视频轨道不具有Navigator.mediaDevices.getSupportedConstraints（）的功能

Question

我在页面上具有来自摄像机的流的视频标签，我想更改focusMode和focusDistance。

问题是我可以在navigator.mediaDevices.getSupportedConstraints（）中看到focusMode和focusDistance，但是我没有看到它们，所以我尝试将它们获取并从实际视频中进行更改，就像这样：

navigator.mediaDevices.getUserMedia(constraints)
      .then((stream) => handleSuccess(stream, canPlay))
      .catch(e => console.log(e))

function handleSuccess(stream, canPlay, video) {
  const video = document.getElementById('video');
  video.srcObject = stream
  const track = stream.getVideoTracks()[0];
  track.getCapabilities() // doesn't return focusMode or focusDistance.
}

总的来说，我想做这样的事情：

function onCapabilitiesReady(capabilities) {
    if (!capabilities.focusDistance) {
      return;
    } else {
      track.applyConstraints({
        advanced : [{focusMode: "manual", focusDistance: 0.3}]
      });
    }
  }