HarmonyOS 鸿蒙Next中读文件时用ArrayBuffer接收数据，转换string后JSON使用问题

您好！

目前系统没开放可变长的buffer，需要自己构建；

我这边写了一个demo；其中/test.txt为：

ArrayBuffer是预先申请的固定空间大小的buffer，并填充了一些数据，将ArrayBuffer转换为string类型，赋值给object中的字段，然后使用JSON把object转换成字符串。 END

如果直接定义arrayBuffer大小，那就回有空值一块返回，我们需要处理一下，请您参考以下demo;

```javascript
export class bufferWithLength {
  public arrays: ArrayBuffer | null = null;
  public lengths: number = 0

  constructor(arrays: ArrayBuffer, lengths: number) {
    this.arrays = arrays
    this.lengths = lengths
  }
}

function mergeArrayBuffer(buffers: bufferWithLength[]) {
  let size = 0
  for (let i = 0; i < buffers.length; i++) {
    size += buffers[i].lengths
  }
  let temp = new Uint8Array(size)
  let length = 0
  for (let i = 0; i < buffers.length; i++) {
    temp.set(new Uint8Array((buffers[i].arrays as ArrayBuffer).slice(0, buffers[i].lengths)), length)
    length += buffers[i].lengths
  }
  return temp.buffer as ArrayBuffer
}

async function savePhotoToGallery(context: common.UIAbilityContext) {
  let filesDir = context.filesDir;
  let file = await fileIo.open(filesDir + '/test.txt', fileIo.OpenMode.READ_ONLY)

  let readLen = 100
  let buffers: bufferWithLength[] = []
  let offset: number = 0

  while (readLen == 100) {
    let arrayBuffer = new ArrayBuffer(100);
    let readOptions: ReadOptions = { offset: offset, length: arrayBuffer.byteLength };
    readLen = fs.readSync(file.fd, arrayBuffer, readOptions);
    offset += readLen
    buffers.push(new bufferWithLength(arrayBuffer, readLen))
  } // 经验证可以得到所有信息,通过marge获得单一完整ArrayBuffer
  let res = mergeArrayBuffer(buffers) // 验证一下此arraybuffer读取到的内容
  let buf = buffer.from(res, 0, res.byteLength)
  console.info("the content of file: " + buf.toString())

  buf.toString().split(`\r\n|\n|\r`).forEach((line) => {
    let options: http.HttpRequestOptions = {
      method: http.RequestMethod.POST,
      extraData: {
        "input": line,
      },
      header: { 'Content-Type': 'application/json' },
      readTimeout: 60000,
      connectTimeout: 5000
    }
    console.info(`line:${line}, options:${JSON.stringify(options)}`)
    fs.closeSync(file);
  })
}

该demo 打印为：

line:ArrayBuffer是预先申请的固定空间大小的buffer，并填充了一些数据，将ArrayBuffer转换为string类型，赋值给object中的字段，然后使用JSON把object转换成字符串。, options:{"method":"POST","extraData":{"input":"ArrayBuffer是预先申请的固定空间大小的buffer，并填充了一些数据，将ArrayBuffer转换为string类型，赋值给object中的字段，然后使用JSON把object转换成字符串。"},"header":{"Content-Type":"application/json"},"readTimeout":60000,"connectTimeout":5000}
line:END, options:{"method":"POST","extraData":{"input":"END"},"header":{"Content-Type":"application/json"},"readTimeout":60000,"connectTimeout":5000}