Add support for lazy transformers.

pkg/barback/lib/src/transform.dart

Index: pkg/barback/lib/src/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transform.dart
index 7f0d217b072f1fd34f03ca87fc5d6b08f86af3ea..26b718d0f47d68a032c19eb351c59ff123400e96 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/transform.dart
@@ -1,35 +1,14 @@
-// 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;
 
-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 'base_transform.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.
-///
-/// 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.
@@ -38,65 +17,11 @@ Transform createTransform(TransformNode node, AssetSet outputs,
 /// facilitate communication between the [Transformer] and the code hosting
 /// the transformation. It lets the [Transformer] access inputs and generate
 /// outputs.
-class Transform {
-  final TransformNode _node;
-  final TransformLogger _logger;
+class Transform extends BaseTransform {
   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)
-    : _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()));
+  Transform(TransformNode node, this._outputs, LogFunction logFunction)

[Bob Nystrom, 2014/01/30 19:33:44]

Transform is in barback's public API, but TransformNode is not, and shouldn't
be. I don't think this constructor should be publicly visible because of that.
(That's why this code previously had the little createTransform() function.

[nweiz, 2014/01/31 03:43:27]

In general I prefer exposing constructors that aren't meant to be invoked to
doing weird private-but-wrapped-in-a-non-exposed-function shenanigans. See for
example the Task constructor in scheduled_test.

In this case in particular, [BaseTransform] already has to expose its
constructor so its subclasses can call it, so it seemed very weird to have that
exposed without this one exposed.

+    : super(node, logFunction);
 
   /// Stores [output] as the output created by this transformation.
   ///