asyncOpen() {
    return new Promise(resolve => {
      navigator.mediaDevices
        .getUserMedia(Camera.constraints)
        .then(MediaStream => {
          // 媒体流跟踪，用于后续关闭
          this.MediaStreamTrack =
            typeof stream.stop === 'function' ? stream : stream.getTracks()[0];

          this.video.srcObject = MediaStream;
          this.video.play();

          resolve(true);
        })
        .catch(err => {
          resolve(false);
        });
    });
  }

  close() {
    this.MediaStreamTrack && this.MediaStreamTrack.stop();
  }

getBase64Image() {
  if (!this.canvas) {
    initCanvas();
  }
  const ctx = this.canvas.getContext('2d');
  ctx.drawImage(video, 0, 0, Camera.constraints.width, Camera.constraints.height);
  return this.canvas.toDataURL();
}

initCanvas() {
  this.canvas = document.createElement('canvas');
  this.canvas.width = Camera.constraints.width;
  this.canvas.height = Camera.constraints.height;
  this.canvas.style.cssText = 'display:none;';
  document.body.appendChild(this.canvas);
}