First pass at build dependency graph for barback.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
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+++ b/pkg/barback/lib/src/transform_node.dart
@@ -0,0 +1,107 @@
+// Copyright (c) 2013, 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_node;
+
+import 'dart:async';
+
+import 'asset.dart';
+import 'asset_graph.dart';
+import 'asset_id.dart';
+import 'asset_node.dart';
+import 'errors.dart';
+import 'phase.dart';
+import 'transform.dart';
+import 'transformer.dart';
+
+/// Describes a transform on a set of assets and its relationship to the build
+/// dependency graph.
+///
+/// Keeps track of whether it's dirty and needs to be run and which assets it
+/// depends on.
+class TransformNode {
+  /// The [Phase] that this transform runs in.
+  final Phase phase;
+
+  /// The [Transformer] to apply to this node's inputs.
+  final Transformer _transformer;
+
+  /// The node for the primary asset this transform depends on.
+  final AssetNode primary;
+
+  /// True if an input has been modified since the last time this transform
+  /// was run.
+  bool get isDirty => _isDirty;
+  var _isDirty = true;
+
+  /// The inputs read by this transform the last time it was run.
+  ///
+  /// Used to tell if an input was removed in a later run.
+  var _inputs = new Set<AssetNode>();
+
+  /// The outputs created by this transform the last time it was run.
+  ///
+  /// Used to tell if an output was removed in a later run.
+  var _outputs = new Set<AssetId>();
+
+  TransformNode(this.phase, this._transformer, this.primary);
+
+  /// Marks this transform as needing to be run.
+  void dirty() {
+    _isDirty = true;
+  }
+
+  /// Applies this transform.
+  ///
+  /// Returns a [TransformOutputs] describing the resulting outputs compared to
+  /// previous runs.
+  Future<TransformOutputs> apply() {
+    var newInputs = new Set<AssetNode>();
+    var newOutputs = new Map<AssetId, Asset>();
+    var transform = createTransform(this, newInputs, newOutputs);
+    return _transformer.apply(transform).catchError((error) {
+      // Catch all transformer errors and pipe them to the results stream.
+      // This is so a broken transformer doesn't take down the whole graph.
+      phase.graph.reportError(error);
+
+      // Don't allow partial results from a failed transform.
+      newOutputs.clear();
+    }).then((_) {
+      _isDirty = false;
+
+      // Stop watching any inputs that were removed.
+      for (var oldInput in _inputs) {
+        oldInput.consumers.remove(this);
+      }
+
+      // Watch any new inputs so this transform will be re-processed when an
+      // input is modified.
+      for (var newInput in newInputs) {
+        newInput.consumers.add(this);
+      }
+
+      _inputs = newInputs;
+
+      // See which outputs are missing from the last run.
+      var outputIds = newOutputs.keys.toSet();
+      var removed = _outputs.difference(outputIds);
+      _outputs = outputIds;
+
+      return new TransformOutputs(newOutputs, removed);
+    });
+  }
+}
+
+/// The result of running a [Transform], compared to the previous time it was
+/// applied.
+class TransformOutputs {
+  /// The outputs that are new or were modified since the last run.
+  final Map<AssetId, Asset> updated;
+
+  /// The outputs that were created by the previous run but were not generated
+  /// by the most recent run.
+  final Set<AssetId> removed;
+
+  TransformOutputs(this.updated, this.removed);
+}