sdk/lib/convert/converter.dart

// Copyright (c) 2013, 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.

part of dart.convert;

/// A [Converter] converts data from one representation into another.
///
/// The [Converter] class provides a default implementation for every method
/// other than [convert].
abstract mixin class Converter<S, T> implements StreamTransformerBase<S, T> {
  const Converter();

  /// Adapts [source] to be a `Converter<TS, TT>`.
  ///
  /// This allows [source] to be used at the new type, but at run-time it
  /// must satisfy the requirements of both the new type and its original type.
  ///
  /// Conversion input must be both [SS] and [TS] and the output created by
  /// [source] for those input must be both [ST] and [TT].
  static Converter<TS, TT> castFrom<SS, ST, TS, TT>(Converter<SS, ST> source) =>
      CastConverter<SS, ST, TS, TT>(source);

  /// Converts [input] and returns the result of the conversion.
  T convert(S input);

  /// Fuses `this` with [other].
  ///
  /// Encoding with the resulting converter is equivalent to converting with
  /// `this` before converting with `other`.
  Converter<S, TT> fuse<TT>(Converter<T, TT> other) {
    return _FusedConverter<S, T, TT>(this, other);
  }

  /// Starts a chunked conversion.
  ///
  /// The returned sink serves as input for the long-running conversion. The
  /// given [sink] serves as output.
  Sink<S> startChunkedConversion(Sink<T> sink) {
    throw UnsupportedError(
      "This converter does not support chunked conversions: $this",
    );
  }

  Stream<T> bind(Stream<S> stream) {
    return Stream<T>.eventTransformed(
      stream,
      (EventSink sink) => _ConverterStreamEventSink(this, sink),
    );
  }

  /// Provides a `Converter<RS, RT>` view of this stream transformer.
  ///
  /// The resulting transformer will check at run-time that all conversion
  /// inputs are actually instances of [S],
  /// and it will check that all conversion output produced by this converter
  /// are actually instances of [RT].
  Converter<RS, RT> cast<RS, RT>() => Converter.castFrom<S, T, RS, RT>(this);
}

/// Fuses two converters.
///
/// For a non-chunked conversion converts the input in sequence.
class _FusedConverter<S, M, T> extends Converter<S, T> {
  final Converter<S, M> _first;
  final Converter<M, T> _second;

  _FusedConverter(this._first, this._second);

  T convert(S input) => _second.convert(_first.convert(input));

  Sink<S> startChunkedConversion(Sink<T> sink) {
    return _first.startChunkedConversion(_second.startChunkedConversion(sink));
  }
}