我在 dart 中使用 BLE，我需要向特定特性发送 9 个字节，其中第一个字节是 5，其余是纪元 [英] I am working with BLE in dart, where I need to send 9 bytes to the specific characteristic , where first byte is 5 and remaining is epoch

问题描述

我尝试向特定特征发送 9 个字节，其中第一个字节是 0x05 ，即 5 ，接下来的 8 个字节作为以秒为单位的纪元，

Hi I trying to send 9 bytes to specific characteristic, where first byte is 0x05 , i.e 5 ,and next 8 bytes as epoch in seconds,

我试过了，

  List<int> timeDataForBLEWrite = [0x5, 0, 0, 0, 0, 0, 0, 0, 0 ];  // here 0 will be replaced by 8 bytes of epoch

为了在几秒钟内获得纪元，我试过这个，

to get epoch in seconds, I tried this,

  int timestampEpochInSeconds = DateTime.now().millisecondsSinceEpoch ~/ 1000; // 1623331779

将纪元转换为字节我已经试过了，

to convert epoch into bytes I have tried this,

 List<int> bytes = utf8.encode(timestampEpochInSeconds.toString());

但在这里我得到 10 个字节，因为 timestampEpochInSeconds 是 1623331779//10 位

but here I am getting 10 bytes because timestampEpochInSeconds is 1623331779 // 10 digits

 print(bytes); // [49, 54, 50, 51, 51, 51, 49, 55, 55, 57]

如何从秒纪元中获取 8 个整数，以便可以向特征发送总共 9 个字节.如下图，

how can I get 8 integers from the seconds epoch so that I can send total 9 bytes to the characteristic. like below,

 characteristic.write(timeDataForBLEWrite);

推荐答案

我假设您不想要以字节为单位的字符串，而是以字节为单位的值.

I am assuming that you don't want the string in bytes but the values in bytes.

蓝牙中的大多数数据都在 Little Endian 中，因此我假设时间戳为字节.

Most data in Bluetooth is in Little Endian so I have made that assumption about the timestamp as bytes.

我在 DartPad 上做了以下示例:

I did the following as an example on DartPad:

import 'dart:typed_data';

List<int> epoch() {
  var timestamp = DateTime.now().millisecondsSinceEpoch ~/ 1000;
  var sendValueBytes = ByteData(9);
  sendValueBytes.setUint8(0, 5);
  // setUint64 not implemented on some systems so use setUint32 in
  // those cases. Leading zeros to pad to equal 64 bit.
  // Epoch as 32-bit good until 2038 Jan 19 @ 03:14:07
  try {
    sendValueBytes.setUint64(1, timestamp.toInt(), Endian.little);
  } on UnsupportedError {
    sendValueBytes.setUint32(1, timestamp.toInt(), Endian.little);
  }
  return sendValueBytes.buffer.asUint8List();
}

void main() {
  print('Epoch Bytes (plus 0x05): ${epoch()}');
}

给出了以下输出:

Epoch Bytes (plus 0x05): [5, 167, 60, 194, 96, 0, 0, 0, 0]

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