Media Session

Editor’s Draft,

More details about this document
This version:
https://w3c.github.io/mediasession/
Latest published version:
https://www.w3.org/TR/mediasession/
Feedback:
GitHub
Editors:
(Google Inc.)
(Apple Inc.)
Former Editors:
(Google Inc.)
(Google Inc.)
(Google Inc.)
(Opera)
Version History:
https://github.com/w3c/mediasession/commits

Abstract

This specification enables web developers to show customized media metadata 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 section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

Feedback and comments on this specification are welcome. GitHub Issues are preferred for discussion on this specification. Alternatively, you can send comments to the Media Working Group’s mailing-list, public-media-wg@w3.org (archives). This draft highlights some of the pending issues that are still to be discussed in the working group. No decision has been taken on the outcome of these issues including whether they are valid.

This document was published by the Media Working Group as an Editor’s Draft. This document is intended to become a W3C Recommendation.

Publication as an Editor’s Draft does not imply endorsement by W3C and its Members.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 03 November 2023 W3C Process Document.

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 notifications, 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 that may come from platform UI or media keys, thereby improving the user experience.

2. Security and Privacy Considerations

This section is non-normative.

The API introduced in this specification has 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 needs to use this data with care. Another part of the API allows a website to receive 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.

2.1. 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 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 chances 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.

2.2. 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.

2.3. Media Session Actions

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

3. Model

3.1. 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.

3.2. 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 traversable and decendent navigables, and each navigable could have a MediaSession object.

The user agent MUST select at most one of the MediaSession objects to present 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 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.

3.3. 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 an 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 algorithms 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 type is "default", attempt to decode the resource as an image.
    3. If the image format is supported, use the image as the artwork for display in platform UI. Otherwise the fetch image algorithm fails and terminates.

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.

3.4. 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 a source can be the platform or the UI surfaces created by the user agent.

A media session action source has an optional target which should be the recipient of any media session action created by the media session action source. If a media session action source’s target is null, the active media session is the recipient of all media session action source’s actions.

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

  • play: the action’s intent is to resume the playback.
  • pause: the action’s intent is to pause the currently active playback.
  • seekbackward: the action’s intent is to move the playback time backward by a short period (eg. a few seconds).
  • seekforward: the action’s intent is to move the playback time forward by a short period (eg. a few seconds).
  • previoustrack: the action’s 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’s intent is to move to the playback to the next item in the playlist if the playback has a notion of playlist.
  • skipad: the action’s intent is to skip the advertisement that is currently playing.
  • stop: the action’s intent is to stop the playback and clear the state if appropriate.
  • seekto: the action’s intent is to move the playback time to a specific time.
  • togglemicrophone: the action’s intent is to mute or unmute the user’s microphone.
  • togglecamera: the action’s intent is to turn the user’s active camera on or off.
  • togglescreenshare: the action’s intent is to turn the user’s active screenshare on or off.
  • hangup: the action’s intent is to end a call.
  • previousslide: the action’s intent is to go back to the previous slide when presenting slides.
  • nextslide: the action’s intent is to go to the next slide when presenting slides.
  • enterpictureinpicture: the action’s intent is to open the media session in a picture-in-picture window.

All MediaSessions 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 named source that a media session action named action has been triggered, the user agent MUST queue a task, using the user interaction task source, to run the following handle media session action steps:

  1. Let session be source’s target.
  2. If session is null, set session to the active media session.
  3. If session is null, abort these steps.
  4. Let actions be session’s supported media session actions.
  5. If actions does not contain the key action, abort these steps.
  6. Let handler be the MediaSessionActionHandler associated with the key action in actions.
  7. Run handler with the details parameter set to: MediaSessionActionDetails.
  8. Run the activation notification steps in the browsing context associated with session.

When the user agent receives a joint command for play and pause, such as a headset button click, it MUST queue a task, using the user interaction task source, to 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 user agent MAY implement a default handler for the togglemicrophone, togglecamera, or togglescreenshare, or hangup media session actions if none was provided for the active media session.

A user agent MAY expose microphone, camera, and screenshare state to web pages via MediaStreamTrack's muted attribute in addition to togglemicrophone, togglecamera or togglescreenshare media session action. In that case, the user agent MUST execute the corresponding MediaSessionActionHandler before running, as different tasks, the steps defined to set a track’s muted state.

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 action 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.

3.5. Position State

A user agent MAY display the current playback position and duration of a media session in the platform UI depending on platform conventions. The position state is the combination of the following:

  • The duration of the media in seconds.
  • The playback rate of the media. It is a coefficient.
  • The last reported playback position of the media. This is the playback position of the media in seconds when the position state was created.

The position state is represented by a MediaPositionState which MUST always be stored with the last position updated time. This is the time the position state was last updated in seconds.

The RECOMMENDED way to determine the position state is to monitor the media elements whose node document’s browsing context is the browsing context.

The actual playback rate is a coefficient computed in the following way:

The current playback position in seconds is computed in the following way:

4. 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",
  "skipad",
  "stop",
  "seekto",
  "togglemicrophone",
  "togglecamera",
  "togglescreenshare",
  "hangup",
  "previousslide",
  "nextslide",
  "enterpictureinpicture"
};

callback MediaSessionActionHandler = undefined(MediaSessionActionDetails details);

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

  attribute MediaSessionPlaybackState playbackState;

  undefined setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);

  undefined setPositionState(optional MediaPositionState state = {});

  Promise<undefined> setMicrophoneActive(boolean active);

  Promise<undefined> setCameraActive(boolean active);

  Promise<undefined> setScreenshareActive(boolean active);
};

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

The setPositionState(state) method, when invoked MUST perform the following steps:

The setMicrophoneActive(active) method indicates to the user agent the microphone capture state desired by the page (e.g. if the microphone is considered "inactive" by the page since it is no longer sending audio through a call, the page can invoke setMicrophoneActive(false)). When invoked, it MUST perform the following steps:

  1. Let document be this's relevant global object's associated Document.
  2. Let captureKind be "microphone".
  3. Return the result of running the update capture state algorithm with document, active and captureKind.

Similarly, the setCameraActive(active) method indicates to the user agent the camera capture state desired by the page. When invoked, it MUST perform the following steps:

  1. Let document be this's relevant global object's associated Document.
  2. Let captureKind be "camera".
  3. Return the result of running the update capture state algorithm with document, active and captureKind.

Similarly, the setScreenshareActive(active) method indicates to the user agent the screenshare capture state desired by the page. When invoked, it MUST perform the following steps:

  1. Let document be this's relevant global object's associated Document.
  2. Let captureKind be "screenshare".
  3. Return the result of running the update capture state algorithm with document, active and captureKind.

The update capture state algorithm, when invoked with document, active and captureKind, MUST perform the following steps:

  1. If document is not fully active, return a promise rejected with InvalidStateError.
  2. If active is true and document’s visibility state is not "visible", the user agent MAY return a promise rejected with InvalidStateError.
  3. Let p be a new promise.
  4. In parallel, run the following steps:
    1. Let applyPausePolicy be true if the user agent implements a policy of pausing all input sources of type captureKind in response to UI and false otherwise.
    2. If applyPausePolicy is true, run the following substeps:
      1. Let currentlyActive be false if the user agent is currently pausing all input sources of type captureKind and true otherwise.
      2. If active is currentlyActive, resolve p with undefined and abort these steps.
      3. If active is true, the user agent MAY wait to proceed, for instance to prompt the user.
      4. If the user agent denies the request to update the capture state, reject p with a NotAllowedError and abort these steps.
    3. Update the user agent capture state UI according to captureKind and active.
    4. Resolve p with undefined.
    5. If applyPausePolicy is true, run the following substeps:
      1. Let newMutedState be true if active is false and false otherwise.
      2. For each MediaStreamTrack whose source is of type captureKind, queue a task to set a track’s muted state to newMutedState.
  5. Return p.

The setMicrophoneActive(active), setCameraActive(active) and setScreenshareActive(active) methods can reject based on user agent specific heuristics. This might in particular happen when the web page asks to activate (unmute) the microphone, camera or screenshare. The user agent could decide to require transient activation in that case. It might also require user input through a prompt to make the actual decision.

The user agent MAY display UI which invokes handlers for media session actions.

5. The MediaMetadata interface

[Exposed=Window]
interface MediaMetadata {
  constructor(optional MediaMetadataInit init = {});
  attribute DOMString title;
  attribute DOMString artist;
  attribute DOMString album;
  attribute FrozenArray<MediaImage> artwork;
  [SameObject] readonly attribute FrozenArray<ChapterInformation> chapterInfo;
};

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

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 has an associated list of chapter information.

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. Let chapters be an empty list of type ChapterInformation.
  7. For each entry in init’s chapterInfo, create a ChapterInformation from entry and append it to chapters.
  8. Set metadata’s chapter information to the result of creating a frozen array from chapters.
  9. 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 (which is a MediaImage list), 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 freeze(O) 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.

6. The ChapterInformation interface

[Exposed=Window]
interface ChapterInformation {
  readonly attribute DOMString title;
  readonly attribute double startTime;
  [SameObject] readonly attribute FrozenArray<MediaImage> artwork;
};

dictionary ChapterInformationInit {
  DOMString title = "";
  double startTime = 0;
  sequence<MediaImage> artwork = [];
};

A ChapterInformation object is a representation of metadata for an individual chapter, such as the title of the section, its timestamp, and screenshot image data of this section, that can be used by user agents to provide a customized user interface.

A ChapterInformation can have an associated media metadata.

A ChapterInformation has an associated title which is DOMString.

A ChapterInformation has an associated startTime which is double.

A ChapterInformation has an associated list of artwork images.

To create a ChapterInformation with init, run the following steps:

  1. Let chapterInfo be a new ChapterInformation object.
  2. Set chapterInfo’s title to init’s title.
  3. Set chapterInfo’s startTime to init’s startTime. If the startTime is negative or greater than duration, throw a TypeError.
  4. Let artwork be the result of running the convert artwork algorithm.
  5. Set chapterInfo’s artwork images to the result of creating a frozen array from artwork.
  6. Return chapterInfo.

The title attribute reflects the ChapterInformation's title. On getting, it MUST return the ChapterInformation's title.

The startTime attribute reflects the ChapterInformation's startTime in seconds. On getting, it MUST return the ChapterInformation's startTime.

The artwork attribute reflects the ChapterInformation's artwork images. On getting, it MUST return the ChapterInformation's artwork images.

7. The MediaImage dictionary

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

The MediaImage dictionary members are inspired by ImageResource in [IMAGE-RESOURCE].

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 the 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.

8. The MediaPositionState dictionary

dictionary MediaPositionState {
  unrestricted double duration;
  double playbackRate;
  double position;
};

The MediaPositionState dictionary is a representation of the current playback position associated with a MediaSession that can be used by user agents to provide a user interface that displays the current playback position and duration.

The duration dictionary member is used to specify the duration in seconds. It should always be positive and positive infinity can be used to indicate media without a defined end such as live playback.

The playbackRate dictionary member is used to specify the playback rate. It can be positive to represent forward playback or negative to represent backwards playback. It should not be zero.

The position dictionary member is used to specify the last reported playback position in seconds. It should always be positive.

9. The MediaSessionActionDetails dictionary

dictionary MediaSessionActionDetails {
  required MediaSessionAction action;
};

dictionary MediaSessionSeekActionDetails : MediaSessionActionDetails {
  double seekOffset;
};

dictionary MediaSessionSeekToActionDetails : MediaSessionActionDetails {
  required double seekTime;
  boolean fastSeek;
};

dictionary MediaSessionCaptureActionDetails : MediaSessionActionDetails {
  boolean isActivating;
};

The MediaSessionActionHandler MUST be run with the details parameter whose dictionary type is:

The action dictionary member is used to specify the media session action that the MediaSessionActionHandler is associated with.

The seekOffset dictionary member is the time in seconds to move the playback time by. If present, it should always be positive. If it is not provided then the site should choose a sensible time (e.g. a few seconds).

The seekTime dictionary member is the time in seconds to move the playback time to.

The fastSeek dictionary member will be true if the seek action is being called multiple times as part of a sequence and this is not the last call in that sequence.

The isActivating dictionary member will be false if the user agent is about to pause all input sources related to the capture action and true otherwise. This dictionary member MUST be present if the user agent implements a policy of pausing all input sources.

10. Permissions Policy Integration

This specification defines a policy-controlled feature identified by the string "mediasession". Its default allowlist is *.

A document’s permissions policy determines whether any content in that document is allowed to use the MediaSession API. If disabled in the document, the User Agent MUST NOT select the document’s media session as the active media session.

11. 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"}],
  chapterInfo: [
    {title: "Chapter 1", startTime: 0, artwork: [{src: "chapter1.jpg"}]},
    {title: "Chapter 2", startTime: 120, artwork: [{src: "chapter2.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 (the same for the artwork in chapterInfo):

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"}
  ],
  chapterInfo: [
    {title: "Chapter 1", startTime: 0, artwork: [
       {src: "chapter1_a.jpg", sizes: "128x128", type: "image/jpeg"},
       {src: "chapter1_b.png", sizes: "256x256", type: "image/png"}
     ]},
    {title: "Chapter 2", startTime: 120, artwork: [
       {src: "chapter2_a.jpg", sizes: "128x128", type: "image/jpeg"},
       {src: "chapter2_b.png", sizes: "256x256", type: "image/png"}
     ]}
  ]
});

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];

function 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();
});

navigator.mediaSession.setActionHandler("seekto", function(details) {
  audio.currentTime = details.seekTime;
});
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();
}
Setting position state:
// Media is loaded, set the duration.
navigator.mediaSession.setPositionState({
  duration: 60
});

// Media starts playing at the beginning.
navigator.mediaSession.playbackState = "playing";

// Media starts playing at 2x 10 seconds in.
navigator.mediaSession.setPositionState({
  duration: 60,
  playbackRate: 2,
  position: 10
});

// Media is paused.
navigator.mediaSession.playbackState = "paused";

// Media is reset.
navigator.mediaSession.setPositionState(null);
Using video conferencing actions:
var isMicrophoneActive = false;
var isCameraActive = false;

navigator.mediaSession.setMicrophoneActive(isMicrophoneActive);
navigator.mediaSession.setCameraActive(isCameraActive);

navigator.mediaSession.setActionHandler("togglemicrophone", function() {
  if (isMicrophoneActive) {
    // Mute the microphone. Implementation omitted.
  } else {
    // Unmute the microphone. Implementation omitted.
  }
  isMicrophoneActive = !isMicrophoneActive;
  navigator.mediaSession.setMicrophoneActive(isMicrophoneActive);
});

navigator.mediaSession.setActionHandler("togglecamera", function() {
  if (isCameraActive) {
    // Disable the camera. Implementation omitted.
  } else {
    // Enable the camera. Implementation omitted.
  }
  isCameraActive = !isCameraActive;
  navigator.mediaSession.setCameraActive(isCameraActive);
});

navigator.mediaSession.setActionHandler("hangup", function() {
  // End the call. Implementation omitted.
});
Handling presenting slide actions:
var currentSlideIndex = 0;

navigator.mediaSession.setActionHandler("previousslide", function() {
  currentSlideIndex--;
  // Set current slide. Implementation omitted.
});

navigator.mediaSession.setActionHandler("nextslide", function() {
  currentSlideIndex++;
  // Set current slide. Implementation omitted.
});
Handling picture-in-picture:
navigator.mediaSession.setActionHandler("enterpictureinpicture", function() {
  remoteVideo.requestPictureInPicture();
});

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.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformant Algorithms

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort 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 can 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 understand and are not intended to be performant. Implementers are encouraged to optimize.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[ECMASCRIPT]
ECMAScript Language Specification. URL: https://tc39.es/ecma262/multipage/
[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/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[MEDIACAPTURE-STREAMS]
Cullen Jennings; et al. Media Capture and Streams. URL: https://w3c.github.io/mediacapture-main/
[MIMESNIFF]
Gordon P. Hemsley. MIME Sniffing Standard. Living Standard. URL: https://mimesniff.spec.whatwg.org/
[PERMISSIONS-POLICY-1]
Ian Clelland. Permissions Policy. URL: https://w3c.github.io/webappsec-permissions-policy/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/
[WEBIDL]
Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/

Informative References

[IMAGE-RESOURCE]
Aaron Gustafson; Rayan Kanso; Marcos Caceres. Image Resource. URL: https://w3c.github.io/image-resource/

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",
  "skipad",
  "stop",
  "seekto",
  "togglemicrophone",
  "togglecamera",
  "togglescreenshare",
  "hangup",
  "previousslide",
  "nextslide",
  "enterpictureinpicture"
};

callback MediaSessionActionHandler = undefined(MediaSessionActionDetails details);

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

  attribute MediaSessionPlaybackState playbackState;

  undefined setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);

  undefined setPositionState(optional MediaPositionState state = {});

  Promise<undefined> setMicrophoneActive(boolean active);

  Promise<undefined> setCameraActive(boolean active);

  Promise<undefined> setScreenshareActive(boolean active);
};

[Exposed=Window]
interface MediaMetadata {
  constructor(optional MediaMetadataInit init = {});
  attribute DOMString title;
  attribute DOMString artist;
  attribute DOMString album;
  attribute FrozenArray<MediaImage> artwork;
  [SameObject] readonly attribute FrozenArray<ChapterInformation> chapterInfo;
};

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

[Exposed=Window]
interface ChapterInformation {
  readonly attribute DOMString title;
  readonly attribute double startTime;
  [SameObject] readonly attribute FrozenArray<MediaImage> artwork;
};

dictionary ChapterInformationInit {
  DOMString title = "";
  double startTime = 0;
  sequence<MediaImage> artwork = [];
};


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

dictionary MediaPositionState {
  unrestricted double duration;
  double playbackRate;
  double position;
};

dictionary MediaSessionActionDetails {
  required MediaSessionAction action;
};

dictionary MediaSessionSeekActionDetails : MediaSessionActionDetails {
  double seekOffset;
};

dictionary MediaSessionSeekToActionDetails : MediaSessionActionDetails {
  required double seekTime;
  boolean fastSeek;
};

dictionary MediaSessionCaptureActionDetails : MediaSessionActionDetails {
  boolean isActivating;
};