First pass at build dependency graph for barback.

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
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+// 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.phase;
+
+import 'dart:async';
+
+import '../barback.dart';
+import '../transformer.dart';
+import 'asset_graph.dart';
+import 'asset_node.dart';
+import 'transform_node.dart';
+
+/// The transforms in a processing graph are organized into a series of phases.
+/// Each phase can access outputs from previous phases and can in turn pass
+/// outputs to later phases.
+///
+/// Phases are processed strictly serially. All transforms in a phase will be
+/// complete before moving on to the next phase. Within a single phase, all
+/// transforms will be run in parallel.
+///
+/// Building can be interrupted between phases. For example, an source is added

[nweiz, 2013/06/18 23:14:46]

"a source"

[Bob Nystrom, 2013/06/20 00:23:59]

Done.

+/// which starts the background process. Sometime during phase 2 (which is

[nweiz, 2013/06/18 23:14:46]

It's not clear what "the background process" refers to here, or why we're
running phase 2 rather than phase 1.

[Bob Nystrom, 2013/06/20 00:23:59]

Arbitrary number for the example.

+/// running asynchronously) that source is modified. When the process queue
+/// goes to advance to phase 3, it will see that modification and start the
+/// waterfall from the beginning again.
+class Phase {
+  /// The graph that owns this phase.
+  final AssetGraph graph;
+
+  /// This phase's position relative to the other phases. Zero-based.
+  final int index;

[nweiz, 2013/06/18 23:14:46]

Is this actually used anywhere? It seems weird to store it.

[Bob Nystrom, 2013/06/20 00:23:59]

I end up using it for debug printing. I can remove it if you like, but it's kind
of handy. At some point, I'll put a decent toString() on this.

+
+  /// The transformers that can access [inputs]. Their outputs will be
+  /// available to the next phase.
+  final List<Transformer> transformers;
+
+  /// The inputs that are available for transforms in this phase to consume.
+  /// For the first phase, these will be the source assets. For all other
+  /// phases, they will be the outputs from the previous phase.
+  final inputs = new Map<AssetId, AssetNode>();
+
+  /// The transforms currently applicable to assets in [inputs]. These are the
+  /// transforms that have been "wired up": they represent a repeatable
+  /// transformation of a single concrete set of inputs. "dart2js" is a
+  /// transformer. "dart2js on web/main.dart" is a transform.
+  final transforms = new Set<TransformNode>();
+
+  /// The nodes that are new in this phase since the last time [process] was
+  /// called. When we process, we'll check these to see if we can hang new
+  /// transforms off them.
+  final newInputs = new Set<AssetNode>();

[nweiz, 2013/06/18 23:14:46]

Seems like most of these fields should be private.

[Bob Nystrom, 2013/06/20 00:23:59]

Done.

+
+  /// The phase after this one. Outputs from this phase will be passed to it.
+  Phase next;

[nweiz, 2013/06/18 23:14:46]

It's weird that this is mutable. If you construct phases backwards from last to
first, couldn't you make this final?

[Bob Nystrom, 2013/06/20 00:23:59]

Done.

+
+  Phase(this.graph, this.index, this.transformers);
+
+  /// Updates the phase's inputs with [updated] and removes [removed]. This
+  /// marks any affected [transforms] as dirty or discards them if their inputs
+  /// are removed.
+  void updateInputs(Map<AssetId, Asset> updated, Set<AssetId> removed) {
+    // Remove any nodes that are no longer being output. Handle removals first
+    // in case there are assets that were removed by one transform but updated
+    // by another. In that case, the update should win.
+    for (var id in removed) {
+      var node = inputs.remove(id);
+
+      // Every transform that was using it is dirty now.
+      if (node != null) {
+        node.consumers.forEach((consumer) => consumer.isDirty = true);
+      }
+    }
+
+    // Update and new or modified assets.
+    updated.forEach((id, asset) {
+      var node = inputs.putIfAbsent(id, () => new AssetNode(id));
+
+      // If it's a new node, remember that so we can see if any new transforms
+      // will consume it.
+      if (node.asset == null) newInputs.add(node);
+
+      node.updateAsset(asset);
+    });
+  }
+
+  /// Processes this phase. For all new inputs, it tries to see if there are
+  /// transformers that can consume them. Then all applicable transforms are
+  /// applied.
+  ///
+  /// Returns a future that completes when processing is done. If there is
+  /// nothing to process, returns `null`.
+  Future process() {
+    var future = _processNewInputs();
+    if (future == null) {
+      return _processTransforms();
+    }
+
+    return future.then((_) => _processTransforms());
+  }
+
+  /// Creates new transforms for any new inputs that are applicable.
+  Future _processNewInputs() {
+    if (newInputs.isEmpty) return null;
+
+    var futures = [];
+    for (var node in newInputs) {
+      for (var transformer in transformers) {
+        futures.add(transformer.isPrimary(node.id).then((isPrimary) {
+          if (!isPrimary) return;
+          var transform = new TransformNode(this, transformer, node);
+          node.consumers.add(transform);
+          transforms.add(transform);
+        }));
+      }
+    }
+
+    newInputs.clear();
+
+    return Future.wait(futures);
+  }
+
+  /// Applies all currently wired up and dirty transforms. Passes their outputs
+  /// to the next phase.
+  Future _processTransforms() {
+    var dirtyTransforms = transforms.where((transform) => transform.isDirty);
+    if (dirtyTransforms.isEmpty) return null;
+
+    return Future.wait(dirtyTransforms.map(
+        (transform) => transform.apply())).then((transformOutputs) {

[nweiz, 2013/06/18 23:14:46]

I think this would be a little cleaner formatted like so:

    return Future.wait(dirtyTransforms.map((transform) => transform.apply()))
        .then((transformOutputs) {

[Bob Nystrom, 2013/06/20 00:23:59]

Done.

+      // Collect all of the outputs. Since the transforms are run in parallel,
+      // we have to be careful here to ensure that the result is deterministic
+      // and not influenced by the order that transforms complete.
+      var updated = new Map<AssetId, Asset>();
+      var removed = new Set<AssetId>();
+      var collisions = new Set<AssetId>();
+
+      // Handle the generated outputs of all transforms first.
+      for (var outputs in transformOutputs) {
+        // Collect the outputs of all transformers together.
+        outputs.updated.forEach((id, asset) {
+          if (updated.containsKey(id)) {
+            // Report a collision.
+            collisions.add(id);
+          } else {
+            // TODO(rnystrom): In the case of a collision, the asset that
+            // "wins" is chosen non-deterministically. Do something better.
+            updated[id] = asset;
+          }
+        });
+
+        // Track any assets no longer output by this transform. We don't
+        // handle the case where *another* transform generates the asset
+        // no longer generated by this one. updateInputs() handles that.
+        removed.addAll(outputs.removed);
+      }
+
+      // Report any collisions in deterministic order.
+      collisions = collisions.toList();
+      collisions.sort((a, b) => a.toString().compareTo(b.toString()));
+      for (var collision in collisions) {
+        graph.reportError(new AssetCollisionException(collision));
+        // TODO(rnystrom): Define what happens after a collision occurs.
+      }
+
+      // Pass the outputs to the next phase.
+      next.updateInputs(updated, removed);
+    });
+  }
+}