Dart IO and Streams the right way

io_expedition_iter3.dart

library io_testing;

import 'dart:async';
import 'dart:io';
import 'dart:convert';

class UpperCaseConverter extends Converter<String, String> {
  @override
  String convert(String input) => input.toUpperCase();

  @override
  Sink<String> startChunkedConversion(Sink<String> sink) {
    return StringEventConverterSink(sink, this);
  }
}

class StringEventConverterSink extends StringConversionSinkBase {
  EventSink<String> innerSink;
  Converter<String, String> converter;

  StringEventConverterSink(Sink<String> sink, Converter<String, String> converter) {
    this.innerSink = sink;
    this.converter = converter;
  }

  @override
  void addSlice(String str, int start, int end, bool isLast) {
    innerSink.add(converter.convert(str));
  }

  @override
  void close() {
    innerSink.close();
  }
}

Future main() async {
  var config = File(Platform.script.toFilePath());
  Stream<List<int>> inputStream = config.openRead();

  var lines = inputStream
      .transform(utf8.decoder)
      .transform(LineSplitter())
      .transform(UpperCaseConverter());
  try {
    await for (var line in lines) {
      print('Got ${line.length} characters from stream');
      print(line);
    }
    print('file is now closed');
  } catch (e) {
    print(e);
  }
}

io_expedition_iter4.dart

library io_expedition;

import 'dart:async';
import 'dart:io';
import 'dart:convert';

class StringConverter extends Converter<String, String> {
  String Function(String x) convertFunction;

  StringConverter(this.convertFunction);

  @override
  String convert(String input) => convertFunction(input);

  @override
  Sink<String> startChunkedConversion(Sink<String> sink) => StringEventConverterSink(sink, this);
}

class UpperCaseConverter extends Converter<String, String> {
  @override
  String convert(String input) => input.toUpperCase();

  @override
  Sink<String> startChunkedConversion(Sink<String> sink) => StringEventConverterSink(sink, this);
}

class StringEventConverterSink extends StringConversionSinkBase {
  EventSink<String> innerSink;
  Converter<String, String> converter;

  StringEventConverterSink(this.innerSink, this.converter);

  @override
  void addSlice(String str, int start, int end, bool isLast) => innerSink.add(converter.convert(str));

  @override
  void close() => innerSink.close();
}

Future main() async {
  var config = File(Platform.script.toFilePath());
  Stream<List<int>> inputStream = config.openRead();

  var lines = inputStream
      .transform(utf8.decoder)
      .transform(LineSplitter())
      .transform(StringConverter((String x) => x.toUpperCase()));
  try {
    await for (var line in lines) {
      print('Got ${line.length} characters from stream');
      print(line);
    }
    print('file is now closed');
  } catch (e) {
    print(e);
  }
}