**
 * 圖片壓縮工具類
 * 最大高度和最大寬度都為 500，如果超出大小將等比例縮放。
 *
 * 注意可能出現壓縮后比原圖更大的情況，在調用的地方自己判斷大小并決定上傳壓縮前或壓縮后的圖到服務器。
 */
// 將base64轉換為blob
export function convertBase64UrlToBlob(urlData) {
 let arr = urlData.split(',')
 let mime = arr[0].match(/:(.*?);/)[1]
 let bstr = atob(arr[1])
 let n = bstr.length
 let u8arr = new Uint8Array(n)
 while (n--) {
  u8arr[n] = bstr.charCodeAt(n)
 }
 return new Blob([u8arr], {type: mime})
}

// 壓縮圖片
export function compressImage(path) {
 //最大高度
 const maxHeight = 500;
 //最大寬度
 const maxWidth = 500;
 return new Promise((resolve, reject) => {
  let img = new Image();
  img.src = path;
  img.onload = function () {
   const originHeight = img.height;
   const originWidth = img.width;
   let compressedWidth = img.height;
   let compressedHeight = img.width;
   if ((originWidth > maxWidth) && (originHeight > maxHeight)) {
    // 更寬更高，
    if ((originHeight / originWidth) > (maxHeight / maxWidth)) {
     // 更加嚴重的高窄型，確定最大高，壓縮寬度
     compressedHeight = maxHeight
     compressedWidth = maxHeight * (originWidth / originHeight)
    } else {
     //更加嚴重的矮寬型, 確定最大寬，壓縮高度
     compressedWidth = maxWidth
     compressedHeight = maxWidth * (originHeight / originWidth)
    }
   } else if (originWidth > maxWidth && originHeight <= maxHeight) {
    // 更寬，但比較矮，以maxWidth作為基準
    compressedWidth = maxWidth
    compressedHeight = maxWidth * (originHeight / originWidth)
   } else if (originWidth <= maxWidth && originHeight > maxHeight) {
    // 比較窄，但很高，取maxHight為基準
    compressedHeight = maxHeight
    compressedWidth = maxHeight * (originWidth / originHeight)
   } else {
    // 符合寬高限制，不做壓縮
   }
   // 生成canvas
   let canvas = document.createElement('canvas');
   let context = canvas.getContext('2d');
   canvas.height = compressedHeight;
   canvas.width = compressedWidth;
   context.clearRect(0, 0, compressedWidth, compressedHeight);
   context.drawImage(img, 0, 0, compressedWidth, compressedHeight);
   let base64 = canvas.toDataURL('image/*', 0.8);
   let blob = convertBase64UrlToBlob(base64);
   // 回調函數返回blob的值。也可根據自己的需求返回base64的值
   resolve(blob)
  }
 })
}

<template>
 <div>
  <van-uploader v-model="fileList" :after-read="afterRead" />
 </div>
</template>
<script>
 import {compressImage} from '../../utils/CompressImageUtils'
 export default {
  components: {},
  methods: {
   //讀取完圖片后
   afterRead(file) {
    console.log('afterRead------', file);
    this._compressAndUploadFile(file);
   },
   //壓縮圖片上傳
   _compressAndUploadFile(file) {
    compressImage(file.content).then(result => {
     console.log('壓縮后的結果', result); // result即為壓縮后的結果
     console.log('壓縮前大小', file.file.size);
     console.log('壓縮后大小', result.size);
     if (result.size > file.file.size){
      console.log('上傳原圖');
      //壓縮后比原來更大，則將原圖上傳
      this._uploadFile(file.file, file.file.name);
     } else {
      //壓縮后比原來小，上傳壓縮后的
      console.log('上傳壓縮圖');
      this._uploadFile(result, file.file.name)
     }
    })
   },
   //上傳圖片
   _uploadFile(file, filename) {
    let params = new FormData();
    params.append("file", file, filename);
    this.$api.uploadImage(params).then(res => {
     console.log('uploadImage', res);
   //上傳成功，寫自己的邏輯
    }).catch(err => {
     console.log('err', err);
    });
   }, 
  }
 }
</script>

//上傳圖片
 uploadImage(params){
  return axios.post(`${base}/api/v1/file`, params, {
   headers: {'content-type': 'multipart/form-data'}
  })
 },