<canvas canvas-id='attendCanvasId' class='myCanvas'></canvas>

// 点击加_压缩
 takePhoto: function () {
  var that = this;
  let imgViewList = that.data.imgViewList; //这个是用来承载页面循环展示图片的
   //拍照、从相册选择上传
   wx.chooseImage({
    count: 4,  //这个是上传的最大数量，默认为9
    sizeType: ['compressed'],  //这个可以理解为上传的图片质量类型（官方给的），虽然没什么卵用，要不然还要我们自己写压缩做什么
    sourceType: ['album', 'camera'],  //这个是图片来源，相册或者相机
    success: function (res) {
     var tempFilePaths = res.tempFilePaths  //这个是选择后返回的图片列表
     that.getCanvasImg(0, 0, tempFilePaths);  //进行压缩
    } 
   });
 },
 //压缩并获取图片，这里用了递归的方法来解决canvas的draw方法延时的问题
 getCanvasImg: function (index,failNum, tempFilePaths){
  var that = this;
  if (index < tempFilePaths.length){
   const ctx = wx.createCanvasContext('attendCanvasId');
   ctx.drawImage(tempFilePaths[index], 0, 0, 300, 150);
   ctx.draw(true, function () {
    index = index + 1;//上传成功的数量，上传成功则加1
    wx.canvasToTempFilePath({
     canvasId: 'attendCanvasId',
     success: function success(res) {
      that.uploadCanvasImg(res.tempFilePath);
      that.getCanvasImg(index,failNum,tempFilePaths);
     }, fail: function (e) {
      failNum += 1;//失败数量，可以用来提示用户
      that.getCanvasImg(inedx,failNum,tempFilePaths);
     }
    });
   });
  }
 },
 //上传图片
 uploadCanvasImg: function (canvasImg){
  var that = this;
  let imgViewList = that.data.imgViewList;
  var tempImg = canvasImg;
  wx.uploadFile({
   url: app.d.fileServer,//文件服务器的地址
   filePath: tempImg,
   formData: {
    paramPath: "gift"
   },
   name: 'file',
   success: function (res) {
    var json2map = JSON.parse(res.data);
    imgViewList.push(app.d.imageUrlFix + json2map[0].fileUrl);
    that.setData({
     imgViewList: imgViewList,
    })
   }
  })
 },