HarmonyOS canvas如何绘制成图片导出

imageSource.createPixelMap(opts, async (err, pixelMap) => {
  let offScreenCanvas = new OffscreenCanvas(value.size.width, value.size.height);
  let OffScreenContext = offScreenCanvas.getContext("2d");
  this.imageScale = offScreenCanvas.width / display.getDefaultDisplaySync().width;
  OffScreenContext.drawImage(pixelMap, 0, 0, offScreenCanvas.width, offScreenCanvas.height); //进行图像绘制

  let linearGradient = offScreenCanvas.getContext("2d", {
    antialias: true
  }).createLinearGradient(50, 0, 300, 0);
  linearGradient.addColorStop(0.0, '#ff0000');
  linearGradient.addColorStop(0.1, '#ffff00');
  linearGradient.addColorStop(1.0, '#00ff00');
  offScreenCanvas.getContext("2d", {
    antialias: true
  }).fillStyle = linearGradient;

  OffScreenContext.fillRect(30, 30, 100, 100)
  saveToFile(OffScreenContext.getPixelMap(0, 0, offScreenCanvas.width, offScreenCanvas.height), uri)
    .then(() => {
      this.showSuccess();
    });
})