Add BaseTransform.consumePrimary to barback.

pkg/barback/lib/src/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.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'errors.dart';
 import 'log.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.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.
+  ///
+  /// This is exposed via [BaseTransformController].
+  bool _consumePrimary = false;
+
   /// The controller for the stream of log entries emitted by the transformer.
   ///
   /// This is exposed via [BaseTransformController].
   ///
   /// This is synchronous because error logs can cause the transform to fail, so
   /// we need to ensure that their processing isn't delayed until after the
   /// transform or build has finished.
   final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
 
   /// A logger so that the [Transformer] can report build details.
@@ skipping 50 matching lines @@
     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()));
+
+  /// 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() {
+    _consumePrimary = true;
+  }
 }
 
 /// 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 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();
   }
 }