HMAC-MD5比MD5多一个key，我们用到的是对Map加密，所以需要对Map里的字段进行解析拼接
trim.dart

class SJTrim {
  static List<T> trimList<T>(List<T> list) {
    return (null != list) ? list : List<T>();
  }

  static Map<K, V> trimMap<K, V>(Map<K, V> map) {
    return (null != map) ? map : Map<K, V>();
  }

  static int trimInt(int value, [int defaultValue = 0]) {
    return (null != value) ? value : defaultValue;
  }

  static bool trimBool(bool value, [bool defaultValue = false]) {
    return (null != value) ? value : defaultValue;
  }

  static double trimDouble(double value, [double defaultValue = 0]) {
    return (null != value) ? value : defaultValue;
  }

  static String trimString(String value, [String defaultValue = '']) {
    return (null != value) ? value : defaultValue;
  }
}

trackinfo_singer.dart

class SJTrackInfoSinger {
  final _trackMapInfo = SplayTreeMap<String, String>();

  void addInt(String key, int value) {
    if (0 == SJTrim.trimInt(value)) {
      return;
    }
    _trackMapInfo[key] = '$value';
  }

  void addString(String key, String value) {
    if (0 == SJTrim.trimString(value).length) {
      return;
    }
    _trackMapInfo[key] = value;
  }

  String toStringInfo() {
    final stringBuffer = StringBuffer();
    _trackMapInfo.forEach((key, value) {
      stringBuffer.write(value);
    });
    return stringBuffer.toString();
  }
}

调用

String generateMD5(SJRouteInfoV4 routInfo) {
    final singer = SJTrackInfoSinger();
    singer.addInt('sj', routInfo.sj);
    if (routInfo.point != null && routInfo.point.length > 0) {
      final stringBuffer = StringBuffer();
      routInfo.point.forEach((location) {//routInfo.point是list，location是map类型
        final singerTmp = SJTrackInfoSinger();
        singerTmp.addString('speed', location.speed);
        singerTmp.addInt('locationType', location.locationType);
        stringBuffer.write(singerTmp.toStringInfo());
      });
      singer.addString('point', stringBuffer.toString());
    }
    final string = singer.toStringInfo();

    final encryptKey = utf8.encode('88888888');
    final ecnryptData = utf8.encode(string);

    final hmacSha256 = new Hmac(sha256, encryptKey);
    final digest = hmacSha256.convert(ecnryptData);
    return digest.toString();
  }