微信小程序canvas画布绘制；canvas画布图片保存

wx.createSelectorQuery()
.select('#canvas')
.fields({ node:  true, size:  true  })
.exec(res  =>  {
let  ctx  =  res[0].node.getContext('2d');//getContext返回Canvas 的绘图上下文
let  canvas  =  res[0].node;
img=canvas;
console.log(img);
// 通过设备的像素比等重新绘制宽高
const  dpr  =  wx.getSystemInfoSync().pixelRatio
canvas.width  =  500  *  dpr
canvas.height  =  900  *  dpr
ctx.scale(dpr,  dpr)
console.log(res[0].width  *  dpr);
console.log(res[0].height  *  dpr);
let  image  =  canvas.createImage();//创建iamge实例
image.src  =  'https://xxxxxxxxx';
image.onload  =  function  (rrr)  {
ctx.drawImage(image,  0,  40,  500,  820);
}
let  image1  =  canvas.createImage();//创建iamge实例
image1.src  =  'https://xxxxxxxx';
image1.onload  =  function  (rrr)  {
ctx.drawImage(image1,  110,  435,  290,  290);
}
})
wx.createSelectorQuery().select('#canvas').boundingClientRect(function(res)  {
console.log(res);
}).exec();

wx.canvasToTempFilePath({
// 把画布转化成临时文件
x:  0,
y:  0,
width:  500,  // 截取的画布的宽
height:  900,  // 截取的画布的高
destWidth:  500,  // 保存成的画布宽度
destHeight:  900,  // 保存成的画布高度
fileType:  'jpg',  // 保存成的文件类型
quality:  1,  // 图片质量
canvas:img,  // 画布ID
success(res)  {
console.log(res.tempFilePath)
// 2-保存图片至相册
wx.saveImageToPhotosAlbum({
// 存成图片至手机
filePath:  res.tempFilePath,
success(res2)  {
wx.showToast({
title:  '保存成功',
duration:  2000
});
},
fail(res3)  {
if (res3.errMsg  ===  'saveImageToPhotosAlbum:fail auth deny') {
wx.showToast({
title:  '保存失败，稍后再试',
duration:  2000,
icon:  'none'
});
}  else  {
wx.showToast({
title:  '保存失败，稍后再试',
duration:  2000,
icon:  'none'
});
}
}
});
},
fail(error)  {
console.log(error);
wx.showToast({
title:  '保存失败，稍后再试',
duration:  2000,
icon:  'none'
});
}
});