Add aggregate transformer types to barback.

pkg/barback/lib/src/transformer/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.transformer.base_transform;
 
 import 'dart:async';
 
+import '../asset/asset_id.dart';
 import '../graph/transform_node.dart';
 import '../log.dart';
 import 'transform_logger.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;
 
-  /// Whether the primary input should be consumed.
+  /// The ids of primary inputs that should be consumed.
   ///
-  /// This is exposed via [BaseTransformController].
-  bool _consumePrimary = false;
+  /// This is exposed by [BaseTransformController].
+  final _consumedPrimaries = new Set<AssetId>();
 
   /// Whether the transformer logged an error.
   ///
   /// This is exposed via [BaseTransformController].
   bool _loggedError = false;
 
   /// The controller for the stream of log entries emitted by the transformer.
   ///
   /// This is exposed via [BaseTransformController].
   ///
@@ skipping 10 matching lines @@
     _logger = new TransformLogger((asset, level, message, span) {
       if (level == LogLevel.ERROR) _loggedError = true;
 
       // If the log isn't already associated with an asset, use the primary.
       if (asset == null) asset = _node.primary.id;
       var entry = new LogEntry(_node.info, asset, level, message, span);
       _onLogController.add(entry);
     });
   }
 
-  /// Consume the primary input so that it doesn't get processed by future
+  /// Consume a 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
+  /// Normally each 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
+  /// allows the transformer to tell barback not to forward a primary input
   /// even if it's not overwritten.
-  void consumePrimary() {
-    _consumePrimary = true;
+  void consumePrimary(AssetId id) {
+    // TODO(nweiz): throw an error if an id is consumed that wasn't listed as a
+    // primary input.
+    _consumedPrimaries.add(id);
   }
 }
 
 /// The base class for controllers of subclasses of [BaseTransform].
 ///
 /// Controllers are used so that [TransformNode]s can get values from a
 /// [BaseTransform] without exposing getters in the public API.
 abstract class BaseTransformController {
   /// The [BaseTransform] controlled by this controller.
   final BaseTransform transform;
 
-  /// Whether the primary input should be consumed.
-  bool get consumePrimary => transform._consumePrimary;
+  /// The ids of primary inputs that should be consumed.
+  Set<AssetId> get consumedPrimaries => transform._consumedPrimaries;
 
   /// Whether the transform logged an error.
   bool get loggedError => transform._loggedError;
 
   /// The stream of log entries emitted by the transformer during a run.
   Stream<LogEntry> get onLog => transform._onLogController.stream;
 
   BaseTransformController(this.transform);
 
   /// Notifies the [BaseTransform] that the transformation has finished being
   /// applied.
   ///
   /// This will close any streams and release any resources that were allocated
   /// for the duration of the transformation.
   void close() {
     transform._onLogController.close();
   }
 }