VideoTrackList

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

length (en-US) 只读

The number of tracks in the list.

selectedIndex (en-US) 只读

The index of the currently selected track, if any, or −1 otherwise.

addtrack (en-US)

Fired when a new video track has been added to the media element. Also available via the onaddtrack property.

change (en-US)

Fired when a video track has been made active or inactive. Also available via the onchange property.

removetrack (en-US)

Fired when a new video track has been removed from the media element. Also available via the onremovetrack property.

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

getTrackById() (en-US)

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

addtrack (en-US)

Fired when a new video track has been added to the media element. Also available via the onaddtrack (en-US) property.

change (en-US)

Fired when a video track has been made active or inactive. Also available via the onchange (en-US) property.

removetrack (en-US)

Fired when a new video track has been removed from the media element. Also available via the onremovetrack (en-US) 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 (en-US) and removetrack (en-US) events, so that you can detect when tracks are added to or removed from the media element's stream.

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

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

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 (en-US) and removetrack (en-US) events are set up.

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

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