// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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

import 'package:async/async.dart';

import '../stream_channel.dart';

/// A [StreamChannelTransformer] transforms the events being passed to and
/// emitted by a [StreamChannel].
///
/// This works on the same principle as [StreamTransformer] and
/// [StreamSinkTransformer]. Each transformer defines a [bind] method that takes
/// in the original [StreamChannel] and returns the transformed version.
///
/// Transformers must be able to have `bind` called multiple times.
class StreamChannelTransformer<S, T> {
  /// The transformer to use on the channel's stream.
  final StreamTransformer<T, S> _streamTransformer;

  /// The transformer to use on the channel's sink.
  final StreamSinkTransformer<S, T> _sinkTransformer;

  /// Creates a [StreamChannelTransformer] from existing stream and sink
  /// transformers.
  const StreamChannelTransformer(
      this._streamTransformer, this._sinkTransformer);

  /// Creates a [StreamChannelTransformer] from a codec's encoder and decoder.
  ///
  /// All input to the inner channel's sink is encoded using [Codec.encoder],
  /// and all output from its stream is decoded using [Codec.decoder].
  StreamChannelTransformer.fromCodec(Codec<S, T> codec)
      : this(
          typedStreamTransformer(codec.decoder),
          StreamSinkTransformer.typed(
              new StreamSinkTransformer.fromStreamTransformer(codec.encoder)));

  /// Transforms the events sent to and emitted by [channel].
  ///
  /// Creates a new channel. When events are passed to the returned channel's
  /// sink, the transformer will transform them and pass the transformed
  /// versions to `channel.sink`. When events are emitted from the
  /// `channel.straem`, the transformer will transform them and pass the
  /// transformed versions to the returned channel's stream.
  StreamChannel<S> bind(StreamChannel<T> channel) =>
      new StreamChannel<S>(
          channel.stream.transform(_streamTransformer),
          _sinkTransformer.bind(channel.sink));
}