Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/transformer/aggregate_transformer.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.transformer.aggregate_transformer;
 
 import '../asset/asset_id.dart';
 import 'aggregate_transform.dart';
 
 /// An alternate interface for transformers that want to perform aggregate
@@ skipping 14 matching lines @@
 /// If possible, aggregate transformers should implement
 /// [DeclaringAggregateTransformer] as well to help barback optimize the package
 /// graph.
 abstract class AggregateTransformer {
   /// Classifies an asset id by returning a key identifying which group the
   /// asset should be placed in.
   ///
   /// All assets for which [classifyPrimary] returns the same key are passed
   /// together to the same [apply] call.
   ///
-  /// Any string can be used to classify an asset. If possible, though, this
-  /// should return a path-like string to aid in logging. If [classifyPrimary]
-  /// needs to do asynchronous work, it can also return a [Future] that
-  /// completes to the key.
+  /// This may return [Future<String>] or, if it's entirely synchronous,
+  /// [String]. Any string can be used to classify an asset. If possible,
+  /// though, this should return a path-like string to aid in logging. If
+  /// [classifyPrimary] needs to do asynchronous work, it can also return a
+  /// [Future] that completes to the key.

[Bob Nystrom, 2014/05/08 20:30:48]

This last sentence seems redundant with the first. Maybe just reword the first
sentence to clarify the async case?

[nweiz, 2014/05/08 21:12:36]

Done.

   ///
   /// A return value of `null` indicates that the transformer is not interested
   /// in an asset. Assets with a key of `null` will not be passed to any [apply]
   /// call; this is equivalent to [Transformer.isPrimary] returning `false`.
-  String classifyPrimary(AssetId id);
+  classifyPrimary(AssetId id);
 
   /// Runs this transformer on a group of primary inputs specified by
   /// [transform].
   ///
   /// If this does asynchronous work, it should return a [Future] that completes
   /// once it's finished.
   ///
   /// This may complete before [AggregateTransform.primarInputs] is closed. For
   /// example, it may know that each key will only have two inputs associated
   /// with it, and so use `transform.primaryInputs.take(2)` to access only those
   /// inputs.
   apply(AggregateTransform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }