vue+elementUI图片同比例压缩的方法

/** 图片压缩，默认同比例压缩
 * @param {Object} fileObj
 * 图片对象
 * 回调函数有一个参数，base64的字符串数据
 */
export function compress(fileObj, callback) {
 try {
 const image = new Image()
 image.src = URL.createObjectURL(fileObj)
 image.onload = function() {
  const that = this
  // 默认按比例压缩
  let w = that.width
  let h = that.height
  const scale = w / h
  w = fileObj.width || w
  h = fileObj.height || (w / scale)
  let quality = 0.7 // 默认图片质量为0.7
  // 生成canvas
  const canvas = document.createElement('canvas')
  const ctx = canvas.getContext('2d')
  // 创建属性节点
  const anw = document.createAttribute('width')
  anw.nodeValue = w
  const anh = document.createAttribute('height')
  anh.nodeValue = h
  canvas.setAttributeNode(anw)
  canvas.setAttributeNode(anh)
  ctx.drawImage(that, 0, 0, w, h)
  // 图像质量
  if (fileObj.quality && fileObj.quality <= 1 && fileObj.quality > 0) {
  quality = fileObj.quality
  }
  // quality值越小，所绘制出的图像越模糊
  const data = canvas.toDataURL('image/jpeg', quality)
  // 压缩完成执行回调
  const newFile = convertBase64UrlToBlob(data)
  callback(newFile)
 }
 } catch (e) {
 console.log('压缩失败!')
 }
}
function convertBase64UrlToBlob(urlData) {
 const bytes = window.atob(urlData.split(',')[1]) // 去掉url的头，并转换为byte
 // 处理异常,将ascii码小于0的转换为大于0
 const ab = new ArrayBuffer(bytes.length)
 const ia = new Uint8Array(ab)
 for (let i = 0; i < bytes.length; i++) {
 ia[i] = bytes.charCodeAt(i)
 }
 return new Blob([ab], { type: 'image/png' })
}

<el-form-item ref="uploadElement" prop="picUrl" class="upload-img-form" label-width="0">
 <el-upload
 ref="uploadxls"
 class="avatar-uploader upload-img"
 :disabled="disabled"
 :auto-upload="false"
 :
 action="aaa"
 ::limit="1"
 :show-file-list="false"
 :on-change="handlePictureCardPreview"
 :before-upload="beforeupload"
 accept="image/png,image/gif,image/jpg,image/jpeg"
 >
 <!--<img v-if="dialogImageUrl" :src="dialogImageUrl" class="avatar">-->
 <i v-if="!dialogImageUrl" class="el-icon-plus avatar-uploader-icon" />
 <!--<i v-show="!dialogImageUrl" class="upload-icon" />
 <div v-show="!dialogImageUrl" slot="tip" class="el-upload__text upload__tip">上传实景图</div>-->
 <div v-if="showDelete" class="remove-img"><i class="el-icon-delete" @click.stop="removeImg" /></div>
 <div slot="tip" class="el-upload__tip">
  <p><span >*</span>上传楼宇实景图</p>
  <p>支持：.jpg .png .gif格式 建议比例：16:9，小于10M</p>
 </div>
 </el-upload>
</el-form-item>

// 图片预览
handlePictureCardPreview(file) {
 const _that = this
 const isLt10M = file.size / 1024 / 1024 < 10
 if (!isLt10M) {
 this.$message.error('上传图片大小不能超过 10M!')
 return false
 } else {
 this.dialogImageUrl = URL.createObjectURL(file.raw)
 compress(file.raw, function(val) {
  _that.theForm.picUrl = val
  _that.imgFile = val
  _that.showDelete = true
  _that.$refs['addBuildingForm'].validateField('picUrl')
 })
 }
}

// 上传前判断
  public async beforeUpload(file: any) {
    const is1M = file.size / 1024 / 1024 < 3; // 限制小于3M
    if (!is1M) {
      this.$message.error('图片尺寸限制最小为270 x 270，大小不可超过3MB，比例为1:1');
      return false;
    } else {
      const isSize = new Promise((resolve, reject) => {
        const width = 270;
        const height = 270;
        const _URL = window.URL || window.webkitURL;
        const img = new Image();
        img.onload = () => {
          const valid = img.width >= width && img.height >= height && img.width === img.height;
          valid &#63; resolve() : reject();
        };
        img.src = _URL.createObjectURL(file);
      }).then(
        () => {
          return file;
        },
        () => {
          this.$message.error('图片尺寸限制最小为270 x 270，大小不可超过3MB，比例为1:1');
          return Promise.reject();
        },
      );
      return isSize;
    }
  }