Factor out an input-handling class from Phase in 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
index c5a620d55864474c38245894989fc6a44e256611..690126a4e77b4102b03760e8af5ec772c448d9a6 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -7,14 +7,13 @@ library barback.phase;
 import 'dart:async';
 import 'dart:collection';
 
-import 'asset.dart';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
 import 'errors.dart';
+import 'phase_input.dart';
 import 'stream_pool.dart';
-import 'transform_node.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
@@ -39,39 +38,11 @@ class Phase {
   /// Their outputs will be available to the next phase.
   final Set<Transformer> _transformers;
 
-  /// The inputs that are available for transforms in this phase to consume.
+  /// The inputs for this phase.
   ///
   /// 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], indexed by
-  /// the ids of their primary 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 Map<AssetId, Set<TransformNode>>();
-
-  /// Controllers for assets that aren't consumed by transforms in this phase.
-  ///
-  /// These assets are passed to the next phase unmodified. They need
-  /// intervening controllers to ensure that the outputs can be marked dirty
-  /// when determining whether transforms apply, and removed if they do.
-  final _passThroughControllers = new Map<AssetId, AssetNodeController>();
-
-  /// Futures that will complete once the transformers that can consume a given
-  /// asset are determined.
-  ///
-  /// Whenever an asset is added or modified, we need to asynchronously
-  /// determine which transformers can use it as their primary input. We can't
-  /// start processing until we know which transformers to run, and this allows
-  /// us to wait until we do.
-  var _adjustTransformersFutures = new Map<AssetId, Future>();
-
-  /// New asset nodes that were added while [_adjustTransformers] was still
-  /// being run on an old version of that asset.
-  var _pendingNewInputs = new Map<AssetId, AssetNode>();
+  final _inputs = new Map<AssetId, PhaseInput>();
 
   /// A map of output ids to the asset node outputs for those ids and the
   /// transforms that produced those asset nodes.
@@ -94,10 +65,7 @@ class Phase {
 
   /// A controller whose stream feeds into [_onDirtyPool].
   ///
-  /// This is used whenever an input is added, changed, or removed. It's
-  /// sometimes redundant with the events collected from [_transforms], but this
-  /// stream is necessary for new and removed inputs, and the transform stream
-  /// is necessary for modified secondary inputs.
+  /// This is used whenever an input is added or transforms are changed.
   final _onDirtyController = new StreamController.broadcast(sync: true);
 
   /// The phase after this one.
@@ -130,51 +98,13 @@ class Phase {
   /// removed and re-created. The phase will automatically handle updated assets
   /// using the [AssetNode.onStateChange] stream.
   void addInput(AssetNode node) {
-    // We remove [node.id] from [inputs] as soon as the node is removed rather
-    // than at the same time [node.id] is removed from [_transforms] so we don't
-    // have to wait on [_adjustTransformers]. It's important that [inputs] is
-    // always up-to-date so that the [AssetCascade] can look there for available
-    // assets.
-    _inputs[node.id] = node;
-    node.whenRemoved.then((_) => _inputs.remove(node.id));
+    if (_inputs.containsKey(node.id)) _inputs[node.id].remove();
 
-    if (!_adjustTransformersFutures.containsKey(node.id)) {
-      _transforms[node.id] = new Set<TransformNode>();
-      _adjustTransformers(node);
-      return;
-    }
-
-    // If an input is added while the same input is still being processed,
-    // that means that the asset was removed and recreated while
-    // [_adjustTransformers] was being run on the old value. We have to wait
-    // until that finishes, then run it again on whatever the newest version
-    // of that asset is.
-
-    // We may already be waiting for the existing [_adjustTransformers] call to
-    // finish. If so, all we need to do is change the node that will be loaded
-    // after it completes.
-    var containedKey = _pendingNewInputs.containsKey(node.id);
-    _pendingNewInputs[node.id] = node;
-    if (containedKey) return;
-
-    // If we aren't already waiting, start doing so.
-    _adjustTransformersFutures[node.id].then((_) {
-      assert(!_adjustTransformersFutures.containsKey(node.id));
-      assert(_pendingNewInputs.containsKey(node.id));
-      _transforms[node.id] = new Set<TransformNode>();
-      _adjustTransformers(_pendingNewInputs.remove(node.id));
-    }, onError: (_) {
-      // If there was a programmatic error while processing the old input,
-      // we don't want to just ignore it; it may have left the system in an
-      // inconsistent state. We also don't want to top-level it, so we
-      // ignore it here but don't start processing the new input. That way
-      // when [process] is called, the error will be piped through its
-      // return value.
-    }).catchError((e) {
-      // If our code above has a programmatic error, ensure it will be piped
-      // through [process] by putting it into [_adjustTransformersFutures].
-      _adjustTransformersFutures[node.id] = new Future.error(e);
-    });
+    var input = new PhaseInput(this, node, _transformers);
+    _inputs[node.id] = input;
+    input.input.whenRemoved.then((_) => _inputs.remove(node.id));
+    _onDirtyPool.add(input.onDirty);
+    _onDirtyController.add(null);
   }
 
   /// Gets the asset node for an input [id].
@@ -182,8 +112,9 @@ class Phase {
   /// If an input with that ID cannot be found, returns null.
   Future<AssetNode> getInput(AssetId id) {
     return newFuture(() {
-      if (id.package == cascade.package) return _inputs[id];
-      return cascade.graph.getAssetNode(id);
+      if (id.package != cascade.package) return cascade.graph.getAssetNode(id);
+      if (_inputs.containsKey(id)) return _inputs[id].input;
+      return null;
     });
   }
 
@@ -201,43 +132,11 @@ class Phase {
   /// Set this phase's transformers to [transformers].
   void updateTransformers(Iterable<Transformer> transformers) {
     _onDirtyController.add(null);
-
-    var newTransformers = transformers.toSet();
-    var oldTransformers = _transformers.toSet();
-    for (var removedTransformer in
-         oldTransformers.difference(newTransformers)) {
-      _transformers.remove(removedTransformer);
-
-      // Remove old transforms for which [removedTransformer] was a transformer.
-      for (var id in _inputs.keys) {
-        // If the transformers are being adjusted for [id], it will
-        // automatically pick up on [removedTransformer] being gone.
-        if (_adjustTransformersFutures.containsKey(id)) continue;
-
-        _transforms[id].removeWhere((transform) {
-          if (transform.transformer != removedTransformer) return false;
-          transform.remove();
-          return true;
-        });
-
-        if (!_transforms[id].isEmpty) continue;
-        _passThroughControllers.putIfAbsent(id, () {
-          return new AssetNodeController.available(
-              _inputs[id].asset, _inputs[id].transform);
-        });
-      }
+    _transformers.clear();
+    _transformers.addAll(transformers);
+    for (var input in _inputs.values) {
+      input.updateTransformers(_transformers);
     }
-
-    var brandNewTransformers = newTransformers.difference(oldTransformers);
-    if (brandNewTransformers.isEmpty) return;
-    brandNewTransformers.forEach(_transformers.add);
-
-    // If there are any new transformers, start re-adjusting the transforms for
-    // all inputs so we pick up which inputs the new transformers apply to.
-    _inputs.forEach((id, node) {
-      if (_adjustTransformersFutures.containsKey(id)) return;
-      _adjustTransformers(node);
-    });
   }
 
   /// Add a new phase after this one with [transformers].
@@ -252,199 +151,21 @@ class Phase {
     return _next;
   }
 
-  /// Asynchronously determines which transformers can consume [node] as a
-  /// primary input and creates transforms for them.
-  ///
-  /// This ensures that if [node] is modified or removed during or after the
-  /// time it takes to adjust its transformers, they're appropriately
-  /// re-adjusted. Its progress can be tracked in [_adjustTransformersFutures].
-  void _adjustTransformers(AssetNode node) {
-    // Mark the phase as dirty. This may not actually end up creating any new
-    // transforms, but we want adding or removing a source asset to consistently
-    // kick off a build, even if that build does nothing.
-    _onDirtyController.add(null);
-
-    // If there's a pass-through for this node, mark it dirty while we figure
-    // out whether we need to add any transforms for it.
-    var controller = _passThroughControllers[node.id];
-    if (controller != null) controller.setDirty();
-
-    // Once the input is available, hook up transformers for it. If it changes
-    // while that's happening, try again.
-    _adjustTransformersFutures[node.id] = _tryUntilStable(node,
-        (asset, transformers) {
-      var oldTransformers = _transforms[node.id]
-          .map((transform) => transform.transformer).toSet();
-
-      return _removeStaleTransforms(asset, transformers).then((_) =>
-          _addFreshTransforms(node, transformers, oldTransformers));
-    }).then((_) {
-      _adjustPassThrough(node);
-
-      // Now all the transforms are set up correctly and the asset is available
-      // for the time being. Set up handlers for when the asset changes in the
-      // future.
-      node.onStateChange.first.then((state) {
-        if (state.isRemoved) {
-          _onDirtyController.add(null);
-          _transforms.remove(node.id);
-          var passThrough = _passThroughControllers.remove(node.id);
-          if (passThrough != null) passThrough.setRemoved();
-        } else {
-          _adjustTransformers(node);
-        }
-      }).catchError((e) {
-        _adjustTransformersFutures[node.id] = new Future.error(e);
-      });
-    }).catchError((error) {
-      if (error is! AssetNotFoundException || error.id != node.id) throw error;
-
-      // If the asset is removed, [tryUntilStable] will throw an
-      // [AssetNotFoundException]. In that case, just remove all transforms for
-      // the node, and its pass-through.
-      _transforms.remove(node.id);
-      var passThrough = _passThroughControllers.remove(node.id);
-      if (passThrough != null) passThrough.setRemoved();
-    }).whenComplete(() {
-      _adjustTransformersFutures.remove(node.id);
-    });
-
-    // Don't top-level errors coming from the input processing. Any errors will
-    // eventually be piped through [process]'s returned Future.
-    _adjustTransformersFutures[node.id].catchError((_) {});
-  }
-
-  // Remove any old transforms that used to have [asset] as a primary asset but
-  // no longer apply to its new contents.
-  Future _removeStaleTransforms(Asset asset, Set<Transformer> transformers) {
-    return Future.wait(_transforms[asset.id].map((transform) {
-      return newFuture(() {
-        if (!transformers.contains(transform.transformer)) return false;
-
-        // TODO(rnystrom): Catch all errors from isPrimary() and redirect to
-        // results.
-        return transform.transformer.isPrimary(asset);
-      }).then((isPrimary) {
-        if (isPrimary) return;
-        _transforms[asset.id].remove(transform);
-        _onDirtyPool.remove(transform.onDirty);
-        transform.remove();
-      });
-    }));
-  }
-
-  // Add new transforms for transformers that consider [node]'s asset to be a
-  // primary input.
-  //
-  // [oldTransformers] is the set of transformers for which there were
-  // transforms that had [node] as a primary input prior to this. They don't
-  // need to be checked, since their transforms were removed or preserved in
-  // [_removeStaleTransforms].
-  Future _addFreshTransforms(AssetNode node, Set<Transformer> transformers,
-      Set<Transformer> oldTransformers) {
-    return Future.wait(transformers.map((transformer) {
-      if (oldTransformers.contains(transformer)) return new Future.value();
-
-      // If the asset is unavailable, the results of this [_adjustTransformers]
-      // run will be discarded, so we can just short-circuit.
-      if (node.asset == null) return new Future.value();
-
-      // We can safely access [node.asset] here even though it might have
-      // changed since (as above) if it has, [_adjustTransformers] will just be
-      // re-run.
-      // TODO(rnystrom): Catch all errors from isPrimary() and redirect to
-      // results.
-      return transformer.isPrimary(node.asset).then((isPrimary) {
-        if (!isPrimary) return;
-        var transform = new TransformNode(this, transformer, node);
-        _transforms[node.id].add(transform);
-        _onDirtyPool.add(transform.onDirty);
-      });
-    }));
-  }
-
-  /// Adjust whether [node] is passed through the phase unmodified, based on
-  /// whether it's consumed by other transforms in this phase.
-  ///
-  /// If [node] was already passed-through, this will update the passed-through
-  /// value.
-  void _adjustPassThrough(AssetNode node) {
-    assert(node.state.isAvailable);
-
-    if (_transforms[node.id].isEmpty) {
-      var controller = _passThroughControllers[node.id];
-      if (controller != null) {
-        controller.setAvailable(node.asset);
-      } else {
-        _passThroughControllers[node.id] =
-            new AssetNodeController.available(node.asset, node.transform);
-      }
-    } else {
-      var controller = _passThroughControllers.remove(node.id);
-      if (controller != null) controller.setRemoved();
-    }
-  }
-
-  /// Like [AssetNode.tryUntilStable], but also re-runs [callback] if this
-  /// phase's transformers are modified.
-  Future _tryUntilStable(AssetNode node,
-      Future callback(Asset asset, Set<Transformer> transformers)) {
-    var oldTransformers;
-    return node.tryUntilStable((asset) {
-      oldTransformers = _transformers.toSet();
-      return callback(asset, _transformers);
-    }).then((result) {
-      if (setEquals(oldTransformers, _transformers)) return result;
-      return _tryUntilStable(node, callback);
-    });
-  }
-
   /// Processes this phase.
   ///
   /// Returns a future that completes when processing is done. If there is
   /// nothing to process, returns `null`.
   Future process() {
-    if (_adjustTransformersFutures.isEmpty) return _processTransforms();
-    return _waitForInputs().then((_) => _processTransforms());
-  }
-
-  Future _waitForInputs() {
-    if (_adjustTransformersFutures.isEmpty) return new Future.value();
-    return Future.wait(_adjustTransformersFutures.values)
-        .then((_) => _waitForInputs());
-  }
-
-  /// Applies all currently wired up and dirty transforms.
-  Future _processTransforms() {
-    var newPassThroughs = _passThroughControllers.values
-        .map((controller) => controller.node)
-        .where((output) {
-      return !_outputs.containsKey(output.id) ||
-        !_outputs[output.id].contains(output);
-    }).toSet();
-
-    // Convert this to a list so we can safely modify _transforms while
-    // iterating over it.
-    var dirtyTransforms =
-        flatten(_transforms.values.map((transforms) => transforms.toList()))
-        .where((transform) => transform.isDirty).toList();
-
-    if (dirtyTransforms.isEmpty && newPassThroughs.isEmpty) return null;
-
-    var collisions = new Set<AssetId>();
-    for (var output in newPassThroughs) {
-      if (_addOutput(output)) collisions.add(output.id);
-    }
+    if (!_inputs.values.any((input) => input.isDirty)) return null;
 
-    return Future.wait(dirtyTransforms.map((transform) {
-      return transform.apply().then((outputs) {
-        for (var output in outputs) {
-          if (_addOutput(output)) collisions.add(output.id);
-        }
+    return Future.wait(_inputs.values.map((input) {
+      if (!input.isDirty) return new Future.value(new Set());
+      return input.process().then((outputs) {
+        return outputs.where(_addOutput).map((output) => output.id).toSet();
       });
-    })).then((_) {
+    })).then((collisionsList) {
       // Report collisions in a deterministic order.
-      collisions = collisions.toList();
+      var collisions = unionAll(collisionsList).toList();
       collisions.sort((a, b) => a.compareTo(b));
       for (var collision in collisions) {
         // Ensure that there's still a collision. It's possible it was resolved