// 在onLoad中调用
const that = this
wx.getSystemInfo({
 success: function (res) {
  console.log(res)
  that.setData({
   model: res.model,
   screen_width: res.windowWidth/375,
   screen_height: res.windowHeight
  })
 }
})

<canvas canvas-id="PosterCanvas" style="width:{{screen_width*375+'px'}}; height:{{screen_height*1.21+'px'}}"></canvas>
drawPoster(){
  let ctx = wx.createCanvasContext('PosterCanvas'),that=this.data;
  console.log('手机型号' + that.model,'宽'+that.screen_width*375,'高'+ that.screen_height)
  let rpx = that.screen_width
  //这里的rpx是相对不同屏幕宽度的相对单位，实际的宽度测量，就是实际测出的px像素值*rpx就可以了；之后无论实在iPhone5，iPhone6，iPhone7...都可以进行自适应。
  ctx.setFillStyle('#1A1A1A')
  ctx.fillRect(0, 0, rpx * 375, that.screen_height * 1.21)
  ctx.fillStyle = "#E8CDAA";
  ctx.setFontSize(29*rpx)
  ctx.font = 'normal 400 Source Han Sans CN';
  ctx.fillText('Hi 朋友', 133*rpx,66*rpx)
  ctx.fillText('先领礼品再买车', 84*rpx, 119*rpx)
  ctx.drawImage('../../img/sell_index5.png', 26*rpx, 185*rpx, 324*rpx, 314*rpx)
  ctx.drawImage('../../img/post_car2x.png', 66 * rpx, 222 * rpx, 243 * rpx, 145 * rpx)
  ctx.setFontSize(16*rpx)
  ctx.font = 'normal 400 Source Han Sans CN';
  ctx.fillText('长按扫描获取更多优惠', 108*rpx, 545*rpx)
  ctx.drawImage('../../img/code_icon2x.png', 68 * rpx, 575 * rpx, 79 * rpx, 79 * rpx)
  ctx.drawImage('../../img/code2_icon2x.png', 229 * rpx, 575 * rpx, 79 * rpx, 79 * rpx)
  ctx.setStrokeStyle('#666666')
  ctx.setLineWidth(1*rpx)
  ctx.lineTo(187*rpx,602*rpx)
  ctx.lineTo(187*rpx, 630*rpx)
  ctx.stroke()
  ctx.fillStyle = "#fff"
  ctx.setFontSize(13 * rpx)
  ctx.fillText('xxx科技汽车销售公司', 119 * rpx, 663 * rpx)
  ctx.fillStyle = "#666666"
  ctx.fillText('朝阳区·望京xxx科技大厦', 109 * rpx, 689 * rpx)
  ctx.setFillStyle('#fff')
  ctx.draw()
 },

wx.downloadFile({

url: that.data.url,
 success(res) {
  if (res.statusCode === 200) {
   // 活动
   ctx.drawImage(res.tempFilePath, 66 * rpx, 222 * rpx, 243 * rpx, 145 * rpx)
   // 二维码
   ctx.draw()
  }
 }
})

ctx.draw(true, ()=>{

// console.log('画完了')
  wx.canvasToTempFilePath()({
   x: 0,
   y: 0,
   width: rpx * 375,
   height: that.screen_height * 1.21,
   canvasId: 'PosterCanvas',
   success: function (res) {
    // console.log(res.tempFilePath);
    wx.saveImageToPhotosAlbum({
     filePath: res.tempFilePath,
     success: (res) => {
      console.log(res)
     },
     fail: (err) => { }
    })

   }
  }) 
 })

mpvue.saveImageToPhotosAlbum({

filePath: __path,
  success(res) {
   mpvue.showToast({
   title: '保存成功',
   icon: 'success',
   duration: 800,
   mask:true
   });
   },
  fail(res) {
    if (res.errMsg === "saveImageToPhotosAlbum:fail:auth denied" || res.errMsg === "saveImageToPhotosAlbum:fail auth deny" || res.errMsg === "saveImageToPhotosAlbum:fail authorize no response") {

   mpvue.showModal({
      title: '提示',
      content: '需要您授权保存相册',
      showCancel: false,
      success:modalSuccess=>{
       mpvue.openSetting({
        success(settingdata) {
         // console.log("settingdata", settingdata)
         if (settingdata.authSetting['scope.writePhotosAlbum']) {
          mpvue.showModal({
           title: '提示',
           content: '获取权限成功,再次点击图片即可保存',
           showCancel: false,
          })
         } else {
          mpvue.showModal({
           title: '提示',
           content: '获取权限失败，将无法保存到相册哦~',
           showCancel: false,
          })
         }
        },
        fail(failData) {
         console.log("failData",failData)
        },
        complete(finishData) {
         console.log("finishData", finishData)
        }
       })
      }
     })
   }
   }
 });