Add support for lazy transformers.

pkg/barback/lib/src/base_transform.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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.transform;
+library barback.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:source_maps/span.dart';
-
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
-import 'asset_set.dart';
 import 'errors.dart';
-import 'log.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.dart';
 
-typedef void LogFunction(AssetId asset, LogLevel level, String message,
-                         Span span);
-
-/// Creates a [Transform] by forwarding to the private constructor.
+/// The base class for the ephemeral transform objects that are passed to
+/// transformers.
 ///
-/// Lets [TransformNode] create [Transforms] without giving a [Transform]
-/// itself a public constructor, which would be visible to external users.
-/// Unlike the [Transform] class, this function is not exported by barback.dart.
-Transform createTransform(TransformNode node, AssetSet outputs,
-                          LogFunction logFunction) =>
-    new Transform._(node, outputs, logFunction);
-
-/// While a [Transformer] represents a *kind* of transformation, this defines
-/// one specific usage of it on a set of files.
-///
-/// This ephemeral object exists only during an actual transform application to
-/// facilitate communication between the [Transformer] and the code hosting
-/// the transformation. It lets the [Transformer] access inputs and generate
-/// outputs.
-class Transform {
+/// 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;
-  final AssetSet _outputs;
 
   /// 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;
   }
 
-  Transform._(this._node, this._outputs, LogFunction logFunction)
+  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()));
-
-  /// Stores [output] as the output created by this transformation.
-  ///
-  /// A transformation can output as many assets as it wants.
-  void addOutput(Asset output) {
-    // TODO(rnystrom): This should immediately throw if an output with that ID
-    // has already been created by this transformer.
-    _outputs.add(output);
-  }
 }