flutter_asset.dart

class Asset {

  final String assetPath;
  final String filename;

  Asset(this.assetPath, this.filename);

  Future<String> prepare() async {
    Directory directory = await getApplicationDocumentsDirectory();
    String path = join(directory.path, filename);
    bool fileAlreadyExists = await File(path).exists();
    if (fileAlreadyExists) {
      return path;
    }

    // Copy asset into app documents folder so that we can read it
    final data = await rootBundle.load(assetPath);
    List<int> bytes = data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
    await File(path).writeAsBytes(bytes);

    return path;
  }

}