微信小程序中的图片压缩问题

Question

现在在做一个图片上传的功能，但是用户上传图片太大会导致上传时间太长，小程序自带的chooseImage里面的压缩有限，现在找到一种办法，就是canvas里面的toDataURL方法，可以把图片转为base64的编码，但是调用此方法需要选中dom元素，代码如下

compressImage (url) {
  let cvs = document.createElement('canvas')
  let ctx = cvs.getContext('2d')
  let img = new window.Image()
  img.src = url
  img.onload = () => {
    cvs.width = img.width
    cvs.height = img.height
    setTimeout(() => {
      ctx.drawImage(img, 0, 0, cvs.width, cvs.height)
      this.newImageData = cvs.toDataURL('image/jpeg', 0.1)
    }, 0)
    this.showPreviewer = true
  }
},

微信小程序无法选中dom元素，所以想知道1.如何调用此方法
2.还有一种方法就是canvas画图，直接保存。想知道chooseImage里面的压缩，toDataURL和canvas画图保存哪种办法好一点

Answer 1

wxml写入

<view bindtap='uploadImg'>上传</view>
 <image bindtap='uploadImg' data-number="0" src="{{ uploadPic[0] }}" alt="" mode="widthFix"></image>
<canvas style="width: {{cw}}px; height: {{ch}}px;" canvas-id="firstCanvas"></canvas>

JS参考

 uploadImg(e) {
    let that = this;
    console.log(e);
    let index = e.currentTarget.dataset.number;
    let uploadFile = ''; //最后处理完，图片上传的图片地址
    wx.chooseImage({
      success(res) {
        console.log(res)
        const tempFilePaths = res.tempFilePaths;

        //获得原始图片大小
        wx.getImageInfo({
          src: res.tempFilePaths[0],
          success(res) {
            // console.log('获得原始图片大小',res.width)
            //console.log(res.height)
            var originWidth, originHeight;
            originHeight = res.height;
            originWidth = res.width;
            console.log(originWidth);
            //压缩比例
            // 最大尺寸限制
            var maxWidth = 1200,
              maxHeight = 600;
            // 目标尺寸
            var targetWidth = originWidth,
              targetHeight = originHeight;
            //等比例压缩，如果宽度大于高度，则宽度优先，否则高度优先
            if (originWidth > maxWidth || originHeight > maxHeight) {
              if (originWidth / originHeight > maxWidth / maxHeight) {
                // 要求宽度*(原生图片比例)=新图片尺寸
                targetWidth = maxWidth;
                targetHeight = Math.round(maxWidth * (originHeight / originWidth));
              } else {
                targetHeight = maxHeight;
                targetWidth = Math.round(maxHeight * (originWidth / originHeight));
              }
            }
            //尝试压缩文件，创建 canvas
            var ctx = wx.createCanvasContext('firstCanvas');
            ctx.clearRect(0, 0, targetWidth, targetHeight);
            ctx.drawImage(tempFilePaths[0], 0, 0, targetWidth, targetHeight);
            ctx.draw();
            //更新canvas大小
            that.setData({
              cw: targetWidth,
              ch: targetHeight
            });
            //保存图片
            setTimeout(function() {
              wx.canvasToTempFilePath({
                canvasId: 'firstCanvas',
                success: (res) => {
                  //写入图片数组
                  var uploadpic = "uploadPic[" + index + "]";
                  //
                  that.setData({
                    [uploadpic]: res.tempFilePath
                  });
                  uploadFile = res.tempFilePath;
                  //保存到相册
                  // wx.saveImageToPhotosAlbum({
                  //   filePath: res.tempFilePath,
                  //   success: (res) => {
                  //     console.log(res)
                  //   },
                  //   fail: (err) => {
                  //     console.error(err)
                  //   }
                  // })
                  wx.uploadFile({
                    url: 'https://example.weixin.qq.com/upload', //仅为示例，非真实的接口地址
                    filePath: uploadFile,
                    name: 'file',
                    formData: {
                      'user': 'test'
                    },
                    success(res) {
                      const data = res.data
                      //do something
                    }
                  })
                },
                fail: (err) => {
                  console.error(err)
                }
              }, this)
            }, 500);




          }
        })




      }
    })
  }

来源：www.heibaiketang.com.
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