Expand barback's notion of dirtiness to understand declaredness.

pkg/barback/lib/src/node_status.dart

Index: pkg/barback/lib/src/node_status.dart
diff --git a/pkg/barback/lib/src/node_status.dart b/pkg/barback/lib/src/node_status.dart
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+++ b/pkg/barback/lib/src/node_status.dart
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+// 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.node_status;
+
+/// The status of a node in barback's package graph.
+///
+/// A node has three possible statuses: [IDLE], [MATERIALIZING], and [RUNNING].
+/// These are ordered from least dirty to most dirty; the [dirtier] and
+/// [dirtiest] functions make use of this ordering.
+class NodeStatus {
+  /// The node has finished its work and won't do anything else until external
+  /// input causes it to.
+  ///
+  /// For deferred nodes, this may indicate that they're finished declaring
+  /// their outputs and waiting to be forced.
+  static const IDLE = const NodeStatus("idle");
+
+  /// The node has declared its outputs but their concrete values are still
+  /// being generated.
+  ///
+  /// This is only meaningful for nodes that are or contain declaring
+  /// transformers. Note that a lazy transformer that's declared its outputs but
+  /// isn't actively working to generate them is considered [IDLE], not
+  /// [MATERIALIZING].
+  static const MATERIALIZING = const NodeStatus("materializing");
+
+  /// The node is actively working on declaring or generating its outputs.
+  ///
+  /// Declaring transformers are only considered dirty until they're finished
+  /// declaring their outputs; past that point, they're always either
+  /// [MATERIALIZING] or [IDLE]. Non-declaring transformers, by contrast, are
+  /// always either [RUNNING] or [IDLE].
+  static const RUNNING = const NodeStatus("running");
+
+  final String _name;
+
+  /// Returns the dirtiest status in [statuses].
+  static NodeStatus dirtiest(Iterable<NodeStatus> statuses) =>
+      statuses.fold(NodeStatus.IDLE,
+          (status1, status2) => status1.dirtier(status2));
+
+  const NodeStatus(this._name);
+
+  String toString() => _name;
+
+  /// Returns [this] or [other], whichever is dirtier.
+  NodeStatus dirtier(NodeStatus other) {
+    if (this == RUNNING || other == RUNNING) return RUNNING;
+    if (this == MATERIALIZING || other == MATERIALIZING) return MATERIALIZING;
+    return IDLE;
+  }
+}