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 - flutter

Question

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,
I tried this,
List<int> timeDataForBLEWrite = [0x5, 0, 0, 0, 0, 0, 0, 0, 0 ]; // here 0 will be replaced by 8 bytes of epoch
to get epoch in seconds, I tried this,
int timestampEpochInSeconds = DateTime.now().millisecondsSinceEpoch ~/ 1000; // 1623331779
to convert epoch into bytes I have tried this,
List<int> bytes = utf8.encode(timestampEpochInSeconds.toString());
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]
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);

Answer 1

I am assuming that you don't want the string in bytes but the values in bytes.
Most data in Bluetooth is in Little Endian so I have made that assumption about the timestamp as bytes.
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()}');
}
Which gave the following output:
Epoch Bytes (plus 0x05): [5, 167, 60, 194, 96, 0, 0, 0, 0]