Element-UI中Upload上传文件前端缓存处理示例

Element-UI对于文件上传组件的功能点着重于文件传递到后台处理，所以要求action为必填属性。

但是如果需要读取本地文件并在前端直接处理，文件就没有必要传递到后台，比如在本地打开一个JSON文件，利用JSON文件在前端进行动态展示等等。

下面就展示一下具体做法：

首先定义一个jsonContent, 我们的目标是将本地选取的文件转换为JSON赋值给jsonContent

然后我们的模板文件是利用el-dialog和el-upload两个组件组合：这里停止文件自动上传模式:auto-upload="false"

 <el-button type="primary" @click="dialogVisible = true">Load from File</el-button>
 <el-dialog title="Load JSON document from file" :visible.sync="dialogVisible">
  <el-upload :file-list="uploadFiles" action="alert" :auto-upload="false" multiple :on-change="loadJsonFromFile">
   <el-button size="small" type="primary">Select a file</el-button>
   <div slot="tip">upload only jpg/png files, and less than 500kb</div>
  </el-upload>
  <span slot="footer">
   <el-button type="primary" @click="dialogVisible = false">cancel</el-button>
   <el-button type="primary" @click="loadJsonFromFileConfirmed">confirm</el-button>
  </span>
 </el-dialog>

最后通过html5的filereader对变量uploadFiles中的文件进行读取并赋值给jsonContent

if (this.uploadFiles) {
    for (let i = 0; i < this.uploadFiles.length; i++) {
     let file = this.uploadFiles[i]
     console.log(file.raw)
     if (!file) continue
     let reader = new FileReader()
     reader.onload = async (e) => {
      try {
       let document = JSON.parse(e.target.result)
       console.log(document)
      } catch (err) {
       console.log(`load JSON document from file error: ${err.message}`)
       this.showSnackbar(`Load JSON document from file error: ${err.message}`, 4000)
      }
     }
     reader.readAsText(file.raw)
    }
   }

为方便测试，以下是完整代码：

<template>
 <div>
  <el-button type="primary" @click="dialogVisible = true">Load from File</el-button>
 <el-dialog title="Load JSON document from file" :visible.sync="dialogVisible">
  <el-upload :file-list="uploadFiles" action="alert" :auto-upload="false" multiple :on-change="loadJsonFromFile">
   <el-button size="small" type="primary">Select a file</el-button>
   <div slot="tip">upload only jpg/png files, and less than 500kb</div>
  </el-upload>
  <span slot="footer">
   <el-button type="primary" @click="dialogVisible = false">cancel</el-button>
   <el-button type="primary" @click="loadJsonFromFileConfirmed">confirm</el-button>
  </span>
 </el-dialog>
</div>

</template>

<script>
export default {
 data () {
  return {
   // data for upload files
   uploadFilename: null,
   uploadFiles: [],
   dialogVisible: false
  }
 },
 methods: {
  loadJsonFromFile (file, fileList) {
   this.uploadFilename = file.name
   this.uploadFiles = fileList
  },
  loadJsonFromFileConfirmed () {
   console.log(this.uploadFiles)
   if (this.uploadFiles) {
    for (let i = 0; i < this.uploadFiles.length; i++) {
     let file = this.uploadFiles[i]
     console.log(file.raw)
     if (!file) continue
     let reader = new FileReader()
     reader.onload = async (e) => {
      try {
       let document = JSON.parse(e.target.result)
       console.log(document)
      } catch (err) {
       console.log(`load JSON document from file error: ${err.message}`)
       this.showSnackbar(`Load JSON document from file error: ${err.message}`, 4000)
      }
     }
     reader.readAsText(file.raw)
    }
   }
   this.dialogVisible = false
  }
 }
}
</script>

PS： 相关阅读

https://developer.mozilla.org/zh-CN/docs/Web/API/FileReader

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