const ctx = canvas.getContext('2d');
let img = canvas.createImage();
img.src = imgUrl;
img.onload = (e) => {
  ctx.drawImage(img, 0, 0, 60, 60);
  wx.canvasGetImageData({
    canvasId: canvasId,
   x: 0,
   y: 0,
   width: 60,
   height: 60,
    fail(e){
        // 输出canvas内容是 empty， 要如何获取imageData
    }
  })
};