当前位置：文江博客知识库 en-US 文档 Web API TextTrackList

TextTrackList - Web APIs 编辑

The TextTrackList interface is used to represent a list of the text tracks defined by the <track> element, with each track represented by a separate textTrack object in the list.

Retrieve an instance of this object with HTMLMediaElement.textTracks. 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.

Event handlers

onaddtrack: An event handler to be called when the addtrack event is fired, indicating that a new text track has been added to the media element.
onchange: An event handler to be called when the change event occurs.
onremovetrack: An event handler to call when the removetrack event is sent, indicating that a text track has been removed from the media element.

Methods

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

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

Events

addtrack: Fired when a new text track has been added to the media element.
Also available via the onaddtrack property.
change: Fired when a text track has been made active or inactive.
Also available via the onchange property.
removetrack: Fired when a new text 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 text tracks present on a media element, TextTrackList 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 video element's text track list

To get a media element's TextTrackList, use its textTracks property.

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

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.

textTracks.onaddtrack = updateTrackCount;
textTracks.onremovetrack = updateTrackCount;

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