Media Session Standard

Editor’s Draft,

This version:
https://wicg.github.io/mediasession
Editors:
(Google Inc.)
(Google Inc.)
Former Editors:
(Opera)
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Version History:
https://github.com/WICG/mediasession/commits

Abstract

This specification enables web developers to show customize media metadata shown on platform UI, customize available platform media controls, and access platform media keys such as hardware keys found on keyboards, headsets, remote controls, and software keys found in notification areas and on lock screens of mobile devices.

Status of this document

This specification was published by the Web Platform Incubator Community Group. It is not a W3C Standard nor is it on the W3C Standards Track. Please note that under the W3C Community Contributor License Agreement (CLA) there is a limited opt-out and other conditions apply. Learn more about W3C Community and Business Groups.

1. Introduction

This section is non-normative.

Media is used extensively today, and the Web is one of the primary means of consuming media content. Many platforms can display media metadata, such as title, artist, album and album art on various UI elements such as notification, media control center, device lockscreen and wearable devices. This specification aims to enable web pages to specify the media metadata to be displayed in platform UI, and respond to media controls which may come from platform UI or media keys, therefore improves the user experience.

2. Conformance

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.

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. [RFC2119]

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 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.)

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

When a method or an attribute is said to call another method or attribute, the user agent must invoke its internal API for that attribute or method so that e.g. the author can’t change the behavior by overriding attributes or methods with custom properties or functions in JavaScript.

Unless otherwise stated, string comparisons are done in a case-sensitive manner.

3. Dependencies

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

4. Security and Privacy Considerations

This section is non-normative.

The API introduced in this specification have very low impact with regards to security and privacy. Part of the API allows a website to expose metadata that can be used by the user agent. The user agent obviously need to use this data with care. Another part of the API allow a website to received commands from the user via buttons or other form of controls which might sometimes introduce a new input layer between the user and the website.

4.1. User interface guidelines

The MediaMetadata introduced in this specification allows a website to offer more information with regards to what is being played. The user agent is meant to use this information in any UI related to media playback, either internal to the user agent or within the platform.

The MediaMetadata are expected to be used in the context of media playback, making spoofing harder but because the MediaMetadata has text fields and image fields, a malicious website could try to spoof another website’s identity. It is recommended that the user agent offers a way to find the origin or clearly expose the origin of the website from which the metadata are coming from.

If a user agent offers a mechanism to go back to a website from a UI element created based on the MediaMetadata, it is recommended that the action should not be noticeable by the website, thus reducing the changes of spoofing.

In general, all security and privacy considerations related to the display of notifications from a website should apply here. It is worth noting that the MediaMetadata offer less customization than regular web notifications, thus would be harder to spoof.

4.2. Incognito mode

For privacy purposes, when in incognito mode, the user agent should be careful when sharing the information from MediaMetadata with the system and make sure they will not be used in a way that would harm the user. Displaying this information in a way that is very visible would be against the user’s intent of browsing in incognito mode. When available, the UI elements should be advertized as private to the platform.

4.3. Media Session Actions

Media session actions are exposing a new input layer to the web platform. User agents should make sure users are aware that there actions might be routed to the website with the active media session. Especially, when the actions are coming from remote devices such as headset or other remote devices. It is recommended for the user agent to follow the platform conventions when listening to these inputs in order to facilitate the user understanding.

5. Model

5.1. Playback State

In order to make play and pause actions work properly, the user agent SHOULD be able to determine if a browsing context of the active media session is playing media or not, which is called the guessed playback state. The RECOMMENDED way for determining the guessed playback state is to monitor the media elements whose node document’s browsing context is the browsing context. The browsing context’s guessed playback state is playing if any of them is potentially playing and not muted, and is paused otherwise. Other information SHOULD also be considered, such as WebAudio and plugins.

The playbackState attribute specifies the declared playback state from the browsing context. The state is combined with the guessed playback state to compute the actual playback state, which is a finalized state and will be used for play and pause actions.

The actual playback state is computed in the following way:

The playbackState attribute could be useful when the page wants to do some preparation steps when the media is paused but it allows the preparation steps to be interrupted by pause action. See Setting playbackState for example.

When the actual playback state of the active media session changes, the user agent MUST run the media session actions update algorithm.

5.2. Routing

There could be multiple MediaSession objects existing at the same time since the user agent could have multiple tabs, each tab could contain a top-level browsing context and multiple nested browsing contexts, and each browsing context could have a MediaSession object.

The user agent MUST select at most one of the MediaSession objects to present it to the user, which is called the active media session. The active media session may be null. The selection is up to the user agent and SHOULD be based on preferred user experience. Note that the playbackState attribute MUST not affect media session routing. It only takes effect for the active media session.

It is RECOMMENDED that the user agent selects the active media session by managing audio focus. A tab or browsing context is said to have audio focus if it is currently playing audio or the user expects to control the media in it. The AudioFocus API targets at this area and could be used once it’s finished.

Whenever the active media session is changed, the user agent MUST run the media session actions update algorithm and the update metadata algorithm.

5.3. Metadata

The media metadata for the active media session MAY be displayed in the platform UI depending on platform conventions. Whenever the active media session changes or setting metadata of the active media session, the user agent MUST run the update metadata algorithm. The steps are as follows:

  1. If the active media session is null, unset the media metadata presented to the platform, and terminate these steps.
  2. If the metadata of the active media session is an empty metadata, unset the media metadata presented to the platform, and terminate these steps.
  3. Update the media metadata presented to the platform to match the metadata for the active media session.
  4. If the user agent wants to display artwork image, it is RECOMMENDED to run the fetch image algorithm.

The RECOMMENDED fetch image algorithm is as follows:

  1. If there are other fetch image algorithm running, cancel existing algorithm execution instances.
  2. If metadata’s artwork of the active media session is empty, then terminate these steps.
  3. If the platform supports displaying media artwork, select a preferred artwork image from metadata’s artwork of the active media session.
  4. Fetch the preferred artwork image’s src.

    Then, in parallel:

    1. Wait for the response.
    2. If the response’s internal response’s type is default, attempt to decode the resource as image.
    3. If the image format is supported, use the image as the artwork for display in platform UI. Otherwise the fetch image algorithm fail and terminate.

If no images are fetched in the fetch image algorithm, the user agent MAY have fallback behavior such as displaying a default image as artwork.

5.4. Actions

A media session action is an action that the page can handle in order for the user to interact with the MediaSession. For example, a page can handle some actions that will then be triggered when the user presses buttons from a headset or other remote device.

A media session action source is a source that might produce a media session action. Such source can be the platform or the UI surfaces created by the user agent.

A media session action is represented by a MediaSessionAction which can have one of the following value:

  • play: the action intent is to resume the playback.
  • pause: the action intent is to pause a currently active playback.
  • seekbackward: the action intent is to move the playback time backward by a short period (eg. few seconds).
  • seekforward: the action intent is to move the playback time forward by a short period (eg. few seconds).
  • previoustrack: the action intent is to either start the current playback from the beginning if the playback has a notion of beginning or move to the previous item in the playlist if the playback has a notion of playlist.
  • nexttrack: the action is to move to the playback to the next item in the playlist if the playback has a notion of playlist.

All MediaSession have a map of supported media session actions with, as a key, a media session action and as a value a MediaSessionActionHandler.

When the update action handler algorithm on a given MediaSession with action and handler parameters is invoked, the user agent MUST run the following steps:

  1. If handler is null, remove action from the supported media session actions for MediaSession and abort these steps.
  2. Add action to the supported media session actions for MediaSession and associate to it the handler.

When the supported media session actions are changed, the user agent SHOULD run the media session actions update algorithm. The user agent MAY queue a task in order to run the media session actions update algorithm in order to avoid UI flickering when multiple actions are modified in the same event loop.

When the user agent is notified by a media session action source that a media session action named action has been triggered, the user agent MUST run the handle media session action steps as follow and consider it triggered by user activation:

  1. If the active media session is null, abort these steps.
  2. Let actions be the active media session’s supported media session actions.
  3. If actions does not contain the key action, abort these steps.
  4. Let handler be the MediaSessionActionHandler associated with the key action in actions.
  5. Run handler.

When the user agent receives a joint command for play and pause, such as a headset button click, it MUST run the following steps:

  1. If the active media session is null, abort these steps.
  2. Let action be a media session action.
  3. If the actual playback state of the active media session is playing, set action to pause.
  4. Otherwise, set action to play.
  5. Run the handle media session action steps with action.

It is RECOMMENDED for user agents to implement a default handler for the play and pause media session actions if none was provided for the active media session.

A page should only register a MediaSessionActionHandler for a media session action when it can handle the action given that the user agent will list this as a supported media session actions and update the media session action sources.

When the media session actions update algorithm is invoked, the user agent MUST run the following steps:

  1. Let available actions be an array of media session actions.
  2. If the active media session is null, set available actions to the empty array.
  3. Otherwise, set the available actions to the list of keys available in the active media session’s supported media session actions.
  4. For each media session action source source, run the following substeps:
    1. Optionally, if the active media session is not null:
      1. If the active media session’s actual playback state is playing, remove play from available actions.
      2. Otherwise, remove pause from available actions.
    2. If the source is a UI element created by the user agent, it MAY remove some elements from available actions if there are too many of them compared to the available space.
    3. Notify the source with the updated list of available actions.

6. The MediaSession interface

[Exposed=Window]
partial interface Navigator {
  [SameObject] readonly attribute MediaSession mediaSession;
};

enum MediaSessionPlaybackState {
  "none",
  "paused",
  "playing"
};

enum MediaSessionAction {
  "play",
  "pause",
  "seekbackward",
  "seekforward",
  "previoustrack",
  "nexttrack",
};

callback MediaSessionActionHandler = void();

[Exposed=Window]
interface MediaSession {
  attribute MediaMetadata? metadata;

  attribute MediaSessionPlaybackState playbackState;

  void setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
};

A MediaSession objects represents a media session for a given document and allows a document to communicate to the user agent some information about the playback and how to handle it.

A MediaSession has an associated metadata object represented by a MediaMetadata. It is initially null.

The mediaSession attribute MUST return the MediaSession instance associated with the Navigator object.

The metadata attribute reflects the MediaSession's metadata. On getting, it MUST return the MediaSession's metadata. On setting, it MUST run the following steps with value being the new value being set:

  1. If the MediaSession's metadata is not null, set its media session to null.
  2. Set the MediaSession's metadata to value.
  3. If the MediaSession's metadata is not null, set its media session to the current MediaSession.
  4. In parallel, run the update metadata algorithm.

The playbackState attribute represents the declared playback state of the media session, by which the session declares whether its browsing context is playing media or not. The initial value is none. On setting, the user agent MUST set the IDL attribute to the new value if it is a valid MediaSessionPlaybackState value. On getting, the user agent MUST return the last valid value that was set. The playbackState attribute is a hint for the user agent to determine whether the browsing context is playing or paused.

Setting playbackState may cause the actual playback state to change and run the media session actions update algorithm.

The MediaSessionPlaybackState enum is used to indicate whether a browsing context is playing media or not, the values are described as follows:

The setActionHandler() method, when invoked, MUST run the update action handler algorithm with action and handler on the MediaSession.

7. The MediaMetadata interface

[Constructor(optional MediaMetadataInit init), Exposed=Window]
interface MediaMetadata {
  attribute DOMString title;
  attribute DOMString artist;
  attribute DOMString album;
  attribute FrozenArray<MediaImage> artwork;
};

dictionary MediaMetadataInit {
  DOMString title = "";
  DOMString artist = "";
  DOMString album = "";
  sequence<MediaImage> artwork = [];
};

A MediaMetadata object is a representation of the metadata associated with a MediaSession that can be used by user agents to provide customized user interface.

A MediaMetadata can have an associated media session.

A MediaMetadata has an associated title, artist and album which are DOMString.

A MediaMetadata has an associated list of artwork images.

A MediaMetadata is said to be an empty metadata if it is equal to null or all the following conditions are true:

The MediaMetadata(init) constructor, when invoked, MUST run the following steps:

  1. Let metadata be a new MediaMetadata object.
  2. Set metadata’s title to init’s title.
  3. Set metadata’s artist to init’s artist.
  4. Set metadata’s album to init’s album.
  5. Run the convert artwork algorithm with init’s artwork as input and set metadata’s artwork images as the result if it succeeded.
  6. Return metadata.

When the convert artwork algorithm with input parameter is invoked, the user agent MUST run the following steps:

  1. Let output be an empty list of type MediaImage.
  2. For each entry in input’s artwork, perform the following steps:
    1. Let image be a new MediaImage.
    2. Let baseURL be the API base URL specified by the entry settings object.
    3. Parse entry’s src using baseURL. If it does not return failure, set image’s src to the return value. Otherwise, throw a TypeError and abort these steps.
    4. Set image’s sizes to entry’s sizes.
    5. Set image’s type to entry’s type.
    6. Append image to the output.
  3. Return output as result.

The title attribute reflects the MediaMetadata's title. On getting, it MUST return the MediaMetadata's title. On setting, it MUST set the MediaMetadata's title to the given value.

The artist attribute reflects the MediaMetadata's artist. On getting, it MUST return the MediaMetadata's artist. On setting, it MUST set the MediaMetadata's artist to the given value.

The album attribute reflects the MediaMetadata's album. On getting, it MUST return the MediaMetadata's album. On setting, it MUST set the MediaMetadata's album to the given value.

The artwork attribute reflects the MediaMetadata's artwork images. On getting, it MUST return the result of the following steps:

  1. Let frozenArtwork be an empty list of type MediaImage.
  2. For each entry in the MediaMetadata's artwork images, perform the following steps:
    1. Let image be a new MediaImage.
    2. Set image’s src to entry’s src.
    3. Set image’s sizes to entry’s sizes.
    4. Set image’s type to entry’s type.
    5. Call Object.freeze on image, to prevent accidental mutation by scripts.
    6. Append image to frozenArtwork.
  3. Create a frozen array from frozenArtwork.
On setting, it MUST run the convert artwork algorithm with the new value as input, and set the MediaMetadata's artwork images as the result if it succeeded.

When MediaMetadata's title, artist, album or artwork images are modified, the user agent MUST run the following steps:

  1. If the instance has no associated media session, abort these steps.
  2. Otherwise, queue a task to run the following substeps:
    1. If the instance no longer has an associated media session, abort these steps.
    2. Otherwise, in parallel, run the update metadata algorithm.

8. The MediaImage dictionary

dictionary MediaImage {
  required USVString src;
  DOMString sizes = "";
  DOMString type = "";
};

The MediaImage dictionary members are inspired from the image objects in Web App Manifest.

The src dictionary member is used to specify the MediaImage object’s source. It is a URL from which the user agent can fetch the image’s data.

The sizes dictionary member is used to specify the MediaImage object’s sizes. It follows the spec of sizes attribute in HTML link element, which is a string consisting of an unordered set of unique space-separated tokens which are ASCII case-insensitive that represents the dimensions of an image. Each keyword is either an ASCII case-insensitive match for the string "any", or a value that consists of two valid non-negative integers that do not have a leading U+0030 DIGIT ZERO (0) character and that are separated by a single U+0078 LATIN SMALL LETTER X or U+0058 LATIN CAPITAL LETTER X character. The keywords represent icon sizes in raw pixels (as opposed to CSS pixels). When multiple image objects are available, a user agent MAY use the value to decide which icon is most suitable for a display context (and ignore any that are inappropriate). The parsing steps for the sizes attribute MUST follow the parsing steps for HTML link element sizes attribute.

The type dictionary member is used to specify the MediaImage object’s MIME type. It is a hint as to the media type of the image. The purpose of this attribute is to allow a user agent to ignore images of media types it does not support.

9. Examples

This section is non-normative.

Setting metadata:
navigator.mediaSession.metadata = new MediaMetadata({
  title: "Episode Title",
  artist: "Podcast Host",
  album: "Podcast Title",
  artwork: [{src: "podcast.jpg"}]
});

Alternatively, providing multiple artwork images in the metadata can let the user agent be able to select different artwork images for different display purposes and better fit for different screens:

navigator.mediaSession.metadata = new MediaMetadata({
  title: "Episode Title",
  artist: "Podcast Host",
  album: "Podcast Title",
  artwork: [
    {src: "podcast.jpg", sizes: "128x128", type: "image/jpeg"},
    {src: "podcast_hd.jpg", sizes: "256x256"},
    {src: "podcast_xhd.jpg", sizes: "1024x1024", type: "image/jpeg"},
    {src: "podcast.png", sizes: "128x128", type: "image/png"},
    {src: "podcast_hd.png", sizes: "256x256", type: "image/png"},
    {src: "podcast.ico", sizes: "128x128 256x256", type: "image/x-icon"}
  ]
});

For example, if the user agent wants to use an image as icon, it may choose "podcast.jpg" or "podcast.png" for a low-pixel-density screen, and "podcast_hd.jpg" or "podcast_hd.png" for a high-pixel-density screen. If the user agent wants to use an image for lockscreen background, "podcast_xhd.jpg" will be preferred.

Changing metadata:

For playlists or chapters of an audio book, multiple media elements can share a single media session.

var audio1 = document.createElement("audio");
audio1.src = "chapter1.mp3";

var audio2 = document.createElement("audio");
audio2.src = "chapter2.mp3";

audio1.play();
audio1.addEventListener("ended", function() {
  audio2.play();
});

Because the session is shared, the metadata must be updated to reflect what is currently playing.

function updateMetadata(event) {
  navigator.mediaSession.metadata = new MediaMetadata({
    title: event.target == audio1 ? "Chapter 1" : "Chapter 2",
    artist: "An Author",
    album: "A Book",
    artwork: [{src: "cover.jpg"}]
  });
}

audio1.addEventListener("play", updateMetadata);
audio2.addEventListener("play", updateMetadata);
Handling media session actions:
var tracks = ["chapter1.mp3", "chapter2.mp3", "chapter3.mp3"];
var trackId = 0;

var audio = document.createElement("audio");
audio.src = tracks[trackId];

void updatePlayingMedia() {
  audio.src = tracks[trackId];
  // Update metadata (omitted)
}

navigator.mediaSession.setActionHandler("previoustrack", function() {
  trackId = (trackId + tracks.length - 1) % tracks.length;
  updatePlayingMedia();
});

navigator.mediaSession.setActionHandler("nexttrack", function() {
  trackId = (trackId + 1) % tracks.length;
  updatePlayingMedia();
});
Setting playbackState:

When a page pauses its media and plays a third-party ad in an iframe, the UA might consider the session as "not playing", however the page wants to allow the user to pause the ad playback and cancel the pending playback after the ad finishes.

var adFrame;
var audio = document.createElement("audio");
audio.src = "foo.mp3";

function resetActionHandlers() {
  navigator.mediaSession.setActionHandler("play", _ => audio.play());
  navigator.mediaSession.setActionHandler("pause", _ => audio.pause());
}

resetActionHandlers();

// This method will be called when the page wants to play some ad.
function pauseAudioAndPlayAd() {
  audio.pause();
  navigator.mediaSession.playbackState = "playing";
  setUpAdFrame();
  adFrame.contentWindow.postMessage("play_ad");
  navigator.mediaSession.setActionHandler("pause", pauseAd);
}

function pauseAd() {
  adFrame.contentWindow.postMessage("pause_ad");
  navigator.mediaSession.playbackState = "paused";
  navigator.mediaSession.setActionHandler("play", resumeAd);
}

function resumeAd() {
  adFrame.contentWindow.postMessage("resume_ad");
  navigator.mediaSession.playbackState = "playing";
  navigator.mediaSession.setActionHandler("pause", pauseAd);
}

window.onmessage = function(e) {
  if (e.data === "ad finished") {
    removeAdFrame();
    navigator.mediaSession.playbackState = "none";
    resetActionHandlers();
  }
}

function setUpAdFrame() {
  adFrame = document.createElement("iframe");
  adFrame.src = "https://example.com/ad-iframe.html";
  document.body.appendChild(adFrame);
}

function removeAdFrame() {
  adFrame.remove();
}

Acknowledgments

The editors would like to thank Paul Adenot, Jake Archibald, Tab Atkins, Jonathan Bailey, François Beaufort, Marcos Caceres, Domenic Denicola, Ralph Giles, Anne van Kesteren, Tobie Langel, Michael Mahemoff, Jer Noble, Elliott Sprehn, Chris Wilson, and Jörn Zaefferer for their participation in technical discussions that ultimately made this specification possible.

Special thanks go to Philip Jägenstedt and David Vest for their help in designing every aspect of media sessions and for their seemingly infinite patience in working through the initial design issues; Jer Noble for his help in building a model that also works well within the iOS audio focus model; and Mounir Lamouri and Anton Vayvod for their early involvement, feedback and support in making this specification happen.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[APPMANIFEST]
Marcos Caceres; et al. Web App Manifest. URL: https://w3c.github.io/manifest/
[FETCH]
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WEBIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://heycam.github.io/webidl/
[WHATWG-URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/

IDL Index

[Exposed=Window]
partial interface Navigator {
  [SameObject] readonly attribute MediaSession mediaSession;
};

enum MediaSessionPlaybackState {
  "none",
  "paused",
  "playing"
};

enum MediaSessionAction {
  "play",
  "pause",
  "seekbackward",
  "seekforward",
  "previoustrack",
  "nexttrack",
};

callback MediaSessionActionHandler = void();

[Exposed=Window]
interface MediaSession {
  attribute MediaMetadata? metadata;

  attribute MediaSessionPlaybackState playbackState;

  void setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
};

[Constructor(optional MediaMetadataInit init), Exposed=Window]
interface MediaMetadata {
  attribute DOMString title;
  attribute DOMString artist;
  attribute DOMString album;
  attribute FrozenArray<MediaImage> artwork;
};

dictionary MediaMetadataInit {
  DOMString title = "";
  DOMString artist = "";
  DOMString album = "";
  sequence<MediaImage> artwork = [];
};

dictionary MediaImage {
  required USVString src;
  DOMString sizes = "";
  DOMString type = "";
};