pkg/barback/lib/src/base_transform.dart

// Copyright (c) 2014, 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.

library barback.base_transform;

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

import 'asset.dart';
import 'asset_id.dart';
import 'asset_node.dart';
import 'errors.dart';
import 'transform_logger.dart';
import 'transform_node.dart';
import 'utils.dart';

/// The base class for the ephemeral transform objects that are passed to
/// transformers.
///
/// This class provides the transformers with inputs, but its up to the
/// subclasses to provide a means of emitting outputs.
abstract class BaseTransform {
  final TransformNode _node;
  final TransformLogger _logger;

  /// A logger so that the [Transformer] can report build details.
  TransformLogger get logger => _logger;

  /// Gets the primary input asset.
  ///
  /// While a transformation can use multiple input assets, one must be a
  /// special "primary" asset. This will be the "entrypoint" or "main" input
  /// file for a transformation.
  ///
  /// For example, with a dart2js transform, the primary input would be the
  /// entrypoint Dart file. All of the other Dart files that that imports
  /// would be secondary inputs.
  ///
  /// This method may fail at runtime if called asynchronously after the
  /// transform begins running. The primary input may become unavailable while
  /// this transformer is running due to asset changes earlier in the graph.
  /// You can ignore the error if this happens: the transformer will be re-run
  /// automatically for you.
  Asset get primaryInput {
    if (_node.primary.state != AssetState.AVAILABLE) {
      throw new AssetNotFoundException(_node.primary.id);
    }

    return _node.primary.asset;
  }

  BaseTransform(this._node, LogFunction logFunction)
    : _logger = new TransformLogger(logFunction);

  /// Gets the asset for an input [id].
  ///
  /// If an input with that ID cannot be found, throws an
  /// [AssetNotFoundException].
  Future<Asset> getInput(AssetId id) => _node.getInput(id);

  /// A convenience method to the contents of the input with [id] as a string.
  ///
  /// This is equivalent to calling `getInput()` followed by `readAsString()`.
  ///
  /// If the asset was created from a [String] the original string is always
  /// returned and [encoding] is ignored. Otherwise, the binary data of the
  /// asset is decoded using [encoding], which defaults to [UTF8].
  Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
    if (encoding == null) encoding = UTF8;
    return getInput(id).then((input) => input.readAsString(encoding: encoding));
  }

  /// A convenience method to the contents of the input with [id].
  ///
  /// This is equivalent to calling `getInput()` followed by `read()`.
  ///
  /// If the asset was created from a [String], this returns its UTF-8 encoding.
  Stream<List<int>> readInput(AssetId id) =>
      futureStream(getInput(id).then((input) => input.read()));
}