First pass at build dependency graph for barback.

pkg/barback/lib/src/transform.dart

Index: pkg/barback/lib/src/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transform.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8023d0edb65818296f6200a214ea9f163af3bf0d
--- /dev/null
+++ b/pkg/barback/lib/src/transform.dart
@@ -0,0 +1,79 @@
+// 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;
+
+import 'dart:async';
+
+import 'asset.dart';
+import 'asset_id.dart';
+import 'asset_node.dart';
+import 'errors.dart';
+import 'transform_node.dart';
+
+/// Creates a [Transform] by forwarding to the private constructor.
+///
+/// Lets [TransformNode] create [Transforms] without giving a [Transform]
+/// itself a public constructor, which would be visible to external users.
+/// Unlike the [Transform] class, this function is not exported by barback.dart.
+Transform createTransform(TransformNode node, Set<AssetNode> inputs,
+                          Map<AssetId, Asset> outputs) =>
+    new Transform._(node, inputs, outputs);
+
+/// While a [Transformer] represents a *kind* of transformation, this defines
+/// one specific usage of it on a set of files.
+///
+/// This ephemeral object exists only during an actual transform application to
+/// facilitate communication between the [Transformer] and the code hosting
+/// the transformation. It lets the [Transformer] access inputs and generate
+/// outputs.
+class Transform {
+  final TransformNode _node;
+
+  final Set<AssetNode> _inputs;
+  final Map<AssetId, Asset> _outputs;
+
+  /// Gets the ID of the primary input for this transformation.
+  ///
+  /// While a transformation can use multiple input assets, one must be a
+  /// special "primary" asset. This will be the "entrypoint" or "main" input
+  /// file for a transformation.
+  ///
+  /// For example, with a dart2js transform, the primary input would be the
+  /// entrypoint Dart file. All of the other Dart files that that imports
+  /// would be secondary inputs.
+  AssetId get primaryId => _node.primary.id;
+
+  /// Gets the asset for the primary input.
+  Future<Asset> get primaryInput => getInput(primaryId);
+
+  Transform._(this._node, this._inputs, this._outputs);
+
+  /// Gets the asset for for an input [id].
+  ///
+  /// If an input with that ID cannot be found, throws an
+  /// [AssetNotFoundException].
+  Future<Asset> getInput(AssetId id) {
+    return new Future(() {
+      var node = _node.phase.inputs[id];
+      // TODO(rnystrom): Need to handle passthrough where an asset from a
+      // previous phase can be found.
+
+      // Throw if the input isn't found. This ensures the transformer's apply
+      // is exited. We'll then catch this and report it through the proper
+      // results stream.
+      if (node == null) throw new MissingInputException(id);
+
+      // Keep track of which assets this transform depends on.
+      _inputs.add(node);
+      return node.asset;
+    });
+  }
+
+  /// Stores [output] as the output created by this transformation for asset
+  /// [id]. A transformation can output as many assets as it wants.
+  void addOutput(AssetId id, Asset output) {
+    _outputs[id] = output;
+  }
+}