manual big endian conversion in Dart

writer.dart

library writer;

import 'dart:typed_data';

/// Writer wraps a fixed size Uint8List and writes values into it using
/// big-endian byte order.
class Writer {
  /// Output buffer.
  final Uint8List out;

  /// Current position within [out].
  var position = 0;

  Writer._create(this.out);

  factory Writer(size) {
    final out = new Uint8List(size);
    if (Endianness.HOST_ENDIAN == Endianness.LITTLE_ENDIAN) {
      return new _WriterForLEHost._create(out);
    } else {
      return new _WriterForBEHost._create(out);
    }
  }

  writeFloat64(double v);

  writeFloat32(double v);

  writeInt32(int v) {
    out[position + 3] = v;
    out[position + 2] = (v >> 8);
    out[position + 1] = (v >> 16);
    out[position + 0] = (v >> 24);
    position += 4;
  }

  writeInt16(int v) {
    out[position + 1] = v;
    out[position + 0] = (v >> 8);
    position += 2;
  }

  writeInt8(int v) {
    out[position] = v;
    position++;
  }

  writeString(String str) {
    out.setAll(position, str.codeUnits);
    position += str.codeUnits.length;
  }
}

/// Lists used for data convertion (alias each other).
final Uint8List _convU8 = new Uint8List(8);
final Float32List _convF32 = new Float32List.view(_convU8.buffer);
final Float64List _convF64 = new Float64List.view(_convU8.buffer);

/// Writer used on little-endian host.
class _WriterForLEHost extends Writer {
  _WriterForLEHost._create(out) : super._create(out);

  writeFloat64(double v) {
    _convF64[0] = v;
    out[position + 7] = _convU8[0];
    out[position + 6] = _convU8[1];
    out[position + 5] = _convU8[2];
    out[position + 4] = _convU8[3];
    out[position + 3] = _convU8[4];
    out[position + 2] = _convU8[5];
    out[position + 1] = _convU8[6];
    out[position + 0] = _convU8[7];
    position += 8;
  }

  writeFloat32(double v) {
    _convF32[0] = v;
    out[position + 3] = _convU8[0];
    out[position + 2] = _convU8[1];
    out[position + 1] = _convU8[2];
    out[position + 0] = _convU8[3];
    position += 4;
  }
}


/// Writer used on the big-endian host.
class _WriterForBEHost extends Writer {
  _WriterForBEHost._create(out) : super._create(out);

  writeFloat64(double v) {
    _convF64[0] = v;
    out[position + 0] = _convU8[0];
    out[position + 1] = _convU8[1];
    out[position + 2] = _convU8[2];
    out[position + 3] = _convU8[3];
    out[position + 4] = _convU8[4];
    out[position + 5] = _convU8[5];
    out[position + 6] = _convU8[6];
    out[position + 7] = _convU8[7];
    position += 8;
  }

  writeFloat32(double v) {
    _convF32[0] = v;
    out[position + 0] = _convU8[0];
    out[position + 1] = _convU8[1];
    out[position + 2] = _convU8[2];
    out[position + 3] = _convU8[3];
    position += 4;
  }
}

If you want to understand fundamentals then you should study how different types are represented in memory and have a grasp of things like big/little endian byte orders.

For parsing you can use ByteData class, it allows you to extract specific types from a sequence of bytes.

If data arrives to you as hex string you can convert it into a List<int> using hex codec from package:convert, after that you can create ByteData from it, e.g.

import 'dart:typed_data';
import 'package:convert/convert.dart';

void parse(String s) {
  // Convert hex from the given string to a sequence of bytes (first two characters become first byte and so on). 
  final bd = ByteData.sublistView(Uint8List.fromList(hex.decode(s)));
  print(bd.getInt32(10, Endian.little)); // print timestamp (should print  1615217400)
}

void main() {
  
 parse('534D627480650300A201F84246600000F0420000F2420000F4420000803F000000400000404000007A4400002041005043470050C3466666663FCDCC4C3F3333333F00007042333333333333E33F9A9999999999D93F40016001200100E700E700E700E700E700E7B803BD03B30332');
}

@mraleph thank you so much!