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<!DOCTYPE html>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title>
Remote Playback API
</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c-common" async
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edDraftURI: 'https://w3c.github.io/remote-playback/',
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{
w3cid: 45389,
name: 'Mounir Lamouri',
company: 'Google',
},
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name: 'Anton Vayvod',
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</head>
<body>
<section id="abstract">
<p>
This specification defines an API extending the
<code><a>HTMLMediaElement</a></code> that enables controlling remote
playback of media from a web page.
</p>
</section>
<section id="sotd">
<p>
This document builds on the group's experience on presenting web
content on external presentation-type displays, and re-uses patterns
and design considerations from the Presentation API specification
whenever appropriate [[PRESENTATION-API]].
</p>
<p>
Although this document is still a <strong>work in progress</strong> and
is subject to change, the Working Group believes that the API surface is
stable. Most of the remaining issues listed on the <a href=
"https://github.com/w3c/remote-playback/issues/">issue tracker</a> are
considered minor at this stage except for
Issue <a href="https://github.com/w3c/remote-playback/issues/41">#41</a>.
</p>
<p>
Issue <a href="https://github.com/w3c/remote-playback/issues/41">#41</a>
discusses the set of media playback features that remote playback
devices are expected to support. The group will seek further developer
feedback and implementation experience to identify any interoperability
issues around these features when used during remote playback, and will
further clarify the specification based on feedback received.
</p>
<p>
For other issues or concerns, it is possible to <a href=
'https://github.com/w3c/remote-playback/issues/new'>file a bug</a> or
send an email to the <a href=
'https://lists.w3.org/Archives/Public/public-secondscreen/'>mailing
list</a>. For small editorial changes like typos, sending a pull request
is appreciated.
</p>
<p>
The Working Group invites everyone to review this document, and will
work with relevant groups at W3C to conduct horizontal reviews on
accessibility, internationalization, privacy, security and technical
architecture principles.
</p>
<p>
No feature has been identified as being <strong>at risk</strong>.
</p>
<p>
The Second Screen Working Group will develop a test suite for the
Remote Playback API during the Candidate Recommendation period and
prepare an implementation report. For this specification to advance to
Proposed Recommendation, two independent, interoperable implementations
of each feature must be demonstrated, as detailed in the <a href=
"#candidate-recommendation-exit-criteria">Candidate Recommendation exit
criteria</a> section.
</p>
</section>
<section id='conformance'>
<p>
This specification defines conformance criteria that apply to a single
product: the <dfn>user agent</dfn> that implements the interfaces that
it contains.
</p>
<p>
Implementations that use ECMAScript to expose the APIs defined in this
specification MUST implement them in a manner consistent with the
ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]].
</p>
</section>
<section class="informative">
<h2>
Introduction
</h2>
<p>
This specification aims to make <a>remote playback devices</a> such as
connected TVs, projectors or audio-only speakers, available to the Web
and takes into account playback devices 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 or quiet speakers lack the ability to
playback media content to a larger audience, for example, a group of
colleagues in a conference room, or friends and family at home. Playing
media content on an external larger and/or louder <a>remote playback
device</a> helps to improve the perceived quality and impact of the
played media.
</p>
<p>
At its core, this specification enables a page that acts as the
<a>browsing context</a> to initiate and control remote playback of a
particular media element on a selected <a>remote playback device</a>.
How the remoting is initiated and controlled is left to the UA in order
to allow the use of <a>remote playback devices</a> that can be attached
in a wide variety of ways. For example, when a <a>remote playback
device</a> is attached using HDMI or Miracast, the same UA that acts as
the <a>browsing context</a> renders the remote media. Instead of
playing the media on the same device, however, it can use whatever
means the operating system provides for using the external <a>remote
playback device</a>. In such a case, both the <a>browsing context</a>
and the media player run on the same UA and the operating system is
used to route the player output to the <a>remote playback device</a>.
This is commonly referred to as the <dfn><b id="media-mirroring">media
mirroring</b></dfn> case. This specification imposes no requirements on
the <a>remote playback devices</a> connected in such a manner.
</p>
<p>
If the <a>remote playback device</a> is able to play the media and
communicate with the <a>browsing context</a> but is unable to fetch the
media, the <a>browsing context</a> needs to fetch the media data and
pass it on to the <a>remote playback device</a> for rendering. This is
commonly referred to as <dfn><b id="media-remoting">media
remoting</b></dfn> case.
</p>
<p>
If the <a>remote playback device</a> is able to fetch and play the
media and communicate with the <a>browsing context</a>, the <a>browsing
context</a> does not need to fetch or render the remoted media. In this
case, the UA acts as a proxy that requests the <a>remote playback
device</a> to play the media itself by passing the necessary data like
the media source. This is commonly referred to as the <dfn><b id=
"media-flinging">media flinging</b></dfn> 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 remote playback
devices could choose to implement.
</p>
<p>
The API defined here is intended to be used with UAs that attach to
<a>remote playback device</a> devices through any of the above means.
</p>
</section>
<section class='informative'>
<h2>
Use cases and requirements
</h2>
<p>
The use cases and requirements of this specification are captured in a
separate document available <a href=
'https://github.com/w3c/remote-playback/blob/gh-pages/use-cases.md'>here</a>.
</p>
</section>
<section>
<h2>
Dependencies
</h2>
<p>
The following concepts and interfaces are defined in [[!HTML]]:
</p>
<ul>
<li>
<dfn><a href=
'https://html.spec.whatwg.org/multipage/embedded-content.html#htmlmediaelement'>HTMLMediaElement</a></dfn>, <dfn><a href=
'https://html.spec.whatwg.org/multipage/embedded-content.html#media-element'>
media element</a></dfn>, <dfn data-lt=
"resource selection algorithm"><a href=
"https://html.spec.whatwg.org/multipage/embedded-content.html#concept-media-load-algorithm">
selecting media resource</a></dfn>, <dfn data-lt=
"currentSrc"><a href="https://html.spec.whatwg.org/multipage/embedded-content.html#dom-media-currentsrc">
current source</a></dfn>,
<dfn><a href="https://html.spec.whatwg.org/multipage/media.html#dom-media-fastseek">fastSeek</a></dfn>,
<dfn><a href="https://html.spec.whatwg.org/multipage/media.html#dom-media-seeking">seeking</a></dfn>,
<dfn data-lt="media resources"><a href=
"https://html.spec.whatwg.org/multipage/embedded-content.html#location-of-the-media-resource">
media resource</a></dfn>, <dfn data-lt="exposing a user interface to the user"><a href=
"https://html.spec.whatwg.org/multipage/embedded-content.html#expose-a-user-interface-to-the-user">
expose a user interface to the user</a></dfn>,
<dfn><a href="https://html.spec.whatwg.org/multipage/media.html#the-track-element">
<code>track</code></a></dfn>, and
<dfn><a href="https://html.spec.whatwg.org/multipage/media.html#attr-track-srclang">
<code>srclang</code></a></dfn>,
<dfn><a href="https://html.spec.whatwg.org/multipage/media.html#allowed-to-play">
allowed to play</a></dfn>.
</li>
<li>
<dfn><a href=
'https://html.spec.whatwg.org/multipage/browsers.html#browsing-context'>
browsing context</a></dfn>, <dfn><a href=
"https://www.w3.org/TR/html5/browsers.html#nested-browsing-contexts">nested
browsing context</a></dfn>, and <dfn><a href=
'https://html.spec.whatwg.org/multipage/browsers.html#allowed-to-show-a-popup'>
allowed to show a popup</a></dfn>
</li>
<li>
<dfn data-lt='EventHandler'><a href=
'https://html.spec.whatwg.org/multipage/webappapis.html#event-handlers'>
event handler</a></dfn>, <dfn><a href=
'https://html.spec.whatwg.org/multipage/webappapis.html#event-handler-event-type'>
event handler event type</a></dfn>, <dfn><a href=
'https://html.spec.whatwg.org/multipage/webappapis.html#fire-a-simple-event'>
fire a simple event</a></dfn> and <dfn><a href=
'https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-task'>
queue a task</a></dfn>
</li>
<li>
<dfn><a href='https://dom.spec.whatwg.org/#interface-eventtarget'>EventTarget</a></dfn>
<li>
<dfn data-lt='fire|fires'><a href=
'https://html.spec.whatwg.org/multipage/infrastructure.html#concept-event-fire'>
firing an event</a></dfn> and <dfn><a href=
'https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel'>
in parallel</a></dfn>,
<dfn><a href="https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#reflecting-content-attributes-in-idl-attributes">reflect</a></dfn>,
</li>
<li>
<dfn><a href=
'https://html.spec.whatwg.org/multipage/scripting.html#cereactions'><code>
CEReactions</code></a></dfn>
</li>
<li>
<dfn><a href=
'https://html.spec.whatwg.org/multipage/webappapis.html#task-source'>
task source</a></dfn>
</li>
<li>
<dfn><a href=
'https://html.spec.whatwg.org/multipage/media.html#media-element-event-task-source'>
media element event task source</a></dfn>
</li>
</ul>
<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>throw</dfn> in this specification is used as defined in
[[!WEBIDL]].
</p>
<p>
The interface <dfn><code>PresentationRequest</code></dfn> is defined in
[[PRESENTATION-API]].
</p>
<p>
The term <dfn><a href=
"https://www.w3.org/TR/secure-contexts/#potentially-trustworthy-origin">
potentially trustworthy origin</a></dfn> is defined in [[!SECURE-CONTEXTS]].
</p>
<p>
The following exception names are defined by [[!WEBIDL]] and used by
this specification:
</p>
<ul>
<li>
<dfn><code>InvalidAccessError</code></dfn>
</li>
<li>
<dfn><code>InvalidStateError</code></dfn>
</li>
<li>
<dfn><code>NotAllowedError</code></dfn>
</li>
<li>
<dfn><code>NotFoundError</code></dfn>
</li>
<li>
<dfn><code>NotSupportedError</code></dfn>
</li>
<li>
<dfn><code>OperationError</code></dfn>
</li>
</ul>
</section>
<section>
<h2>
Examples
</h2>
<p>
This section shows code examples that highlight the usage of the main
features of the Remote Playback API. In these examples,
<code>player.html</code> implements the player page controlling the
remote playback and <code>media.ext</code> is the media file to be
played remotely. Both the page and the media are served from the domain
<code>https://example.org</code>. Please refer to the comments in the
code examples for further details.
</p>
<section>
<h3>
Monitor availability of remote playback devices example
</h3>
<pre class="example">
<!-- player.html -->
<!-- The video element with custom controls that supports remote playback. -->
<video id="videoElement" src="https://example.org/media.ext" />
<button id="deviceBtn" style="display: none;">Pick device</button>
<script>
// The "Pick device" button is visible if at least one remote playback device is available.
var deviceBtn = document.getElementById("deviceBtn");
var videoElem = document.getElementById("videoElement");
function availabilityCallback(available) {
// Show or hide the device picker button depending on device availability.
deviceBtn.style.display = available ? "inline" : "none";
}
videoElem.remote.watchAvailability(availabilityCallback).catch(function() {
// Availability monitoring is not supported by the platform, so discovery of
// remote playback devices will happen only after remote.prompt() is called.
// Pretend the devices are available for simplicity; or, one could implement
// a third state for the button.
deviceBtn.style.display = "inline";
});
</script>
</pre>
</section>
<section>
<h3>
Starting remote playback of a video example
</h3>
<pre class="example">
<!-- player.html -->
<script>
devicesBtn.onclick = function() {
// Request the user to select a remote playback device.
videoElem.remote.prompt()
// Update the UI and monitor the connected state.
.then(updateRemotePlaybackState);
// Otherwise, the user cancelled the selection UI or no screens were found.
};
<script>
</pre>
</section>
<section>
<h3>
Monitoring remote playback state changes
</h3>
<pre class="example">
<!-- player.html -->
<script>
// The remote playback may be initiated by the user agent,
// so check the initial state to sync the UI with it.
if (videoElem.remote.state == "disconnected")
switchToLocalUI();
else
switchToRemoteUI();
videoElem.remote.onconnecting = switchToRemoteUI;
videoElem.remote.onconnect = swithToRemoteUI;
videoElem.remote.ondisconnect = switchToLocalUI;
// Handles both 'connecting' and 'connected' state. Calling more than once
// is a no-op.
function switchToRemoteUI() {
// Indicate that the state is 'connecting' or 'connected' to the user.
// For example, hide the video element as only controls are needed.
videoElem.style.display = "none";
// Stop monitoring the availability of remote playback devices.
videoElem.remote.cancelWatchAvailability();
};
function switchToLocalUI() {
// Show the video element.
videoElem.style.display = "inline";
// Start watching the device availability again.
videoElem.remote.watchAvailability(availabilityCallback);
};
<script>
</pre>
</section>
</section>
<section>
<h2>
API
</h2>
<section>
<h3>
Common idioms
</h3>
<p>
A <dfn>local playback device</dfn> is the device the <a>browsing
context</a> is running on along with the default video/audio outputs
the device has.
</p>
<div class="note">
A <a>local playback device</a> might have extra outputs, like an
external display or speakers/headphones. As long as the switch of
what output to use happens outside of the <a>user agent</a> on the
system level, the playback is considered to happen on a <a>local
playback device</a> for the purpose of this spec.
</div>
<p></p>
<p>
A <dfn data-lt="remote playback devices">remote playback device</dfn>
is any other device but the <a>local playback device</a> that the
<a>browsing context</a> can use to play media on.
</p>
<p>
A <dfn>media element state</dfn> is the set of all single <a>media
element</a> properties observable by the page and/or the user via the
<a>user agent</a> implementation. The new properties introduced by
this spec are not considered part of the <a>media element state</a>
for convenience.
</p>
<div class="example">
For example, the <code>paused</code> attribute or the pause/resume
button reflecting that state on the default controls of the media
element would be a part of <a>media element state</a>.
</div>
<p></p>
<p>
A <dfn>local playback state</dfn> is the user agent implementation of
<a>media element state</a> for the particular <a>media element</a>
for playback on the <a>local playback device</a>.
</p>
<p>
A <dfn>remote playback state</dfn> is the user agent implementation
of <a>media element state</a> for the particular <a>media element</a>
for playback on the certain <a>remote playback device</a>.
</p>
<p class="note">
For a good user experience it is important that the <a>media element
state</a> doesn't change unexpectedly when the
<code><a data-link-for="RemotePlayback">state</a></code> changes. It
is also important that <a>remote playback state</a> is in sync with
the <a>media element state</a> so when the media is paused on the
<a>remote playback device</a> it looks paused to both the user and
the page.
</p>
<p>
The <a>task source</a> for the tasks mentioned in this specification
is the <a>media element event task source</a>.
</p>
</section>
<section>
<h3>
<code><a>RemotePlayback</a></code> interface
</h3>
<pre class='idl'>
[Exposed=Window]
interface RemotePlayback : EventTarget {
Promise<long> watchAvailability(RemotePlaybackAvailabilityCallback callback);
Promise<void> cancelWatchAvailability(optional long id);
readonly attribute RemotePlaybackState state;
attribute EventHandler onconnecting;
attribute EventHandler onconnect;
attribute EventHandler ondisconnect;
Promise<void> prompt();
};
enum RemotePlaybackState {
"connecting",
"connected",
"disconnected"
};
callback RemotePlaybackAvailabilityCallback = void(boolean available);
</pre>
<p>
A <dfn>RemotePlayback</dfn> object is an interface allowing the page
to detect availability of, connect to and control playback on
<a>remote playback devices</a>.
</p>
<p>
A <dfn>RemotePlaybackState</dfn> enumeration represents the state of
connection to some <a>remote playback device</a>.
</p>
<p>
A <dfn>RemotePlaybackAvailabilityCallback</dfn> represents the
<a>remote playback device availability</a> information.
</p>
<section>
<h4>
Observing remote playback device availability
</h4>
<p>
A <a><code>RemotePlaybackAvailabilityCallback</code></a> is the way
for the page to obtain the <dfn>remote playback device
availability</dfn> for the corresponding <a>media element</a>. If
the user agent can <a>monitor the list of available remote playback
devices</a> in the background (without a pending request to
<code><a data-link-for="RemotePlayback">prompt</a>()</code>), the
<a><code>RemotePlaybackAvailabilityCallback</code></a> behavior
defined below MUST be implemented by the user agent. Otherwise, the
promise returned by <code><a data-link-for=
"RemotePlayback">watchAvailability</a>()</code> MUST be rejected
with <a>NotSupportedError</a>.
</p>
<section>
<h5>
The set of availability callbacks
</h5>
<p>
The <a>user agent</a> MUST keep track of the <dfn>set of
availability callbacks</dfn> registered with each <a>media
element</a> through the <code><a data-link-for=
"RemotePlayback">watchAvailability</a>()</code> method. The
<a>set of availability callbacks</a> for each
<a>RemotePlayback</a> object is represented as a set of tuples
<em>(<var>callbackId</var>, <var>callback</var>)</em>, initially
empty, where:
</p>
<ol>
<li>
<var>callbackId</var> is a positive integer unique among all ids
returned by
<code><a data-link-for="RemotePlayback">watchAvailability</a>()</code>
in a given <a>browsing context</a>;
</li>
<li>
<var>callback</var> is a
<a>RemotePlaybackAvailabilityCallback</a> object.
</li>
</ol>
<p>
Since there's one and only one <a>RemotePlayback</a> object per
each <a>media element</a>, <a>set of availability callbacks</a>
of a <a>media element</a> is the same set as the <a>set of
availability callbacks</a> of the <a>RemotePlayback</a> object
referred to by the element's <a data-link-for=
"HTMLMediaElement">remote</a> property.
</p>
<p>
The combined set of all <a data-lt=
"set of availability callbacks">sets of availability
callbacks</a> of all <a>RemotePlayback</a> objects known to the
<a>browsing context</a> is referred to as <dfn>global set of
availability callbacks</dfn>.
</p>
</section>
<section>
<h5>
The list of available remote playback devices
</h5>
<p>
The <a>user agent</a> MUST keep a <dfn>list of available remote
playback devices</dfn>. This list contains <a>remote playback
devices</a> 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 remote playback
devices</a> or an empty list if the algorithm hasn't been run
yet.
</p>
<p>
The <a>user agent</a> MAY not support running the algorithm to
<a>monitor the list of available remote playback devices</a>
continuously, for example, because of platform or power
consumption restrictions. In this case the promise returned by
<code><a data-link-for=
"RemotePlayback">watchAvailability</a>()</code> MUST be rejected
with <a>NotSupportedError</a>, the <a>global set of availability
callbacks</a> will be empty and the algorithm to <a>monitor the
list of available remote playback devices</a> will only run as
part of the <a>initiate remote playback</a> algorithm.
</p>
<p>
When the <a>global set of availability callbacks</a> is not
empty, the <a>user agent</a> MUST <a>monitor the list of
available remote playback devices</a> continuously, so that pages
can keep track of the last value received via the registered
callbacks to offer remote playback only when there are available
devices.
</p>
<p class="note">
The user agent is expected not to <a>monitor the list of available
remote playback devices</a> when possible, to satisfy the <a href=
"https://github.com/w3c/remote-playback/blob/gh-pages/use-cases.md#power-saving-friendly">
power saving non-functional requirement</a>. For example, the
<a>user agent</a> might choose not to run the monitoring
algorithm when the <a>global set of availability callbacks</a> is
empty, when every page that has <a data-lt="media element">media
elements</a> with non-empty <a>set of availability callbacks</a>
is in the background.
</p>
<p>
Some <a>remote playback devices</a> may only be able to play a
subset of <a data-link-for="media resource">media resources</a>
because of functional, security or hardware limitations. Examples
are set-top boxes, smart TVs or networked speakers capable of
rendering only certain formats of video and/or audio. We say that
such a device is a <dfn>compatible remote playback device</dfn>
for a <a>media resource</a> if the <a>user agent</a> can
reasonably guarantee that the remote playback of the media
specified by the resource will succeed on that device.
</p>
<p class="note">
The <a>user agent</a> can use the <a><code>srclang</code></a>
attribute of the <a><code>track</code></a> element as a hint of
the language of the text track data to help identify a
<a>compatible remote playback device</a>.
</p>
<p>
The <a>media resources</a> of a <a>media element</a>, that were
considered by the user agent to find a <a>compatible remote
playback device</a>, are called the <dfn>availability sources
set</dfn>.
</p>
<p>
The <a>media resource</a> of a <a>media element</a>, that is used
to <a>initiate remote playback</a> on the selected <a>remote
playback device</a> is called <dfn>remote playback source</dfn>.
Remote playback source MUST belong to <a>availability sources
set</a>.
</p>
<p>
The mechanism of picking the <a>availability sources set</a> and
the <a>remote playback source</a> is implementation-specific. For
example, the user agent MUST either use the <a data-link-for=
"media element">currentSrc</a> of the <a>media element</a> for
both availability monitoring and remote playback or use all the
media resources associated with the media element as the
<a>availability sources set</a> and pick one of the resources as
the <a>remote playback source</a> after user selects the
<a>remote playback device</a>.
</p>
<p>
Remote playback is said to be <dfn>unavailable</dfn> for the
<a>media element</a> if the <a>list of available remote playback
devices</a> is empty or none of them is compatible with any
source from <a>availability sources set</a> for the <a>media
element</a>. The remote playback is said to be
<dfn>available</dfn> otherwise. A <code>boolean</code> set to
<code>false</code> if the remote playback is <a>unavailable</a>
for the <a>media element</a> or <code>true</code> if it is
<a>available</a> is called <dfn>availability</dfn> for the
<a>media element</a>.
</p>
<p>
If the user agent stops
<a data-lt="monitor the list of available remote playback devices">
monitoring the list of available remote playback devices</a>
(for example by a user control or for power saving), it SHOULD
invoke all callbacks in the <a>global set of availability
callbacks</a> with <code>false</code> so that pages can update
their user experience appropriately. It SHOULD also set
the <a>availability</a> value for all <a data-lt="media element">
media elements</a> to <code>false</code> so that availability
information can be propagated correctly if the user agent later
resumes
<a data-lt="monitor the list of available remote playback devices">
monitoring the list of available remote playback devices</a>.
</p>
</section>
<section>
<h5>
Getting the <a>remote playback devices</a> availability
information
</h5>
<p>
When the <code><dfn data-dfn-for=
"RemotePlayback">watchAvailability</dfn>()</code> method is
called, the user agent MUST run the following steps:
</p>
<dl>
<dt>
Input
</dt>
<dd>
<var>callback</var>, the callback that will get fired with
availability information.
</dd>
<dt>
Output
</dt>
<dd>
<var>promise</var>, a promise.
</dd>
</dl>
<ol>
<li>Let <var>promise</var> be a new promise.
</li>
<li>Return <var>promise</var>, and run the following steps below:
</li>
<li>If the <a data-link-for=
"HTMLMediaElement">disableRemotePlayback</a> attribute is present
for the <a>media element</a>, reject the <var>promise</var> with
<a>InvalidStateError</a> and abort all the remaining steps.
</li>
<li>If the user agent is unable to <a>monitor the list of
available remote playback devices</a> for the entire lifetime of
the <a>browsing context</a> (for instance, because the user has
disabled this feature), then run the following steps in parallel:
<ol>
<li>Fulfill <var>promise</var>.
</li>
<li>
<a>Queue a task</a> to invoke the <var>callback</var> with
<code>false</code> as its argument.
</li>
<li>Abort all remaining steps.
</li>
</ol>
</li>
<li>If the user agent is unable to continuously <a>monitor the
list of available remote playback devices</a> but can do it for a
short period of time when <a data-lt="initiate remote playback">
initiating remote playback</a>, then:
<ol>
<li>Reject <var>promise</var> with a <a>NotSupportedError</a>
exception.
</li>
<li>Abort all remaining steps.
</li>
</ol>
</li>
<li>Let <var>callbackId</var> be a positive integer unique among
all the <var>callbackIds</var> previously returned by these
steps in the <a>browsing context</a> of the <a>media
element</a>.
</li>
<li>Create a tuple <em>(<var>callbackId</var>,
<var>callback</var>)</em> and add it to the <a>set of
availability callbacks</a> for this <a>media element</a>.
</li>
<li>Fulfill <var>promise</var> with the <var>callbackId</var> and
run the following steps <a>in parallel</a>:
<ol>
<li>
<a>Queue a task</a> to invoke the <var>callback</var> with
the current <a>availability</a> for the <a>media
element</a>.
</li>
<li>If the <a>user agent</a> is not <a data-lt=
"monitor the list of available remote playback devices">monitoring
the list of available remote playback devices</a>, run the
algorithm to <a>monitor the list of available remote playback
devices</a>.
</li>
</ol>
</li>
</ol>
<p class="note">
A simple algorithm for assigning <var>callbackId</var> values is
to keep a counter for each <a>browsing context</a> and
incrementing it in step 6.
</p>
<div class="note">
To avoid leaking information that could fingerprint the user, the
user agent is expected not to assign a <var>callbackId</var> that
uses any persistent information from the browser profile or a
<a>remote playback device</a>.
</div>
</section>
<section>
<h5>
Monitoring the list of available remote playback devices
</h5>
<p>
If the <a>set of availability callbacks</a> is non-empty, or
there is a pending request to <a data-link-for="RemotePlayback"
data-lt="prompt">initiate remote playback</a>, the <a>user
agent</a> MUST <dfn>monitor the list of available remote playback
devices</dfn> by running the following steps:
</p>
<ol>
<li>Retrieve available remote playback devices (using an
implementation specific mechanism) and let <var>newDevices</var>
be this list.
</li>
<li>For each <a>media element</a> known to the <a>browsing
context</a>:
<ol>
<li>If the <a data-link-for=
"HTMLMediaElement">disableRemotePlayback</a> attribute is
present for <var>mediaElement</var>, abort all the remaining
steps for this tuple and continue to the next one.
</li>
<li>Set <var>newAvailabilityValue</var> to the value of
<a>availability</a> for the <a>media element</a> calculated
using the <var>newDevices</var> list instead of the <a>list
of available remote playback devices</a>.
</li>
<li>If the current <a>availability</a> is not equal to <var>
newAvailabilityValue</var>, then for each
<em>(<var>callbackId</var>, <var>callback</var>)</em> of
the element's <a>set of availability callbacks</a>:
<ol>
<li>
<a>Queue a task</a> to invoke <var>callback</var> with
<var>newAvailabilityValue</var> as its argument.
</li>
</ol>
</li>
</ol>
</li>
<li>Set the <a>list of available remote playback devices</a> to
the value of <var>newDevices</var>.
</li>
</ol>
</section>
<section>
<h5>
Stop observing remote playback devices availability
</h5>
<p>
When a <code><dfn data-dfn-for=
"RemotePlayback">cancelWatchAvailability</dfn>()</code> method is
called, the <a>user agent</a> MUST run the following steps:
</p>
<dl>
<dt>
Input
</dt>
<dd>
<var>id</var>, the callback identifier.
</dd>
<dt>
Output
</dt>
<dd>
<var>promise</var>, a promise.
</dd>
</dl>
<ol>
<li>Let <var>promise</var> be a new promise.
</li>
<li>Return <var>promise</var>, and run the following steps below:
</li>
<li>If the <a data-link-for=
"HTMLMediaElement">disableRemotePlayback</a> attribute is present
for the <a>media element</a>, reject <var>promise</var> with <a>
InvalidStateError</a> and abort all the remaining steps.
</li>
<li>If the parameter <var>id</var> is <code>undefined</code>,
clear the <a>set of availability callbacks</a>.
</li>
<li>Otherwise, if <var>id</var> matches the <var>callbackId</var>
for any entry in the <a>set of availability callbacks</a>, remove
the entry from the set.
</li>
<li>Otherwise, reject <var>promise</var> with
<a>NotFoundError</a> and abort all the remaining steps.
</li>
<li>If the <a>set of availability callbacks</a> is now empty and
there is no pending request to <a data-link-for="RemotePlayback"
data-lt="prompt">initiate remote playback</a>, cancel any
pending task to <a>monitor the list of available remote
playback devices</a> for power saving purposes.
</li>
<li>Fulfill <var>promise</var>.
</li>
</ol>
<div class="note">
The mechanism used to monitor <a>remote playback devices</a>
availability and determine the compatibility of a <a>remote
playback device</a> with the selected <a>availability sources
set</a> is left to the user agent.
</div>
</section>
</section>
<section>
<h4>
Prompt user for changing remote playback state
</h4>
<p>
When the <code><dfn data-dfn-for=
"RemotePlayback">prompt</dfn>()</code> method is called, the
<a>user agent</a> MUST run the following steps:
</p>
<dl>
<dt>
Input
</dt>
<dd>
None, but the algorithm references the <a>media element</a>, its
<a data-lt="RemotePlayback">remote</a> property and its
<a>availability sources set</a>.
</dd>
<dt>
Output
</dt>
<dd>
A promise.
</dd>
</dl>
<ol>
<li>Let <var>promise</var> be a new promise.
</li>
<li>Return <var>promise</var> and continue running these steps <a>
in parallel</a>.
</li>
<li>If the <a data-link-for=
"HTMLMediaElement">disableRemotePlayback</a> attribute is present
for the <a>media element</a>, reject the <var>promise</var> with
<a>InvalidStateError</a> and abort all the remaining steps.
</li>
<li>If there is already an unsettled promise from a previous call
to <a data-link-for="RemotePlayback">prompt</a> for the same
<a>media element</a> or even for the same <a>browsing context</a>,
the user agent MAY reject <var>promise</var> with an
<a>OperationError</a> exception and abort all remaining steps.
<div class="note">
The rationale here is that the user agent might show a dialog
that's modal to either the <a>media element</a> or the
<a>browsing context</a>. In such a case, the second call to
<code><a data-link-for="RemotePlayback">prompt</a>()</code>