elementPlus给el-upload加loading状态

为el-upload添加loading状态，可以通过以下步骤实现：

1. 在el-upload组件中添加一个loading属性，用于控制loading状态的显示和隐藏。

<el-upload
  class="upload-demo"
  action="/upload"
  :show-file-list="false"
  :loading="uploading"
  :on-success="handleSuccess"
  :on-error="handleError">
  <el-button slot="trigger" size="small" type="primary">选取文件</el-button>
  <el-button size="small" type="success" @click="submitUpload">上传到服务器</el-button>
  <div slot="tip" class="el-upload__tip">只能上传jpg/png文件，且不超过500kb</div>
</el-upload>

2. 在Vue组件中定义uploading变量，并在上传前将其设置为true，上传成功或失败后将其设置为false。

export default {
  data() {
    return {
      uploading: false
    };
  },
  methods: {
    submitUpload() {
      this.uploading = true;
      this.$refs.upload.submit();
    },
    handleSuccess(res, file) {
      this.uploading = false;
      this.$message({
        message: '上传成功',
        type: 'success'
      });
    },
    handleError(err, file) {
      this.uploading = false;
      this.$message.error('上传失败');
    }
  }
}

3. 在CSS中定义loading样式，用于显示loading状态。

.el-upload.is-uploading .el-upload__input {
  display: none;
}

.el-upload.is-uploading .el-upload__tip {
  display: none;
}

.el-upload.is-uploading .el-button {
  pointer-events: none;
  opacity: 0.6;
}

.el-upload.is-uploading .el-button.loading:before {
  content: '';
  position: absolute;
  top: calc(50% - 10px);
  left: calc(50% - 10px);
  width: 20px;
  height: 20px;
  border-radius: 50%;
  border: 2px solid #fff;
  border-top-color: #409EFF;
  animation: upload-loading 0.6s linear infinite;
}

@keyframes upload-loading {
  0% {
    transform: rotate(0);
  }
  100% {
    transform: rotate(360deg);
  }
}

这样就可以为el-upload添加loading状态了，上传文件时会显示loading动画。