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VideoTrackList - Web APIs 编辑

The VideoTrackList interface is used to represent a list of the video tracks contained within a <video> element, with each track represented by a separate VideoTrack object in the list.

Retrieve an instance of this object with HTMLMediaElement.videoTracks. The individual tracks can be accessed using array syntax or functions such as forEach() for example.

Properties

This interface also inherits properties from its parent interface, EventTarget.

length Read only: The number of tracks in the list.
selectedIndex Read only: The index of the currently selected track, if any, or −1 otherwise.

Event handlers

onaddtrack: An event handler to be called when the addtrack event is fired, indicating that a new video track has been added to the media element.
onchange: An event handler to be called when the change event occurs — that is, when the value of the selected property for a track has changed, due to the track being made active or inactive.
onremovetrack: An event handler to call when the removetrack event is sent, indicating that a video track has been removed from the media element.

Methods

This interface also inherits methods from its parent interface, EventTarget.

getTrackById(): Returns the VideoTrack found within the VideoTrackList whose id matches the specified string. If no match is found, null is returned.

Events

addtrack: Fired when a new video track has been added to the media element.
Also available via the onaddtrack property.
change: Fired when a video track has been made active or inactive.
Also available via the onchange property.
removetrack: Fired when a new video track has been removed from the media element.
Also available via the onremovetrack property.

In addition to being able to obtain direct access to the video tracks present on a media element, VideoTrackList lets you set event handlers on the addtrack and removetrack events, so that you can detect when tracks are added to or removed from the media element's stream. See onaddtrack and onremovetrack for details and examples.

Examples

Getting a media element's video track list

To get a media element's VideoTrackList, use its videoTracks property.

var videoTracks = document.querySelector("video").videoTracks;

Monitoring track count changes

In this example, we have an app that displays information about the number of channels available. To keep it up to date, handlers for the addtrack and removetrack events are set up.

videoTracks.onaddtrack = updateTrackCount;
videoTracks.onremovetrack = updateTrackCount;

function updateTrackCount(event) {
  trackCount = videoTracks.length;
  drawTrackCountIndicator(trackCount);
}