文件操作 ​

//=========================请求图片并预览
  base64Img: any = null
  getBase64(imgUrl: string): any {
    //判断浏览器是否支持FileReader
    if (window.FileReader) {
      return new Promise((resolve) => {
        fetch(imgUrl)
          .then((respone) => respone.blob()) //blob()是fetch中的方法,返回获取到的Blob内容
          .then((blob) => {
            let reader = new FileReader()
            let _this = this
            reader.onloadend = function () {
              console.log(reader.result)
              _this.base64Img = reader.result
              resolve(reader.result)
            }
            reader.readAsDataURL(blob)
          })
          .catch((e) => {
            throw Error(e)
          })
      })
    } else {
      alert('FileReader Not supported by your browser!')
    }
  }

//=====================上传图片并预览
  base64List: any[] = []
  async selectImg(e: any): Promise<void> {
    let fileList = e.target.files
    let base64List = []
    for (let i = 0; i < fileList.length; i++) {
      base64List.push(await this.getbase64Img(fileList[i]))
    }
    this.base64List.push(...base64List)
    e.target.value = '' //置空input的value，可重复上传
  }
  getbase64Img(file: File): any {
    return new Promise((resolve) => {
      let reader = new FileReader()
      reader.onloadend = function (res: any) {
        resolve(res.target.result)
      }
      reader.readAsDataURL(file)
    })
  }

//==============================================================ArrayBuffer
buffer = new ArrayBuffer(8); //创建一个8字节长度的ArrayBuffer
// console.log(this.buffer.byteLength) //8,因为ArrayBuffer是字节数组，它的元素就是字节，所以有多少字节长度就为多少

//===============================================================TypeArray
//1.Int8Array
buffer1 = new ArrayBuffer(8); //创建一个8字节长度的ArrayBuffer
int8Array1 = new Int8Array(this.buffer1); //创建一个带符号的8位的数组（也就是每一个元素占8个位，就是一字节，也可以和ArrayBuffer一样叫字节数组）
// console.log(this.int8Array1.length) //8,因为Int8Array是字节数组，它的元素就是字节，所以有多少字节长度就为多少
// console.log(this.int8Array1.buffer === this.buffer1) //true

//2.Int16Array
int16Array1 = new Int16Array(this.buffer1); //创建一个带符号的16位的数组（也就是每一个元素占16个位，就是2个字节，也可叫二字节数组）
// console.log(this.int16Array1.length)//4,因为Int16Array是二字节数组，它的每个元素占两个字节，所以 字节数/2 就是它的长度
// console.log(this.int16Array1.buffer === this.buffer1) //true

//3.Int32Array...

//=================================================================DataView
buffer2 = new ArrayBuffer(2); //2个字节的ArrayBuffer
dataView2 = new DataView(this.buffer2);
//dataView2.setInt8(0,1)
//dataView2.setInt8(1,2)
//console.log(this.dataView2.getInt8(0)) //1
//console.log(this.dataView2.getInt8(1)) //2
//console.log(this.dataView2.getInt16(0)) //258，getInt16(0)就是获取第一个双字所表示的十进制数字，与getInt8(0)不同的就是getInt16(0)获取的是第一个字节所表示的十进制数字

//console.log(this.dataView2.buffer === this.buffer2) //true

//=================================================================Blob
  data3 = { name: 'xiaopihong' }
  str3 = JSON.stringify(this.data3)
  blob3 = new Blob([this.str3], { type: 'application/json' })
  //console.log(this.blob3.size) //21,就是json字符串的长度
  //同一个blob可以被读成多种类型
  readBlob(blob: Blob, type: string): Promise<void> {
    return new Promise(function (resolve) {
      let reader = new FileReader()
      reader.onloadend = function (event: any) {
        // console.log(reader.result) //或者这里直接resolve(render.result)，也就是resolve(event.target.result)
        resolve(event.target.result)
      }
      switch (type) {
        case 'ArrayBuffer':
          reader.readAsArrayBuffer(blob) //读成ArrayBuffer格式
          break
        case 'DataURL':
          reader.readAsDataURL(blob) //读成DataURL格式，DataURL就是base64格式的字符串数据
          break
        case 'Text':
          reader.readAsText(blob, 'utf-8') //读成utf-8的文本格式
          break
        default:
          break
      }
    })
  }
  handleReadBlob(): void {
    this.readBlob(this.blob3, 'ArrayBuffer').then((result) => {
      console.log(result)
    })
    this.readBlob(this.blob3, 'DataURL').then((result) => {
      console.log(result)
    })
    this.readBlob(this.blob3, 'Text').then((result) => {
      console.log(result)
    })
  }

//=================================================================Object URL
  downloadByObjectURL(): void {
    let data = { name: 'xiaopihong' }
    let str = JSON.stringify(data)
    let blob = new Blob([str], { type: 'application/json' })
    let a = document.createElement('a')
    a.download = 'user.json'
    a.rel = 'noopener' //不打开
    let url = URL.createObjectURL(blob)
    a.href = url //Object URL可赋值给src url href
    a.dispatchEvent(new MouseEvent('click'))
    URL.revokeObjectURL(url) //销毁Object URL，然后会销毁blob
  }