屏幕捕捉 API

屏幕捕捉 API 使用起来相对简单。其唯一的方法是MediaDevices.getDisplayMedia()，它的任务是以捕获的形式要求用户选择一个屏幕或屏幕的一部分的MediaStream。

要开始从屏幕上捕捉视频，你需要在 getDisplayMedia() 的实例上调用 Media navigator.mediaDevices：

captureStream =
  await navigator.mediaDevices.getDisplayMedia(displayMediaOptions);

The Promise returned by getDisplayMedia() resolves to a MediaStream which streams the captured media.

See the article Using the Screen Capture API for a more in-depth look at how to use the API to capture screen contents as a stream.

屏幕捕捉 API 没有自己的任何接口，而是在现有的 MediaDevices 接口上添加了一个方法。

MediaDevices.getDisplayMedia()

The getDisplayMedia() method is added to the MediaDevices interface. Similar to getUserMedia(), this method creates a promise that resolves with a MediaStream containing the display area selected by the user, in a format that matches the specified options.

The Screen Capture API adds properties to the following dictionaries defined by other specifications.

MediaTrackConstraints.cursor

A ConstrainDOMString indicating whether or not the cursor should be included in the captured display surface's stream, and if it should always be visible or if it should only be visible while the mouse is in motion.

MediaTrackConstraints.displaySurface

A ConstrainDOMString indicating what type of display surface is to be captured. The value is one of application, browser, monitor, or window.

MediaTrackConstraints.logicalSurface

Indicates whether or not the video in the stream represents a logical display surface (that is, one which may not be entirely visible onscreen, or may be completely offscreen). A value of true indicates a logical display surface is to be captured.

MediaTrackSettings.cursor

A string which indicates whether or not the display surface currently being captured includes the mouse cursor, and if so, whether it's only visible while the mouse is in motion or if it's always visible. The value is one of always, motion, or never.

MediaTrackSettings.displaySurface

A string indicating what type of display surface is currently being captured. The value is one of application, browser, monitor, or window.

MediaTrackSettings.logicalSurface

A Boolean value which is true if the video being captured doesn't directly correspond to a single onscreen display area.

MediaTrackSupportedConstraints.cursor

A Boolean which is true if the user agent and device support the MediaTrackConstraints.cursor constraint.

MediaTrackSupportedConstraints.displaySurface

A Boolean which is true if the current environment supports the MediaTrackConstraints.displaySurface constraint.

MediaTrackSupportedConstraints.logicalSurface

A Boolean which is true if the current environment supports the constraint MediaTrackConstraints.logicalSurface.

The following dictionaries are defined by the Screen Capture API.

CursorCaptureConstraint

An enumerated string type used to provide the value for the cursor property for the settings and constraints. The possible values are always, motion, and never.

DisplayCaptureSurfaceType

An enumerated string type which is used to identify the kind of display surface to capture. This type is used for the displaySurface property in the constraints and settings objects, and has the possible values application, browser, monitor, and window.

User agents that support Feature Policy (either using HTTP's Permissions-Policy header or the <iframe> attribute allow) can specify a desire to use the Screen Capture API using the policy control directive display-capture:

<iframe allow="display-capture" src="/some-other-document.html"></iframe>

The default allow list is self, which lets the any content within the document use Screen Capture.

See Using Feature Policy for a more in-depth explanation of how Feature Policy is used.

• Using the Screen Capture API
• MediaDevices.getDisplayMedia()