微信小程序上传图片压缩

上传图片时图片过大，接口请求速度会很慢，利用canvas对图片进行压缩

wxml文件：

<canvas canvas-id="canvas" style="width:{{cWidth}}px;height:{{cHeight}}px;position: absolute;left:-1000px;top:-1000px;"></canvas>

js文件：

data: { 
  cWidth: 0,
  cHeight: 0
}

wx.chooseImage({
    count: 1,
    sizeType: ['compressed'], // 官方压缩设置
    success: function(res) {
      console.log(res);
      wx.getImageInfo({
        src: res.tempFilePaths[0],
        success: function(res) {
          var ratio = 2;
          // 图片原始长宽
          var canvasWidth = res.width;
          var canvasHeight = res.height;
          // 保证宽高在400以内
          while (canvasWidth > 400 || canvasHeight > 400){
              canvasWidth = Math.trunc(res.width / ratio);
              canvasHeight = Math.trunc(res.height / ratio);
              ratio++;
          }
          _this.setData({
              cWidth: canvasWidth,
              cHeight: canvasHeight
          })
          var ctx = wx.createCanvasContext('canvas');
          ctx.drawImage(res.path, 0, 0, canvasWidth, canvasHeight);
          ctx.draw(false, setTimeout(function(){
               wx.canvasToTempFilePath({
                   canvasId: 'canvas',
                   destWidth: canvasWidth,
                   destHeight: canvasHeight,
                   success: function (res) {
                       // 最终图片路径
                       console.log(res.tempFilePath);
                   },
                   fail: function (res) {
                       console.log(res.errMsg);
                  }
              })
          },100))
        }
      })
    }
})