HarmonyOS fs读取本地文件

可以在每次读取之后使用@ohos.buffer (Buffer)中的Buffer对象存储本次读取的值，在全部读取完成后，使用buffer.concat()合并所有Buffer的内容，demo参考如下：

async readLocalFileWithStream() {
  try {
    // 存储每次读取的结果
    let buffers: buffer.Buffer[] = [];
    // 打开文件流
    let inputStream = fs.createStreamSync(this.temp1Path, 'r+');
    // 以流的形式读取源文件内容
    let bufSize = 4096;
    let readSize = 0;
    let buf = new ArrayBuffer(bufSize);
    class Option {
      public offset: number = 0;
      public length: number = bufSize;
    }
    let option = new Option();
    option.offset = readSize;
    let readLen = await inputStream.read(buf, option);
    // 存储当前读取结果
    buffers.push(buffer.from(buf.slice(0, readLen)))
    readSize += readLen;
    while (readLen > 0) {
      option.offset = readSize;
      readLen = await inputStream.read(buf, option);
      // 存储当前读取结果
      buffers.push(buffer.from(buf.slice(0, readLen)))
      readSize += readLen;
    }
    // 关闭文件流
    inputStream.closeSync();
    // 合并内容
    let finalBuf: ArrayBuffer = buffer.concat(buffers).buffer
    console.info(`final ArrayBuffer byteLength is ${finalBuf.byteLength}}`)
  } catch (error) {
    let err = error as BusinessError;
    console.error(`readLocalFileWithStream failed, code is ${err.code},message is ${err.message}`);
  }
}