上传图片，点击更换

做这种效果：

imgUrl判断显示那个样式

```
      <el-form-item label="上传图片" v-model="imgUrl" class="items" prop="imgUrls">
        <div v-show="!imgUrl">
          <el-upload
            action
            list-type="picture-card"
            :limit="1"
            drag
            :show-file-list="false"
            :on-change="on_change"
            accept=".jpg, .png"
            :before-upload="onBeforeUpload"
            :http-request="uploadFile"
            :on-exceed="handlePicExceed"
          >
            <i slot="default" class="el-icon-plus">
              <ul>
                <li>拖拽或点击上传文件</li>
                <li>支持扩展名：png、jpg</li>
              </ul>
            </i>
          </el-upload>
        </div>
        <div v-show="imgUrl" style="width: 148px;height: 148px;position: relative;">
          <input
            type="file"
            ref="input"
            accept="image/png, image/jpeg, image/jpg"
            style="display:none"
            @change="Inputhange"
          />
          <img
            :src="imgUrl"
            alt
            width="148"
            height="148px"
            class="el-upload-list__item-thumbnail imgS"
          />
          <span class="shade" @click="changeClick">点击更改</span>
        </div>
      </el-form-item>
```

　　点击input框更换图片

    // 点击更改图片
    changeClick() {
      this.$refs.input.click();
    },
    Inputhange(e) {
      console.log(e.target.files[0]);
      this.files = e.target.files[0];
      let form = new FormData();
      // 后端接受参数 ，可以接受多个参数
      form.append("file", this.files);
      this.$post(
        "",
        form,
        res => {
          if (res.code === 1) {
            this.$message.error(res.msg);
            return;
          }
          this.imgUrl = res.data;
          this.ruleForm.imgUrls = 2; // 验证图片
          console.log(res);
        },
        err => {
          this.$message.error(err.msg);
          throw err;
        }
      );
    },

　　el-upload中的事件

    // 上传图片的change事件
    on_change(file) {
      let isJPG = file.raw.type === "image/jpeg";
      let isPNG = file.raw.type === "image/png";
      // console.log(isPNG);
      if (isJPG || isPNG) {
        console.log(file);
        return;
      }
      this.$message.error("上传图片只能是 JPG 格式或PNG格式");
    },

　　

    handlePicExceed() {
      this.$message({
        message: "最多上传1张图片",
        type: "warning"
      });
    },

　　

    // 上传前的钩子
    onBeforeUpload(file) {
      // console.log(file);
      const isIMAGE = file.type === "image/jpeg" || file.type === "image/png";
      // console.log(isIMAGE);
      if (!isIMAGE) {
        this.$message.error("上传文件只能是jpg,png格式!");
      }
      return isIMAGE;
    },

　　

    // 自定义上传
    uploadFile(file) {
      console.log(file);
      this.files = file.file;
      let form = new FormData();
      form.append("file", this.files);
      this.$post(
        "",
        form,
        res => {
          if (res.code === 1) {
            this.$message.error(res.msg);
            return;
          }
          // this.imgUrl = res.data;
          this.ruleForm.imgUrls = 2; // 验证图片
          this.imgUrl = res.data;
          console.log(res);
        },
        err => {
          this.$message.error(err.msg);
          throw err;
        }
      );
    },