Add aggregate transformer types to barback.

pkg/barback/lib/src/transformer/declaring_transform.dart

Index: pkg/barback/lib/src/transformer/declaring_transform.dart
diff --git a/pkg/barback/lib/src/transformer/declaring_transform.dart b/pkg/barback/lib/src/transformer/declaring_transform.dart
index e15082fc37ad6d33310bf029315385e20fab4c71..7dff33a83d51f74dec2f4608ba4b25b9ae3dc97e 100644
--- a/pkg/barback/lib/src/transformer/declaring_transform.dart
+++ b/pkg/barback/lib/src/transformer/declaring_transform.dart
@@ -4,21 +4,45 @@
 
 library barback.transformer.declaring_transform;
 
+import 'dart:async';
+
 import '../asset/asset_id.dart';
-import '../graph/transform_node.dart';
-import 'base_transform.dart';
+import 'declaring_aggregate_transform.dart';
+import 'transform_logger.dart';
+
+/// Creates a new [DeclaringTransform] wrapping an
+/// [AggregateDeclaringTransform].
+///
+/// Although barback internally works in terms of
+/// [DeclaringAggregateTransformer]s, most transformers only work on individual
+/// primary inputs in isolation. We want to allow those transformers to
+/// implement the more user-friendly [DeclaringTransformer] interface which
+/// takes the more user-friendly [DeclaringTransform] object. This method wraps
+/// the more general [DeclaringAggregateTransform] to return a
+/// [DeclaringTransform] instead.
+Future<DeclaringTransform> newDeclaringTransform(
+    DeclaringAggregateTransform aggregate) {
+  // A wrapped [Transformer] will assign each primary input a unique transform
+  // key, so we can safely get the first asset emitted. We don't want to wait
+  // for the stream to close, since that requires barback to prove that no more
+  // new assets will be generated.
+  return aggregate.primaryIds.first.then((primaryId) => 
+      new DeclaringTransform._(aggregate, primaryId));
+}
 
-/// A transform for [DeclaringTransform]ers that allows them to declare the ids
+/// A transform for [DeclaringTransformer]s that allows them to declare the ids
 /// of the outputs they'll generate without generating the concrete bodies of
 /// those outputs.
-class DeclaringTransform extends BaseTransform {
-  final _outputIds = new Set<AssetId>();
+class DeclaringTransform {
+  /// The underlying aggregate transform.
+  final DeclaringAggregateTransform _aggregate;
 
   final AssetId primaryId;
 
-  DeclaringTransform._(TransformNode node)
-      : primaryId = node.primary.id,
-        super(node);
+  /// A logger so that the [Transformer] can report build details.
+  TransformLogger get logger => _aggregate.logger;
+
+  DeclaringTransform._(this._aggregate, this.primaryId);
 
   /// Stores [id] as the id of an output that will be created by this
   /// transformation when it's run.
@@ -27,21 +51,14 @@ class DeclaringTransform extends BaseTransform {
   /// [DeclaringTransformer.declareOutputs] declareds a given asset id for a
   /// given input, [Transformer.apply] should emit the corresponding asset as
   /// well.
-  void declareOutput(AssetId id) {
-    // TODO(nweiz): This should immediately throw if an output with that ID
-    // has already been declared by this transformer.
-    _outputIds.add(id);
-  }
-}
-
-/// The controller for [DeclaringTransform].
-class DeclaringTransformController extends BaseTransformController {
-  DeclaringTransform get transform => super.transform;
+  void declareOutput(AssetId id) => _aggregate.declareOutput(id);
 
-  /// The set of ids that the transformer declares it will emit for the given
-  /// primary input.
-  Set<AssetId> get outputIds => transform._outputIds;
-
-  DeclaringTransformController(TransformNode node)
-      : super(new DeclaringTransform._(node));
+  /// Consume the primary input so that it doesn't get processed by future
+  /// phases or emitted once processing has finished.
+  ///
+  /// Normally the primary input will automatically be forwarded unless the
+  /// transformer overwrites it by emitting an input with the same id. This
+  /// allows the transformer to tell barback not to forward the primary input
+  /// even if it's not overwritten.
+  void consumePrimary() => _aggregate.consumePrimary(primaryId);
 }