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MediaTrackConstraints.groupId - Web APIs 编辑

The MediaTrackConstraints dictionary's groupId property is a ConstrainDOMString describing the requested or mandatory constraints placed upon the value of the groupId constrainable property.

If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.groupId as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.

Syntax

var constraintsObject = { groupId: constraint };

constraintsObject.groupId = constraint;

Value

An object based on ConstrainDOMString specifying one or more acceptable, ideal, and/or exact (mandatory) group IDs which are acceptable as the source of media content.

Group IDs are unique for a given origin for the duration of a single browsing session, and are shared by all media sources that come from the same physical device. For example, the microphone and speaker on the same headset would share a group ID. This makes it possible to use the group ID to ensure that the audio and input devices are on the same headset by retrieving the group ID of the input device and specifying it when asking for an output device, perhaps.

However, the value of the groupId is determined by the source of the track's content, and there's no particular format mandated by the specification (although some kind of GUID is recommended). That means that a given track will only return one value for the groupId when you call getCapabilities(), and keep in mind that this value will change for each browsing session.

Because of this, there's no use for the group ID when calling MediaStreamTrack.applyConstraints(), since there is only one possible value, and you can't use it to ensure the same group is used across multiple browsing sessions when calling getUserMedia().