class="remove">
</script>
<script class="remove">
var respecConfig = {
specStatus: 'ED',
edDraftURI: 'http://w3c.github.io/presentation-api/',
shortName: 'presentation-api',
editors: [
{
w3cid: 68454,
name: 'Mark Foltz',
company: 'Google',
mailto: 'mfoltz@google.com'
},
{
w3cid: 63802,
name: 'Dominik Röttsches',
company: 'Intel',
note: 'until April 2015'
}
],
previousMaturity: 'WD',
previousPublishDate: '2015-07-01',
otherLinks: [
{
key: 'Version history',
data: [
{
value: 'GitHub w3c/presentation-api/commits',
href: 'https://github.com/w3c/presentation-api/commits/'
}
]
},
{
key: 'Participate',
data: [
{
value: 'GitHub w3c/presentation-api',
href: 'https://github.com/w3c/presentation-api/'
},
{
value: 'File an issue',
href: 'https://github.com/w3c/presentation-api/issues/new'
},
{
value: 'Open issues',
href: 'https://github.com/w3c/presentation-api/issues/'
},
{
value: 'Mailing-list (public-secondscreen@w3.org)',
href: 'https://lists.w3.org/Archives/Public/public-secondscreen/'
}
]
}
],
wg: 'Second Screen Presentation Working Group',
wgURI: 'http://www.w3.org/2014/secondscreen/',
wgPublicList: 'public-secondscreen',
wgPatentURI: 'http://www.w3.org/2004/01/pp-impl/74168/status',
localBiblio: {
PROMGUIDE: {
title: 'Writing Promise-Using Specifications',
href: 'http://www.w3.org/2001/tag/doc/promises-guide',
authors: [
'Domenic Denicola'
],
status: 'finding',
publisher: 'W3C'
}
},
issueBase: "https://www.github.com/w3c/presentation-api/issues/",
githubAPI: "https://api.github.com/repos/w3c/presentation-api",
// Temp fix for ReSpec issue #483:
// https://github.com/w3c/respec/issues/483
afterEnd: function () {
Array.prototype.forEach.call(
document.querySelectorAll("[for],[dfn-for],[link-for]"),
function (element) {
element.removeAttribute('for');
element.removeAttribute('dfn-for');
element.removeAttribute('link-for');
}
);
}
};
</script>
<style>
/* Note formatting taken from HTML5 spec */
.note { border-left-style: solid; border-left-width: 0.25em; background: none repeat scroll 0 0 #E9FBE9; border-color: #52E052; }
.note em, .warning em, .note i, .warning i { font-style: normal; }
p.note, div.note { padding: 0.5em 2em; }
span.note { padding: 0 2em; }
.note p:first-child { margin-top: 0; }
.note p:last-child { margin-bottom: 0; }
p.note:before { content: 'NOTE: '; }
.non-normative { border-left-style: solid; border-left-width: 0.25em; background: none repeat scroll 0 0 #E9FBE9; border-color: #52E052; }
p.non-normative:before { content: 'Non-normative: '; font-weight: bolder;}
p.non-normative, div.non-normative { padding: 0.5em 2em; }
/* Pre.idl formatting taken from HTML5 spec */
pre.idl { border: solid thin #d3d3d3; background: #FCFCFC; color: black; padding: 0.5em 1em; position: relative; }
pre.idl :link, pre.idl :visited { color: inherit; background: transparent; }
pre.idl::before { content: "IDL"; font: bold small sans-serif;
padding: 0.5em; background: white; position: absolute; top: 0;
margin: -1px 0 0 -4em; width: 1.5em; border: thin solid;
border-radius: 0 0 0 0.5em }
/* .example idl formatting taken from HTML5 nightly spec */
.example {
display: block;
color: #222222;
background: #FCFCFC;
border-left-style: solid;
border-color: #c0c0c0;
border-left-width: 0.25em;
margin-left: 1em;
padding-left: 1em;
padding-bottom: 0.5em;
}
.algorithm li {
margin-bottom: 0.5em;
}
.interface dd, .parameters dt {
margin-bottom: 0.5em;
}
code { color: orangered; }
table { border-collapse: collapse; border-style: hidden hidden none hidden; }
table thead, table tbody { border-bottom: solid; }
table td, table th { border-left: solid; border-right: solid; border-bottom: solid thin; vertical-align: top; padding: 0.2em; }
dfn { font-weight: bolder; font-style: normal; }
.copyright { font-size: small; }
.issue[id^='issue-'] > *:not([role='heading']) { display: none; }
</style>
</head>
<body>
<section id="abstract">
<p>
This specification defines an API to enable web content to access
external presentation-type displays and use them for presenting web
content.
</p>
</section>
<section id="sotd">
<p>
This document is a <b>work in progress</b> and is subject to change.
Some sections are still incomplete or underspecified. <a href=
"#security-and-privacy-considerations">Security and privacy
considerations</a> need to be adjusted based on feedback and
experience. Some open issues are noted inline; please check the group's
<a href="https://github.com/w3c/presentation-api/issues">issue
tracker</a> on GitHub for all open issues. Feedback from early
experimentations is encouraged to allow the Second Screen Presentation
Working Group to evolve the specification based on implementation
issues.
</p>
</section>
<section class="informative">
<h2>
Introduction
</h2>
<p>
This specification aims to make <a>presentation displays</a> such as
projectors or connected TVs, available to the Web and takes into
account displays that are attached using wired (HDMI, DVI, or similar)
and wireless technologies (Miracast, Chromecast, DLNA, AirPlay, or
similar).
</p>
<p>
Devices with limited screen size lack the ability to show content to a
larger audience, for example, a group of colleagues in a conference
room, or friends and family at home. Showing content on an external
large <a>presentation display</a> helps to improve the perceived
quality and impact of the presented content.
</p>
<p>
At its core, this specification enables an exchange of messages between
a page that acts as the <a>controller</a> and another page that
represents the <a data-lt=
"receiving browsing context">presentation</a> shown in the
<a>presentation display</a>. How the messages are transmitted is left
to the UA in order to allow the use of <a>presentation display</a>
devices that can be attached in a wide variety of ways. For example,
when a <a>presentation display</a> device is attached using HDMI or
Miracast, the same UA that acts as the <a>controller</a> renders the
<a data-lt="receiving browsing context">presentation</a>. Instead of
displaying the <a data-lt=
"receiving browsing context">presentation</a> in another window on the
same device, however, it can use whatever means the operating system
provides for using the external <a>presentation displays</a>. In such a
case, both the <a>controller</a> and <a data-lt=
"receiving browsing context">presentation</a> run on the same UA and
the operating system is used to route the <a>presentation display</a>
output to the <a>presentation display</a>. This is commonly referred to
as the <b id="1-ua">1-UA</b> case. This specification imposes no
requirements on the <a>presentation display</a> devices connected in
such a manner.
</p>
<p>
If the <a>presentation display</a> is able to render HTML documents and
communicate with the <a>controller</a>, the <a>controller</a> does not
need to render the <a data-lt=
"receiving browsing context">presentation</a>. In this case, the UA
acts as a proxy that requests the <a>presentation display</a> to show
and render the <a data-lt=
"receiving browsing context">presentation</a> itself. This is commonly
referred to as the <b id="2-ua">2-UA</b> case. This way of attaching to
displays could be enhanced in the future by defining a standard
protocol for delivering these types of messages that display devices
could choose to implement.
</p>
<p>
The API defined here is intended to be used with UAs that attach to
<a>presentation display</a> devices through any of the above means.
</p>
</section>
<section id="use-cases-and-requirements">
<h2>
Use cases and requirements
</h2>
<p>
Use cases and requirements are captured in a separate <a href=
"https://github.com/w3c/presentation-api/blob/gh-pages/uc-req.md">Presentation
API Use Cases and Requirements</a> document.
</p>
</section>
<section id="conformance">
<p>
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.
</p>
<p>
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.)
</p>
</section>
<section>
<h2>
Terminology
</h2>
<p>
The terms <dfn><a href=
"http://www.w3.org/TR/html5/browsers.html#browsing-context">browsing
context</a></dfn>, <dfn><a href=
"http://www.w3.org/TR/html5/webappapis.html#event-handlers">event
handler</a></dfn>, <dfn><a href=
"http://www.w3.org/TR/html5/webappapis.html#event-handler-event-type">event
handler event type</a></dfn>, <dfn data-lt="fire"><a href=
"http://www.w3.org/TR/html5/infrastructure.html#concept-event-fire">firing
an event</a></dfn>, <dfn><a href=
"http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">firing
a simple event</a></dfn> <dfn><a href=
"http://www.w3.org/TR/html5/browsers.html#navigate">navigate</a></dfn>,
<dfn><a href="http://www.w3.org/TR/html5/webappapis.html">queue a
task</a></dfn>, <dfn><a href=
"http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">
trusted event</a></dfn>, <dfn><a href=
"http://www.w3.org/TR/html5/browsers.html#allowed-to-show-a-popup">allowed
to show a popup</a></dfn> are defined in [[!HTML5]].
</p>
<p>
This document provides interface definitions using the Web IDL standard
([[!WEBIDL]]). The terms <dfn><a href=
"https://heycam.github.io/webidl/#idl-promise">Promise</a></dfn>,
<dfn><a href=
"http://heycam.github.io/webidl/#idl-ArrayBuffer">ArrayBuffer</a></dfn>,
<dfn><a href=
"http://heycam.github.io/webidl/#idl-ArrayBufferView">ArrayBufferView</a></dfn>
and <dfn><a href=
"http://heycam.github.io/webidl/#idl-DOMException">DOMException</a></dfn>
are defined in [[!WEBIDL]].
</p>
<p>
The terms <dfn data-lt="resolve"><a href=
"http://www.w3.org/2001/tag/doc/promises-guide#resolve-promise">resolving
a Promise</a></dfn>, and <dfn data-lt="reject"><a href=
"http://www.w3.org/2001/tag/doc/promises-guide#reject-promise">rejecting
a Promise</a></dfn> are used as explained in [[!PROMGUIDE]].
</p>
<p>
The term <dfn><a href="https://url.spec.whatwg.org/#url">URL</a></dfn>
is defined in the WHATWG URL standard [[!URL]].
</p>
<p>
The term <dfn><a href=
"http://dev.w3.org/2006/webapi/FileAPI/#blob">Blob</a></dfn> is defined
in the File API specification [[!FILEAPI]].
</p>
<p>
The term <dfn><a href=
"http://w3c.github.io/webrtc-pc/#idl-def-RTCDataChannel">RTCDataChannel</a></dfn>
is defined in the WebRTC API specification ([[WEBRTC]]).
</p>
</section>
<section>
<h2>
Examples
</h2>
<p>
This section shows example codes that highlight the usage of main
features of the Presentation API. In these examples,
<code>controller.html</code> implements the controller and
<code>presentation.html</code> implements the presentation. Both pages
are served from the domain <code>http://example.org</code>
(<code>http://example.org/controller.html</code> and
<code>http://example.org/presentation.html</code>). Please refer to the
comments in the code examples for further details.
</p>
<section>
<h3>
Monitor availability of presentation displays example
</h3>
<pre class="example highlight">
<!-- controller.html -->
<button id="castBtn" style="display: none;">Cast</button>
<script>
// it is also possible to use relative presentation URL e.g. "presentation.html"
var presUrl = "http://example.com/presentation.html";
// the cast button is visible if at least one presentation display is available
var castBtn = document.getElementById("castBtn");
// show or hide cast button depending on display availability
var handleAvailabilityChange = function(available) {
castBtn.style.display = available ? "inline" : "none";
};
// Promise is resolved as soon as the presentation display availability is known.
var request = new PresentationRequest(presUrl);
request.getAvailability().then(function(availability) {
// availability.value may be kept up-to-date by the 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>
</pre>
</section>
<section>
<h3>
Starting a new presentation session example
</h3>
<pre class="example highlight">
<!-- controller.html -->
<script>
// Start new session.
request.start()
// the new started session will be passed to setSession on success
.then(setSession)
// user cancels the selection dialog or an error is occurred
.catch(endSession);
</script>
</pre>
</section>
<section>
<h3>
Reconnect to a presentation session example
</h3>
<pre class="example highlight">
<!-- controller.html -->
<script>
// read presId from localStorage if exists
var presId = localStorage && localStorage["presId"] || null;
// presId is mandatory when reconnecting to a session.
if (presId) {
request.reconnect(presId)
// The reconnected session will be passed to setSession on success
.then(setSession)
// no session found for presUrl and presId or an error is occurred
.catch(endSession);
}
</script>
</pre>
</section>
<section>
<h3>
Handling an event for a UA initiated presentation session example
</h3>
<pre class="example highlight">
<!-- controller.html -->
<head>
<!-- Setting presentation.defaultRequest allows the page to specify the
PresentationRequest to use when the UA initiates a presentation session. -->
</head>
<script>
navigator.presentation.defaultRequest = new PresentationRequest(defaultUrl);
navigator.presentation.defaultRequest.onsessionconnect = function(evt) {
setSession(evt.session);
};
</script>
</pre>
</section>
<section>
<h3>
Monitor session's state and exchange data example
</h3>
<pre class="example highlight">
<!-- controller.html -->
<script>
var session;
var setSession = function (theSession) {
// end existing session, if any
endSession();
// set the new session
session = theSession;
if (session) {
// save presId in localStorage
localStorage && (localStorage["presId"] = session.id);
// monitor session's state
session.onstatechange = function () {
if (this == session) {
if (this.state == "closed") {
// Offer the user a chance to reconnect(), e.g. if there was a
// network disruption, or the user wishes to resume control.
} else if (this.state == "terminated") {
// The presentation has terminated. Offer the user a chance to
// start a new presentation.
}
};
// register message handler
session.onmessage = function (evt) {
console.log("receive message", evt.data);
};
// send message to presentation page
session.send("say hello");
}
};
var disconnectController = function () {
// close old session if exists
session && session.close();
};
var endPresentation = function () {
session && session.terminate();
// remove old presId from localStorage if exists
localStorage && delete localStorage["presId"];
};
</script>
</pre>
</section>
<section>
<h3>
Presentation: monitor available session(s) and say hello.
</h3>
<pre class="example highlight">
<!-- presentation.html -->
<script>
var addSession = function(session) {
session.onstatechange = function () {
// session.state is either 'connected,' 'closed,' or 'terminated'
console.log("session's state is now", session.state);
};
session.onmessage = function (evt) {
if (evt.data == "say hello")
session.send("hello");
}
};
navigator.presentation.receiver.getSession().then(addSession);
navigator.presentation.receiver.onsessionavailable = function(evt) {
navigator.presentation.receiver.getSessions().then(function(sessions) {
addSession(sessions[sessions.length-1]);
});
};
</script>
</pre>
</section>
</section>
<section>
<h2>
API
</h2>
<section>
<h3>
Common idioms
</h3>
<p>
A <dfn lt="presentation display|presentation displays">presentation
display</dfn> refers to an external screen available to the user
agent via an implementation specific connection technology.
</p>
<p>
A <dfn lt="presentation session|presentation sessions">presentation
session</dfn> is an object relating a <a>controlling browsing
context</a> to its <a>receiving browsing context</a> and enables
two-way-messaging between them. Each <a>presentation session</a> has
a <dfn>presentation session state</dfn>, a <dfn lt=
"presentation session identifier|presentation session identifiers">presentation
session identifier</dfn> to distinguish it from other <a>presentation
sessions</a>, and a <dfn>presentation session URL</dfn> that is a
<a>URL</a> used to create or resume the <a>presentation session</a>.
A <dfn>valid presentation session identifier</dfn> consists of
alphanumeric ASCII characters only, is at least 16 characters long,
and is unique within the <a>set of presentations</a>.
</p>
<p>
A <dfn lt=
"controlling browsing context|controlling browsing contexts">controlling
browsing context</dfn> (or <dfn>controller</dfn> for short) is a
<a>browsing context</a> that has connected to a <a>presentation
session</a> by calling <code><a for=
"PresentationRequest">start</a>()</code> or <code><a for=
"PresentationRequest">reconnect</a>()</code>, or received a
<a>presentation session</a> via a <code>sessionconnect</code> event.
</p>
<p>
The <dfn data-lt=
"receiving browsing context|receiving browsing contexts">receiving
browsing context</dfn> (or <a data-lt=
"receiving browsing context">presentation</a> for short) is the
browsing context responsible for rendering to a <a>presentation
display</a>. A <a>receiving browsing context</a> can reside in the
same user agent as the <a>controlling browsing context</a> or a
different one.
</p>
<p>
The <dfn>set of presentations</dfn>, initially empty, contains the
<a>presentation session</a>s created by the <a>controlling browsing
contexts</a> for the user agent (or a specific user profile within
the user agent). The <a>set of presentations</a> is represented by a
list of tuples, where each tuple consists of a <a>presentation
session URL</a>, a <a>presentation session identifier</a>, and the
<a>presentation session</a> itself. The <a>presentation session
URL</a> and <a>presentation session identifier</a> uniquely identify
the <a>presentation session</a>.
</p>
</section>
<section>
<h3>
Interface <code>PresentationSession</code>
</h3>
<p>
Each <a>presentation session</a> is represented by a
<a>PresentationSession</a> object.
</p>
<pre class="idl">
enum PresentationSessionState { "connected", "closed", "terminated" };
enum BinaryType { "blob", "arraybuffer" };
interface PresentationSession : EventTarget {
readonly attribute DOMString? id;
readonly attribute PresentationSessionState state;
void close();
void terminate();
attribute EventHandler onstatechange;
// Communication
attribute BinaryType binaryType;
attribute EventHandler onmessage;
void send (DOMString message);
void send (Blob data);
void send (ArrayBuffer data);
void send (ArrayBufferView data);
};
</pre>
<div dfn-for="PresentationSession" link-for="PresentationSession">
<p>
The <dfn><code>id</code></dfn> attribute specifies the
<a>presentation session</a>'s <a>presentation session
identifier</a>.
</p>
<p>
The <dfn><code>state</code></dfn> attribute represents the
<a>presentation session</a>'s current state. It can take one of the
values of <a>PresentationSessionState</a> depending on connection
state.
</p>
<p>
When the <code><dfn>send</dfn>()</code> method is called on a
<a>PresentationSession</a> object with a <code>message</code>, the
user agent MUST run the algorithm to <a data-lt=
"algorithm-send">send a message through a
<code>PresentationSession</code></a>.
</p>
<p>
When the <code><dfn>close</dfn>()</code> method is called on a
<a>PresentationSession</a>, the user agent MUST run the algorithm
to <a data-lt="close-algorithm">close a presentation session</a>
with <a>PresentationSession</a>.
</p>
</div>
<section>
<h4>
Sending a message through <code>PresentationSession</code>
</h4>
<div class="note">
No specific transport for the connection between the <a>controlling
browsing context</a> and the <a>receiving browsing context</a> is
mandated, except that for multiple calls to <code><a for=
"PresentationSession">send</a>()</code> it has to be ensured that
messages are delivered to the other end reliably and in sequence.
The transport should function equivalently to an
<a><code>RTCDataChannel</code></a> in reliable mode.
</div>
<p>
Let <dfn>presentation message data</dfn> be the payload data to be
transmitted between two browsing contexts. Let <dfn>presentation
message type</dfn> be the type of that data, one of
<code>text</code> and <code>binary</code>.
</p>
<p>
When the user agent is to <dfn data-lt="algorithm-send">send a
message through a <code>PresentationSession</code> S</dfn>, it MUST
run the following steps:
</p>
<ol link-for="PresentationSession">
<li>If the <a>state</a> property of <a>PresentationSession</a> is
not <code>"connected"</code>, throw an
<code>InvalidStateError</code> exception.
</li>
<li>Let <a>presentation message type</a> <em>messageType</em> be
<code>binary</code> if <code>data</code> is one of
<code>ArrayBuffer</code>, <code>ArrayBufferView</code>, or
<code>Blob</code>. Let <em>messageType</em> be <code>text</code> if
<code>data</code> is of type <code>DOMString</code>)
</li>
<li>Assign the <dfn>destination browsing context</dfn> as follows:
<ol>
<li>Let the <a>destination browsing context</a> be the
<a>controlling browsing context</a> if
<code><a>send</a>()</code> is called in the <a>receiving
browsing context</a>.
</li>
<li>Let <a>destination browsing context</a> be the
<a>receiving browsing context</a> if <code><a>send</a>()</code>
is called from the <a>controlling browsing context</a>.
</li>
</ol>
</li>
<li>Using an implementation specific mechanism, transmit the
contents of the <code>data</code> argument as <a>presentation
message data</a> and <a>presentation message type</a>
<em>messageType</em> to the <a>destination browsing context</a>
side.
</li>
</ol>
</section>
<section>
<h4>
Receiving a message through <code>PresentationSession</code>
</h4>
<p>
When the user agent has received a transmission from the remote
side consisting of <a>presentation message data</a> and
<a>presentation message type</a>, it MUST run the following steps:
</p>
<ol link-for="PresentationSession">
<li>If the <a>state</a> property of <a>PresentationSession</a> is
not <code>"connected"</code>, abort these steps.
</li>
<li>Let <em>event</em> be a newly created <a>trusted event</a> that
uses the <code>MessageEvent</code> interface, with the event type
<code>message</code>, which does not bubble, is not cancelable, and
has no default action.
</li>
<li>Initialize the <em>event's</em> data attribute as follows:
<ol>
<li>If the <a>presentation message type</a> is
<code>text</code>, then initialize <em>event's</em>
<code>data</code> attribute to the contents of <a>presentation
message data</a> of type <code>DOMString</code>.
</li>
<li>If the <a>presentation message type</a> is
<code>binary</code>, and <a>binaryType</a> is set to
<code>blob</code>, then initialize <em>event</em>'s
<code>data</code> attribute to a new <code>Blob</code> object
that represents <a>presentation message data</a> as its raw
data.
</li>
<li>If the <a>presentation message type</a> is
<code>binary</code>, and <a>binaryType</a> is set to
<code>arraybuffer</code>, then initialize <em>event</em>'s
<code>data</code> attribute to a new <code>ArrayBuffer</code>
object whose contents are <a>presentation message data</a>.
</li>
</ol>
</li>
<li>
<a>Queue a task</a> to <a>fire</a> <em>event</em> at
<a><code>PresentationSession</code></a>.
</li>
</ol>
</section>
<section>
<h4>
Closing a <code>PresentationSession</code>
</h4>
<p>
When the user agent is to <dfn data-lt="close-algorithm">close a
presentation session</dfn> using <em>session</em>, it MUST do the
following:
</p>
<ol>
<li>
<a>Queue a task</a> to run the following steps in order:
<ol>
<li>If the <a>presentation session state</a> of
<em>session</em> is not <code>connected</code>, then abort
these steps.
</li>
<li>Set <a>presentation session state</a> of <em>session</em>
to <code>closed</code>.
</li>
<li>
<a>Fire</a> an event named <code>statechange</code> at
<em>session</em>.
</li>
<li>If <em>session</em> is owned by a <a>controlling browsing
context</a>, signal the <a>receiving browsing context</a> to
<a data-lt="close-algorithm">close the presentation session</a>
that was created when <em>session</em> was used to <a>establish
a presentation connection</a> with the presentation via
<em>session</em>, using an implementation-specific mechanism.
</li>
<li>If <em>session</em> is owned by a <a>receiving browsing
context</a>, signal the <a>controlling browsing context</a> to
<a data-lt="close-algorithm">close the presentation session</a>
that was used to <a>establish a presentation connection</a>
with the presentation via <em>session</em>, using an
implementation-specific mechanism.
</li>
</ol>
</li>
</ol>
</section>
<section>
<h4>
Terminating a <code>PresentationSession</code>
</h4>
<p>
When the user agent is to <dfn data-lt=
"terminate-algorithm">terminate a presentation session</dfn> using
<em>session</em>, it MUST run the following steps:
</p>
<ol>
<li>
<a>Queue a task</a> to run the following steps in order:
<ol>
<li>If the <a>presentation session state</a> of
<em>session</em> is not <code>connected</code>, then abort
these steps.
</li>
<li>If <em>session</em> is owned by a <a>receiving browsing
context</a>, close that context (equivalent to
<code>window.close()</code>) and abort the following steps.
</li>
<li>For each <em>known session</em> in the <a>set of
presentations</a>:
<ol>
<li>If the <a>presentation session identifier</a> of
<em>known session</em> and <em>session</em> are equal, and
the <a>presentation session state</a> of <em>known
session</em> is <code>connected</code>, then run the
following steps:
<ol>
<li>Set <a>presentation session state</a> of <em>known
session</em> to <code>terminated</code>.
</li>
<li>
<a>Fire</a> an event named <code>statechange</code>
at <em>known session</em>.
</li>
</ol>
</li>
</ol>
</li>
<li>Close the <a>receiving browsing context</a>, using an
implementation specific mechanism.
</li>
</ol>
</li>
</ol>
<p>
The user agent hosting the <a>receiving browsing context</a> MAY
run the following steps when closing that context in response to
<code>terminate()</code> or <code>window.close()</code>:
</p>
<ol>
<li>
<a>Queue a task</a> to run the following steps:
<ol>
<li>For each <em>session</em> that was connected to the
<a>receiving browsing context</a>:
<ol>
<li>If the <a>presentation session state</a> of
<em>session</em> is not <code>connected</code>, then abort
the following steps.
</li>
<li>Let <em>controllingSession</em> be the <a>presentation
session</a> in the <a>controlling browsing context</a> that
was used to <a>establish a presentation connection</a> via
<em>session</em>.
</li>
<li>Set the <a>presentation session state</a> of
<em>controllingSession</em> to <code>terminated</code>.
</li>
<li>
<a>Fire</a> an event named <code>statechange</code> at
<em>controllingSession</em>.
</li>
</ol>
</li>
</ol>
</li>
</ol>
</section>
<section>
<h4>
Event Handlers
</h4>
<p>
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 <a>PresentationSession</a>
interface:
</p>
<table dfn-for="PresentationSession">
<thead>
<tr>
<th>
Event handler
</th>
<th>
Event handler event type
</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<dfn><code>onmessage</code></dfn>
</td>
<td>
<code>message</code>
</td>
</tr>
<tr>
<td>
<dfn><code>onstatechange</code></dfn>
</td>
<td>
<code>statechange</code>
</td>
</tr>
</tbody>
</table>
</section>
</section>
<section>
<h3>
Interface <code>PresentationAvailability</code>
</h3>
<pre class="idl">
interface PresentationAvailability : EventTarget {
readonly attribute boolean value;
attribute EventHandler onchange;
};
</pre>
<p>
A <a><code>PresentationAvailability</code></a> object is associated
with a <a>presentation display</a> and represents its
<dfn>presentation display availability</dfn>.
</p>
<p>
The <dfn for="PresentationAvailability">value</dfn> attribute MUST
return the last value it was set to. The value is updated by the
<a>monitor the list of available presentation displays</a> algorithm.
</p>
<p>
The <dfn for="PresentationAvailability">onchange</dfn> attribute is
an <a>event handler</a> whose corresponding <a>event handler event
type</a> is <code>change</code>.
</p>
<section>
<h4>
The set of availability objects
</h4>
<p>
The user agent MUST keep track of the <dfn>set of availability
objects</dfn> requested through the <code><a for=
"PresentationRequest">getAvailability</a>()</code> method. The
<a>set of availability objects</a> is represented as a set of
tuples <em>(A, availabilityUrl)</em>, initially empty, where:
</p>
<ol>
<li>
<em>A</em> is a live <a>PresentationAvailability</a> object;
</li>
<li>
<em>availabilityUrl</em> is the <code>availabilityUrl</code>
passed to <code><a for=
"PresentationRequest">getAvailability</a>()</code> to create A.
</li>
</ol>
</section>
<section>
<h4>
The list of available presentation displays
</h4>
<p>
The user agent MUST keep a <dfn>list of available presentation
displays</dfn>. This current list of <a>presentation displays</a>
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 <a>monitor the list of
available presentation displays</a>.
</p>
<p>
While there are live <a>PresentationAvailability</a> objects, the
user agent MAY <a>monitor the list of available presentation
displays</a> continuously, so that pages can use the <a for=
"PresentationAvailability">value</a> property of an
<a>PresentationAvailability</a> object to offer presentation only
when there are available displays. However, the user agent may not
support continuous availability monitoring; for example, because of
platform or power consumption restrictions. In this case the
<a>Promise</a> returned by <code>getAvailability()</code> MUST be
<a data-lt="reject">rejected</a> and the algorithm to <a>monitor
the list of available presentation displays</a> will only run as
part of the <a for="PresentationRequest" data-lt="start">start a
presentation session</a> algorithm.
</p>
<p>
When there are no live <a>PresentationAvailability</a> objects
(that is, the <a>set of availability objects</a> is empty), user
agents SHOULD NOT <a>monitor the list of available presentation
displays</a> to satisfy the <a href=
"https://github.com/w3c/presentation-api/blob/gh-pages/uc-req.md#nf-req01-power-saving-friendly">
power saving non-functional requirement</a>. To further save power,
the user agent MAY also keep track of whether the page holding a
<a>PresentationAvailability</a> object is in the foreground. Using
this information, implementation specific discovery of
<a>presentation displays</a> can be resumed or suspended.
</p>
<p>
Some <a>presentation displays</a> 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 a <dfn>compatible presentation display</dfn> for a
<dfn>display availability URL</dfn> if the user agent can
reasonably guarantee that the presentation of the URL on that
display will succeed.
</p>
</section>
<section>
<h4>
Monitor the list of available presentation displays
</h4>
<p>
If <a>set of availability objects</a> is non-empty, or there is a
pending request to <a for="PresentationRequest" data-lt=
"start">start a presentation session</a>, the user agent MUST
<dfn>monitor the list of available presentation displays</dfn> by
running the following steps.
</p>
<ol link-for="PresentationAvailability">
<li>
<a>Queue a task</a> to retrieve available presentation displays
(using an implementation specific mechanism) and let
<em>newDisplays</em> be this list.
</li>
<li>Wait for the completion of that task.
</li>
<li>For each member <em>(A, availabilityUrl)</em> of the <a>set of
availability objects</a>:
<ol>
<li>Set <em>previousAvailability</em> to the value of
<em>A</em>'s <a>value</a> property.
</li>
<li>Let <em>newAvailability</em> be <code>true</code> if
<em>newDisplays</em> is not empty and at least one display in
<em>newDisplays</em> is a <a>compatible presentation
display</a> for <em>availabilityUrl</em>. Otherwise, set
<em>newAvailability</em> to <code>false</code>.
</li>
<li>If <em>previousAvailability</em> is not equal to