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<!DOCTYPE html>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title>
Presentation API
</title>
<style>
@media print {
[data-anolis-spec]::after { content:"[" attr(data-anolis-spec) "]"; font-size:.6em; vertical-align:super; text-transform:uppercase }
}
</style>
<link rel="stylesheet" href=
"http://www.w3.org/community/src/css/spec/cg-final.css" type="text/css">
<style type="text/css">
/* Note formatting taken from HTML5 spec */
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margin-bottom: 0.5em;
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code { color: orangered; }
</style>
</head>
<body>
<div class="head">
<!--logo-->
<h1>
Presentation API
</h1>
<h2 class="no-num no-toc">
Final Report [DATE]
</h2>
<dl>
<dt>
This Version:
</dt>
<dd>
<!--begin-link-->http://webscreens.github.io/presentation-api/
<!--end-link-->
</dd>
<dt>
Latest Published Version
</dt>
<dd>
<!--begin-link-->http://webscreens.github.io/presentation-api/
<!--end-link-->
</dd>
<dt>
<a id="previous-version">Previous Version:</a>
</dt>
<dd>
<a href=
"http://webscreens.github.io/presentation-api/20131112/">http://webscreens.github.io/presentation-api/20131112/</a>
</dd>
<dt>
Participate:
</dt>
<dd>
Send feedback to the community group's mailing list <a href=
"http://lists.w3.org/Archives/Public/public-webscreens/">[email protected]</a>,
or <a href=
"https://github.com/webscreens/presentation-api/issues">create or
browse open issues</a> on GitHub. Also, you may join on the <a href=
"irc://irc.w3.org:6665/#webscreens">CG's IRC channel</a>.
</dd>
<dt>
Version History:
</dt>
<dd>
<!--begin-link-->https://github.com/webscreens/presentation-api/commits/<!--end-link-->
</dd>
<dt>
Editors:
</dt>
<dd>
<a href="mailto:[email protected]">Dominik Röttsches</a>,
Intel
</dd>
</dl>
<p class="copyright">
<a href=
"http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
© 2014 the Contributors to the Presentation API Specification,
published by the <a href=
"http://www.w3.org/community/webscreens/">Second Screen Presentation
Community Group</a> under the <a href=
"https://www.w3.org/community/about/agreements/final/">W3C Community
Final Specification Agreement (CLA)</a>.
</p>
<hr>
</div>
<h2 class="no-num no-toc">
Abstract
</h2>
<p>
This specification defines an API to enable web content to access
external presentation-type displays and use them for presenting web
content.
</p>
<h2 class="no-num no-toc" id="sotd">
Status of this Document
</h2>
<p>
This specification was published by the <a href=
"http://www.w3.org/community/webscreens/">Second Screen Presentation
Community Group</a>. It is not a W3C Standard nor is it on the W3C
Standards Track. Please note that under the <a href=
"http://www.w3.org/community/about/agreements/final/">W3C Community Final
Specification Agreement (FSA)</a> other conditions apply. Learn more
about <a href="http://www.w3.org/community/">W3C Community and Business
Groups</a>.
</p>
<p>
This report documents the use cases, requirements, examples and
interfaces needed to enable web pages to display web content on secondary
screens. It is an evolved version of the <a href=
"http://webscreens.github.io/presentation-api/20131112/">initial
Presentation API</a> that represents the result of discussions within the
Second Screen Presentation Community Group so far. API semantics still
need to be specified. The report may serve as starting point for a
possible Working Group chartered to work on the same topic.
</p>
<h2 class="no-num no-toc">
Table of Contents
</h2><!--toc-->
<h2>
Introduction
</h2><em>This section is non-normative.</em>
<p>
This specification aims to make secondary displays such as a projector or
a connected TV 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
display 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
requesting page and a presentation page shown in the secondary display.
How those messages are transmitted is left to the UA in order to allow
for use of display devices that can be attached in a wide variety of
ways. For example, when a display device is attached using HDMI or
MiraCast, the UA on the requesting device can render the requested
presentation page in that same UA, but instead of displaying in a window
on that same device, it can use whatever means the operating system
provides for using those external displays. In that case, both the
requesting page and the presentation page run on the requesting device
and the operating system is used to route the presentation display output
to the other display device. The second display device doesn't need to
know anything about this spec or that the content involves HTML5.
</p>
<p>
Alternately, some types of external displays may be able to render HTML5
themselves and may have defined their own way to send messages to that
content. In that case, the UA on the requesting device would not need to
render the presentation page itself. Instead, the UA could act as a proxy
translating the request to show a page and the messages into the form
understood by the display device.
</p>
<p>
This way of attaching to displays could be enhanced in the future through
definition of a standard protocol for delivering these types of messages
that display devices could choose to implement.
</p>
<p>
The API defined here is intended be used with UAs that attach to display
devices through any of the above means.
</p>
<h3 id="use-cases">
Use Cases
</h3>
<h4>
Presentations
</h4>
<p>
A user is preparing a set of slides for a talk. Using a web based
service, she is editing her slides and speaker notes on the primary
screen, while the secondary larger screen shows a preview of the current
slide. When the slides are done, her mobile phone allows her to access
them from an online service while on the go. Coming to the conference,
using wireless display technology, she would like to present her slides
on the stage screen from her mobile phone. The phone's touch screen helps
her to navigate slides and presents a slide preview, while the projector
shows her slides to the audience.
</p>
<p>
<b>Requirements:</b> R1, R3, R4, R5, R7
</p>
<h4>
<a id="video-sharing">Video and Image Sharing</a>
</h4>
<p>
Using an online video or image sharing service, a user would like to show
memorable moments to her friends. Using a device with a small screen, it
is impossible to show the content to a large group of people. Connecting
an external TV screen or projector to her device - with a cable or
wirelessly - the online sharing service now makes use of the connected
display, allowing a wider audience to enjoy the content.
</p>
<p>
The web page shows UI elements that allow the user to trigger displaying
content on the secondary display (e.g a "send to second screen" ) only if
there is at least one secondary screen available.
</p>
<p>
<b>Requirements:</b> R1, R3, R4, R5, R7
</p>
<h4>
Gaming
</h4>
<p>
Splitting the gaming experience into a near screen controller and a large
screen visual experience, new gaming experiences can be created.
Accessing the local display on the small screen device and an external
larger display allows for richer web-based gaming experiences.
</p>
<p>
<b>Requirements:</b> R1, R3, R4, R5, R7
</p>
<h4>
Media Flinging to Multiple Screens
</h4>Alice enters a video sharing site using a browser on her tablet. Next,
Alice picks her favorite video from the site, and the video starts to play
on her tablet. While the video is playing Alice clicks a button "Share on
different screen". The browser provides a user interface that lists all the
screens Alice has around her home, asking her to select one. The screens
are identified by names that are familiar to Alice. Alice picks one screen
from the list, "Alice's big TV", and the video playback continues
seamlessly on the selected screen. Next she decides to switch the playback
to a different screen. She clicks the same button "Share on different
screen" provided by the site, and the browser presents the user interface
that lists all the screens. Alice picks another screen from the list,
"Alice's kitchen TV", and the playback resumes on that screen. Video site
also provides a feature to see the action (Alice is watching a soccer game)
from different angle. Alice clicks a button "Select screen for additional
angle", and the browser asks Alice similarly to select the screen to be
used for playback. Alice picks "Alice's Projector" and the soccer game is
shown on the projector from a different angle, in parallel to the content
being played back on "Alice's kitchen TV".
<p>
<b>Requirements:</b> R1, R3, R4, R5, R6, R7
</p>
<h2>
Requirements
</h2>
<h3>
Functional Requirements
</h3>
<ul>
<li>
<b>Discovery / Availability</b>
<ul>
<li>R1: The UA must provide a way to find out whether at least one
secondary screen is available.
</li>
</ul>
</li>
</ul>
<p class="note">
Multi-Screen enumeration and named identification removed, after
discussion on the mailing list, cmp. <a rel="nofollow" class=
"external free" href=
"http://lists.w3.org/Archives/Public/public-webscreens/2014Feb/0021.html">
http://lists.w3.org/Archives/Public/public-webscreens/2014Feb/0021.html</a> :
</p>
<ul>
<li>
<b>Launching Presentation</b>
<ul>
<li>R3: The UA must provide a means of start sending content to the
secondary screen.
</li>
</ul>
</li>
<li>
<b>Resuming Presentation</b>
<ul>
<li>R4: The UA must be able to resume an existing session with
content being displayed on the secondary screen.
</li>
</ul>
</li>
<li>
<b>Communication</b>
<ul>
<li>R5: The UA must enable exchanging data between the primary and
the secondary screen in order to have a control channel between the
primary and secondary page.
</li>
<li>R6: The UA must not make assumptions about the the execution
locality of the user agent of the remote page it communicates with
(i.e. the secondary page might run on a remote user agent and thus
the link between the two pages' UA must be loosely coupled).
</li>
</ul>
</li>
</ul>
<ul>
<li>
<b>Signaling Disconnection</b>
<ul>
<li>R7: The UA must signal disconnection from the presentation page
to the primary page and vice versa.
</li>
</ul>
</li>
</ul>
<h3>
<span id="Non-Functional_Requirements">Non-Functional Requirements</span>
</h3>
<ul>
<li>
<b>Power Saving Friendly</b>
<ul>
<li>All API design decisions must be analyzed from a power efficiency
point of view. Especially when using wireless display technologies or
querying availability over a radio channel, care needs to be taken to
design the API in a way that does not pose obstacles to using radio
resources in an efficient way. For example, powering up the wireless
display detection only when needed.
</li>
</ul>
</li>
</ul>
<h2>
Conformance
</h2>
<p>
All diagrams, examples, and notes in this specification are
non-normative, as are all sections explicitly marked non-normative.
Everything else in this specification is normative.
</p>
<p>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119. For readability, these words do not appear in all uppercase letters
in this specification. <span data-anolis-ref="">RFC2119</span>
</p>
<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 end 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>
<h2>
Terminology
</h2>
<p>
The term <dfn title="presentation display">presentation display</dfn>
refers to an external screen connected to the device that the user agent
runs on.
</p><!-- <p>
The terms <span title="opener-browsing-context" data-anolis-spec=
"w3c-html">opener browsing context</span> and <span title=
"auxiliary browsing context" data-anolis-spec="w3c-html">auxiliary
browsing context</span> are defined in <span data-anolis-ref=
"">HTML5</span>.
</p>
-->
<p>
The terms <span data-anolis-spec="w3c-html">event handlers</span> and
<span title="event handler event type" data-anolis-spec="w3c-html">event
handler event types</span> are defined in <span data-anolis-ref=
"">HTML5</span>.
</p>
<p>
This document provides interface definitions using the
<span data-anolis-ref="">WEBIDL</span> standard.
</p>
<h2>
Example
</h2>
<p>
Running in a compliant user agent, code for presenting a page
<code>example.html</code> on the presentation display looks as follows:
</p>
<pre class="example">
/* controller.html */
<button disabled>Show</button>
<script>
var presentation = navigator.presentation,
showButton = document.querySelector('button');
presentation.onavailablechange = function(e) {
showButton.disabled = !e.available;
showButton.onclick = show;
};
function show() {
var session = presentation.requestSession('http://example.org/');
session.onstatechange = function() {
switch (session.state) {
case 'connected':
session.postMessage(/*...*/);
session.onmessage = function() { /*...*/ };
break;
case 'disconnected':
console.log('Disconnected.');
break;
}
};
}
</script>
</pre>
<p>
The availability monitoring for secondary screens begins when the page
adds an event listener for the <code>availablechange</code> event on the
<code>navigator.presentation</code> object. If there are already
available screens when the page adds the first event listener for the
event, the UA synthesizes a single availablechange event to signal the
availability.
</p>
<p class="open-issue">
Do we want to fire an event immediately after the page registers for it?
What's a best practice method for asynchronous notifications of ths kind?
See below in the <a href="#open-questions">Open Questions</a> section.
</p>
<p class="open-issue">
With additional information from the page the UA may be able to filter out
available screens which cannot support the page's content. For example, the
second screen application of domain A, discovered using DIAL, will not be
capable of rendering content from domain B. It is an open issue how to provide
such information from page to UA.
</p>
<p>
The "Show" button's state (initially disabled) informs the user of the
availability of secondary screen(s), and the button's state is updated if
the availability changes. (The rationale of putting the actual boolean
information into a property of the event <code>e.available</code> is to
allow the implementation to optimize power consumption for network
discovery of remote wireless screens. If this information was provided in
a globally accessible flag, the network discovery could never be
suspended for keeping the flag up to date.)
</p>
<p>
Once the user clicks the "Show" button, the <code>show()</code> function
is called.
</p>
<p>
By calling <code>navigator.presentation.requestSession(url)</code>, the
script on the page tries to launch or resume a presentation on a
secondary screen. Based on the <code>url</code> argument, the UA looks
for existing sessions and available screens, and presents a screen picker
user interface to the user. Out of the list of existing sessions or
available screens the user selects one item. If an existing session was
selected, the session is resumed by establishing a communication channel.
If a new screen was selected, the UA connects to the selected screen,
brings up a new window on the selected screen, starts to show the content
denoted by the url argument, and the UA establishes a communication
channel with this window.
</p>
<p>
When the <code>navigator.presentation.requestSession(url)</code> function
is called, the UA immediately returns a <code>session</code> object to
the script which represents a handle to the current presentation session,
used for communication and state handling.
</p>
<p>
If the user has selected a screen, the content is shown and a
communication channel was established, the state of the session object
changes from <code>"disconnected"</code> to <code>"connected"</code>,
which signals the presentation session is up and running, and the opener
page can communicate with the presentation page. For communication with
the presentation page, the session object's <code>postMessage()</code> is
used to send, and the <code>onmessage</code> event handler to receive
messages.
</p>
<p>
If the user cancels the screen selection and never selects a screen, no
state transition happens and the <code>session</code> object remains in
the <code>"disconnected"</code> state.
</p>
<h4>
Open Questions
</h4>
<p class="open-issue">
Do we need to insert into the description an additional permission prompt
to grant the page access to the "one ore more screens are available"
Information?
</p>
<p class="open-issue">
If there are already connected screens when the page subscribes to the
<code>onavailablechange</code> event, we can handle this in two ways: We
can synthesize one initial event to notify the page about available
screens as soon as the first event handler is installed (as described).
Or we can add another message like
<code>navigator.presentation.getAvailable(function(available) { }
);</code> to notify the page about available screens using this one-time
asynchronous getter. Which way should we go?
</p>
<p class="open-issue">
Do we need an additional state like resumed in order to identify resumed
session? It seems that this could be handled on the page level. The
opener page could ask the presentation page whether it is
<code>"new"</code> or <code>"resumed"</code>.
</p>
<h3>
Usage on Remote Screen
</h3>For addressing the requirement of communication between originating
page and presentation page/screen, we can now use the same
<code>session</code> object on the remote side.
<pre class="example">
navigator.presentation.onpresent = function(e) {
// Communicate with opener page.
e.session.postMessage(/*...*/);
e.session.onmessage = function() {/*...*/};
e.session.onstatechange = function() {
switch (this.state) {
case "disconnected":
// Handle disconnection from opener page.
}
};
};
</pre>
<p>
When the content denoted by the url argument in the
<code>requestSession()</code> example above is loaded, the page on the
presentation screen receives a <code>PresentEvent</code>, with a
<code>session</code> property representing the session. This session is a
similar object as in the first example. Here, its initial state is
<code>"connected"</code>, which means we can use it to communicate with
the opener page using <code>postMessage()</code> and
<code>onmessage</code>.
</p>
<p>
We can also monitor the connection state by listening for
<code>statechange</code> events. When the state changes to
<code>"disconnected"</code> the page is made aware of the fact that
communication with the opener page was lost, but it can continue to
display the current content. The communication can be re-established when
a new present event fires on the <code>navigator.presentation</code>
object.
</p>
<h2>
Interfaces
</h2>
<p>
The interfaces described herein address the requirements outlined in the
<a href="#use-cases">Use Cases</a> section, and specifically, also
consider the <a href="#media-flinging-to-multiple-screens">Media Flinging
to Multiple Screens</a> use case unaddressed by the <a href=
"#previous-version">previous version of the Presentation API</a>. This
section describes the interfaces to the extend discussed in the Second
Screen Presentation Community Group. Readers are encouraged to consult
the <a href="#example">Example</a> section together with this section for
a more complete understanding of the technical parts of this
specification.
</p>
<h3>
<code>NavigatorPresentation</code>
</h3>
<pre class="idl">
interface NavigatorPresentation : EventTarget {
PresentationSession requestSession(DOMString url);
attribute EventHandler onavailablechange;
attribute EventHandler onpresent;
};
partial interface Navigator {
readonly attribute NavigatorPresentation presentation;
};
</pre>
<h3>
<code>AvailableChangeEvent</code>
</h3>
<p>
Fired at the primary screen's <code>NavigatorPresentation</code> object,
when screen availability changes.
</p>
<pre class="idl">
[Constructor(DOMString type, optional AvailableChangeEventInit eventInitDict)]
interface AvailableChangeEvent : Event {
readonly attribute boolean available;
};
dictionary AvailableChangeEventInit : EventInit {
boolean available;
};
</pre>
<h3>
<code>PresentEvent</code>
</h3>Fired at the secondary screen's <code>NavigatorPresentation</code>
object, when the presentation session is established.
<pre class="idl">
[Constructor(DOMString type, optional PresentEventInit eventInitDict)]
interface PresentEvent : Event {
readonly attribute PresentationSession session;
};
dictionary PresentEventInit : EventInit {
PresentationSession session;
};
</pre>
<h3>
<code>PresentationSession</code>
</h3>
<p>
An object representing the established presentation session.
</p>
<pre class="idl">
enum PresentationSessionState { "connected", "disconnected" /*, "resumed" */ };
interface PresentationSession : EventTarget {
readonly attribute PresentationSessionState state;
void postMessage(DOMString message);
void close();
attribute EventHandler onmessage;
attribute EventHandler onstatechange;
};
</pre>
<h2 class="no-num">
References
</h2>
<div id="anolis-references"></div>
<h2 class="no-num">
Acknowledgements
</h2>
<p>
Thanks to Wayne Carr, Louay Bassbous, Anssi Kostiainen, 闵洪波 (Hongbo Min),
Anton Vayvod for help with editing, reviews and feedback to this draft.
</p>
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