MediaDevices.getDisplayMedia() - Web APIs 编辑

The MediaDevices interface's getDisplayMedia() method prompts the user to select and grant permission to capture the contents of a display or portion thereof (such as a window) as a MediaStream. The resulting stream can then be recorded using the MediaStream Recording API or transmitted as part of a WebRTC session.

See Using the Screen Capture API for more details and an example.

Syntax

var promise = navigator.mediaDevices.getDisplayMedia(constraints);

Parameters

constraints Optional

An optional MediaStreamConstraints object specifying requirements for the returned MediaStream. Since getDisplayMedia() requires a video track, the returned stream will have one even if no video track is expressly requested by the constraints object.

Return value

A Promise that resolves to a MediaStream containing a video track whose contents come from a user-selected screen area, as well as an optional audio track.

Note: Browser support for audio tracks varies, both in terms of whether or not they're supported at all by the media recorder and in terms of the which audio source or sourcoes are supported. Check the compatibility table for details for each browser.

Exceptions

Rejections of the returned promise are made by passing a DOMException error object to the promise's failure handler. Possible errors are:

AbortError

An error or failure that doesn't match any of the other exceptions below occurred.

InvalidStateError

The call to getDisplayMedia() was not made from code running due to a user action, such as an event handler. Another potential cause for this event: the document in whose context getDisplayMedia() was called is not fully active; for example, perhaps it is not the frontmost tab.

NotAllowedError

Permission to access a screen area was denied by the user, or the current browsing instance is not permitted access to screen sharing.

NotFoundError

No sources of screen video are available for capture.

NotReadableError

The user selected a screen, window, tab, or other source of screen data, but a hardware or operating system level error or lockout occurred, preventing the sharing of the selected source.

OverconstrainedError

After creating the stream, applying the specified constraints fails because no compatible stream could be generated.

TypeError

The specified constraints include constraints which are not permitted when calling getDisplayMedia(). These unsupported constraints are advanced and any constraints which in turn have a member named min or exact.

Usage notes

Privacy and security

Because getDisplayMedia() could be used in nefarious ways, it can be a source of significant privacy and security concerns. For that reason, the specification details measures browsers are required to take in order to fully support getDisplayMedia().

• The specified constraints can't be used to limit the options available to the user. Instead, they must be applied after the user chooses a source, in order to generate output that matches the constraints.
• The go-ahead permission to use getDisplayMedia() cannot be persisted for reuse. The user must be prompted for permission every time.
• The call to getDisplayMedia() must be made from code which is running in response to a user action, such as in an event handler.
• Browsers are encouraged to provide a warning to users about sharing displays or windows that contain browsers, and to keep a close eye on what other content might be getting captured and shown to other users.

Example

In the example below, a startCapture() method is created which initiates screen capture given a set of options specified by the displayMediaOptions parameter. The options are specified in the form of a MediaStreamConstraints object which specifies the preferred stream configuration and the display surface from which video is to be captured.

async function startCapture(displayMediaOptions) {
  let captureStream = null;

  try {
    captureStream = await navigator.mediaDevices.getDisplayMedia(displayMediaOptions);
  } catch(err) {
    console.error("Error: " + err);
  }
  return captureStream;
}

This uses await to asynchronously wait for getDisplayMedia() to resolve with a MediaStream which contains the display contents as requested by the specified options. The stream is then returned to the caller for use, perhaps for adding to a WebRTC call using RTCPeerConnection.addTrack() to add the video track from the stream.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Screen Capture API
• Using the Screen Capture API
• Media Capture and Streams API
• WebRTC API
• getUserMedia(): Capturing media from a camera and/or microphone