Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/graph/transform_node.dart

Index: pkg/barback/lib/src/graph/transform_node.dart
diff --git a/pkg/barback/lib/src/graph/transform_node.dart b/pkg/barback/lib/src/graph/transform_node.dart
index 75a76a26e22714d8411211ecf35f00762cc65d99..5ef4b8108363930ad4b13acd4852057677bd5a89 100644
--- a/pkg/barback/lib/src/graph/transform_node.dart
+++ b/pkg/barback/lib/src/graph/transform_node.dart
@@ -9,15 +9,15 @@ import 'dart:async';
 import '../asset/asset.dart';
 import '../asset/asset_id.dart';
 import '../asset/asset_node.dart';
+import '../asset/asset_node_set.dart';
 import '../errors.dart';
 import '../log.dart';
 import '../transformer/aggregate_transform.dart';
+import '../transformer/aggregate_transformer.dart';
 import '../transformer/declaring_aggregate_transform.dart';
-import '../transformer/declaring_transform.dart';
-import '../transformer/declaring_transformer.dart';
-import '../transformer/lazy_transformer.dart';
-import '../transformer/transform.dart';
-import '../transformer/transformer.dart';
+import '../transformer/declaring_aggregate_transformer.dart';
+import '../transformer/lazy_aggregate_transformer.dart';
+import '../utils.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'phase.dart';
@@ -34,20 +34,23 @@ 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 [AggregateTransformer] to apply to this node's inputs.
+  final AggregateTransformer transformer;
 
-  /// The node for the primary asset this transform depends on.
-  final AssetNode primary;
+  /// The primary asset nodes this transform runs on.
+  final _primaries = new AssetNodeSet();
 
   /// A string describing the location of [this] in the transformer graph.
   final String _location;
 
-  /// The subscription to [primary]'s [AssetNode.onStateChange] stream.
-  StreamSubscription _primarySubscription;
+  /// The subscription to the [_primaries]' [AssetNode.onStateChange] streams.
+  final _primarySubscriptions = new Map<AssetId, StreamSubscription>();
 
   /// The subscription to [phase]'s [Phase.onAsset] stream.
-  StreamSubscription<AssetNode> _phaseSubscription;
+  StreamSubscription<AssetNode> _phaseAssetSubscription;
+
+  /// The subscription to [phase]'s [Phase.onStatusChange] stream.
+  StreamSubscription<NodeStatus> _phaseStatusSubscription;
 
   /// How far along [this] is in processing its assets.
   NodeStatus get status {
@@ -55,19 +58,28 @@ class TransformNode {
       return NodeStatus.IDLE;
     }
 
-    if (_declaring && _state != _State.DECLARING) {
+    if (_declaring && _state != _State.DECLARING &&
+        _state != _State.NEEDS_DECLARE) {
       return NodeStatus.MATERIALIZING;
     } else {
       return NodeStatus.RUNNING;
     }
   }
 
+  /// The [TransformInfo] describing this node.
+  ///
+  /// [TransformInfo] is the publicly-visible representation of a transform
+  /// node.
+  TransformInfo get info => new TransformInfo(transformer,
+      new AssetId(phase.cascade.package, key));
+
   /// Whether this is a declaring transform.
   ///
-  /// This is usually identical to `transformer is DeclaringTransformer`, but if
-  /// a declaring and non-lazy transformer emits an error during
-  /// `declareOutputs` it's treated as though it wasn't declaring.
-  bool get _declaring => transformer is DeclaringTransformer &&
+  /// This is usually identical to `transformer is
+  /// DeclaringAggregateTransformer`, but if a declaring and non-lazy
+  /// transformer emits an error during `declareOutputs` it's treated as though
+  /// it wasn't declaring.
+  bool get _declaring => transformer is DeclaringAggregateTransformer &&
       (_state == _State.DECLARING || _declaredOutputs != null);
 
   /// Whether this transform has been forced since it last finished applying.
@@ -77,8 +89,9 @@ class TransformNode {
   /// transformers, since they always need to eagerly generate outputs.
   bool _forced;
 
-  /// The subscriptions to each input's [AssetNode.onStateChange] stream.
-  final _inputSubscriptions = new Map<AssetId, StreamSubscription>();
+  /// The subscriptions to each secondary input's [AssetNode.onStateChange]
+  /// stream.
+  final _secondarySubscriptions = new Map<AssetId, StreamSubscription>();
 
   /// The controllers for the asset nodes emitted by this node.
   final _outputControllers = new Map<AssetId, AssetNodeController>();
@@ -87,14 +100,14 @@ class TransformNode {
   /// ran.
   final _missingInputs = new Set<AssetId>();
 
-  /// The controller that's used to pass [primary] through [this] if it's not
-  /// consumed or overwritten.
+  /// The controllers that are used to pass each primary input through [this] if
+  /// it's not consumed or overwritten.
   ///
   /// This needs an intervening controller to ensure that the output can be
   /// marked dirty when determining whether [this] will consume or overwrite it,
-  /// and be marked removed if it does. [_passThroughController] will be null
-  /// if the asset is not being passed through.
-  AssetNodeController _passThroughController;
+  /// and be marked removed if it does. No pass-through controller will exist
+  /// for primary inputs that are not being passed through.
+  final _passThroughControllers = new Map<AssetId, AssetNodeController>();
 
   /// The asset node for this transform.
   final _streams = new NodeStreams();
@@ -103,7 +116,7 @@ class TransformNode {
   Stream<LogEntry> get onLog => _streams.onLog;
 
   /// The current state of [this].
-  var _state = _State.DECLARING;
+  var _state = _State.DECLARED;
 
   /// Whether [this] has been marked as removed.
   bool get _isRemoved => _streams.onAssetController.isClosed;
@@ -112,60 +125,87 @@ class TransformNode {
   // can run [apply] even if [force] hasn't been called, since [transformer]
   // should run eagerly if possible.
   bool get _canRunDeclaringEagerly =>
-      _declaring && transformer is! LazyTransformer &&
-      primary.state.isAvailable;
+      _declaring && transformer is! LazyAggregateTransformer &&
+      _primaries.every((input) => input.state.isAvailable);
 
-  /// Whether the most recent run of this transform has declared that it
-  /// consumes the primary input.
+  /// Which primary inputs the most recent run of this transform has declared
+  /// that it consumes.
   ///
-  /// Defaults to `false`. This is not meaningful unless [_state] is
-  /// [_State.APPLIED] or [_State.DECLARED].
-  bool _consumePrimary = false;
+  /// This starts out `null`, indicating that the transform hasn't declared
+  /// anything yet. This is not meaningful unless [_state] is [_State.APPLIED]
+  /// or [_State.DECLARED].
+  Set<AssetId> _consumedPrimaries;
 
   /// The set of output ids that [transformer] declared it would emit.
   ///
-  /// This is only non-null if [transformer] is a [DeclaringTransformer] and its
-  /// [declareOutputs] has been run successfully.
+  /// This is only non-null if [transformer] is a
+  /// [DeclaringAggregateTransformer] and its [declareOutputs] has been run
+  /// successfully.
   Set<AssetId> _declaredOutputs;
 
-  TransformNode(this.phase, Transformer transformer, AssetNode primary,
-      this._location)
-      : key = primary.id.path,
-        transformer = transformer,
-        primary = primary {
-    _forced = transformer is! DeclaringTransformer;
-    if (_forced) primary.force();
+  /// The controller for the currently-running
+  /// [DeclaringAggregateTransformer.declareOutputs] call's
+  /// [DeclaringAggregateTransform].
+  ///
+  /// This will be non-`null` when
+  /// [DeclaringAggregateTransformer.declareOutputs] is running. This means that
+  /// it's always non-`null` when [_state] is [_State.DECLARING], sometimes
+  /// non-`null` when it's [_State.NEEDS_DECLARE], and always `null` otherwise.
+  DeclaringAggregateTransformController _declareController;
+
+  /// The controller for the currently-running [AggregateTransformer.apply]
+  /// call's [AggregateTransform].
+  ///
+  /// This will be non-`null` when [AggregateTransform.apply] is running, which
+  /// means that it's always non-`null` when [_state] is [_State.APPLYING] or
+  /// [_State.NEEDS_APPLY], sometimes non-`null` when it's
+  /// [_State.NEEDS_DECLARE], and always `null` otherwise.
+  AggregateTransformController _applyController;
 
-    _primarySubscription = primary.onStateChange.listen((state) {
-      if (state.isRemoved) {
-        remove();
-      } else {
-        if (_forced) primary.force();
-        _dirty();
-      }
-    });
+  TransformNode(this.phase, this.transformer, this.key, this._location) {
+    _forced = transformer is! DeclaringAggregateTransformer;
 
-    _phaseSubscription = phase.previous.onAsset.listen((node) {
+    _phaseAssetSubscription = phase.previous.onAsset.listen((node) {
       if (!_missingInputs.contains(node.id)) return;
       if (_forced) node.force();
       _dirty();
     });
 
-    _declareOutputs().then((_) {
-      if (_forced || _canRunDeclaringEagerly) {
-        _apply();
-      } else {
-        _state = _State.DECLARED;
-        _streams.changeStatus(NodeStatus.IDLE);
-      }
+    _phaseStatusSubscription = phase.previous.onStatusChange.listen((status) {
+      if (status == NodeStatus.RUNNING) return;
+
+      _maybeFinishDeclareController();
+      _maybeFinishApplyController();
     });
+
+    _run();
   }
 
-  /// The [TransformInfo] describing this node.
-  ///
-  /// [TransformInfo] is the publicly-visible representation of a transform
-  /// node.
-  TransformInfo get info => new TransformInfo(transformer, primary.id);
+  /// Adds [input] as a primary input for this node.
+  void addPrimary(AssetNode input) {
+    _primaries.add(input);
+    if (_forced) input.force();
+
+    _primarySubscriptions[input.id] = input.onStateChange
+        .listen((_) => _onPrimaryStateChange(input));
+
+    if (_state == _State.DECLARING && !_declareController.isDone) {
+      // If we're running `declareOutputs` and its id stream isn't closed yet,
+      // pass this in as another id.
+      _declareController.addId(input.id);
+      _maybeFinishDeclareController();
+    } else if (_state == _State.APPLYING) {
+      // If we're running `apply`, we need to wait until [input] is available
+      // before we pass it into the stream. If it's available now, great; if
+      // not, [_onPrimaryStateChange] will handle it.
+      if (!input.state.isAvailable) return;
+      _onPrimaryStateChange(input);
+      _maybeFinishApplyController();
+    } else {
+      // Otherwise, a new input means we'll need to re-run `declareOutputs`.
+      _restartRun();
+    }
+  }
 
   /// Marks this transform as removed.
   ///
@@ -175,41 +215,58 @@ class TransformNode {
   /// valid even if its primary input still exists.
   void remove() {
     _streams.close();
-    _primarySubscription.cancel();
-    _phaseSubscription.cancel();
-    _clearInputSubscriptions();
+    _phaseAssetSubscription.cancel();
+    _phaseStatusSubscription.cancel();
+    if (_declareController != null) _declareController.cancel();
+    if (_applyController != null) _applyController.cancel();
+    _clearSecondarySubscriptions();
     _clearOutputs();
-    if (_passThroughController != null) {
-      _passThroughController.setRemoved();
-      _passThroughController = null;
+
+    for (var subscription in _primarySubscriptions.values) {
+      subscription.cancel();
+    }
+    _primarySubscriptions.clear();
+
+    for (var controller in _passThroughControllers.values) {
+      controller.setRemoved();
     }
+    _passThroughControllers.clear();
   }
 
   /// If [this] is deferred, ensures that its concrete outputs will be
   /// generated.
   void force() {
     if (_forced || _state == _State.APPLIED) return;
-    primary.force();
+    for (var input in _primaries) {
+      input.force();
+    }
+
     _forced = true;
-    if (_state == _State.DECLARED) _dirty();
+    if (_state == _State.DECLARED) _apply();
   }
 
   /// Marks this transform as dirty.
   ///
-  /// This causes all of the transform's outputs to be marked as dirty as well.
+  /// Specifically, this should be called when one of the transform's inputs'
+  /// contents change, or when a secondary input is removed. Primary inputs
+  /// being added or removed are handled by [addInput] and
+  /// [_onPrimaryStateChange].
   void _dirty() {
-    // If we're in the process of running [declareOutputs], we already know that
-    // [apply] needs to be run so there's nothing we need to mark as dirty.
-    if (_state == _State.DECLARING) return;
+    if (_state == _State.DECLARING || _state == _State.NEEDS_DECLARE ||
+        _state == _State.NEEDS_APPLY) {
+      // If we already know that [_apply] needs to be run, there's nothing to do
+      // here.
+      return;
+    }
 
     if (!_forced && !_canRunDeclaringEagerly) {
       // [forced] should only ever be false for a declaring transformer.
       assert(_declaring);
 
-      // If we've finished applying, transition to MATERIALIZING, indicating
-      // that we know what outputs [apply] will emit but we're waiting to emit
-      // them concretely until [force] is called. If we're still applying, we'll
-      // transition to MATERIALIZING once we finish.
+      // If we've finished applying, transition to DECLARED, indicating that we
+      // know what outputs [apply] will emit but we're waiting to emit them
+      // concretely until [force] is called. If we're still applying, we'll
+      // transition to DECLARED once we finish.
       if (_state == _State.APPLIED) _state = _State.DECLARED;
       for (var controller in _outputControllers.values) {
         controller.setLazy(force);
@@ -218,11 +275,6 @@ class TransformNode {
       return;
     }
 
-    if (_passThroughController != null) _passThroughController.setDirty();
-    for (var controller in _outputControllers.values) {
-      controller.setDirty();
-    }
-
     if (_state == _State.APPLIED) {
       if (_declaredOutputs != null) _emitDeclaredOutputs();
       _apply();
@@ -233,21 +285,136 @@ class TransformNode {
     }
   }
 
+  /// The callback called when [input]'s state changes.
+  void _onPrimaryStateChange(AssetNode input) {
+    if (input.state.isRemoved) {
+      _primarySubscriptions.remove(input.id);
+
+      if (_primaries.isEmpty) {
+        // If there are no more primary inputs, there's no more use for this
+        // node in the graph. It will be re-created by its
+        // [TransformerClassifier] if a new input with [key] is added.
+        remove();
+        return;
+      }
+
+      // Any change to the number of primary inputs requires that we re-run the
+      // transformation.
+      _restartRun();
+    } else if (input.state.isAvailable) {
+      if (_state == _State.DECLARED && _canRunDeclaringEagerly) {
+        // If [this] is fully declared but hasn't started applying, this input
+        // becoming available may mean that all inputs are available, in which
+        // case we can run apply eagerly.
+        _apply();
+        return;
+      }
+
+      // If we're not actively passing concrete assets to the transformer, the
+      // distinction between a dirty asset and an available one isn't relevant.
+      if (_state != _State.APPLYING) return;
+
+      if (_applyController.isDone) {
+        // If we get a new asset after we've closed the asset stream, we need to
+        // re-run declare and then apply.
+        _restartRun();
+      } else {
+        // If the new asset comes before the asset stream is done, we can just
+        // pass it to the stream.
+        _applyController.addInput(input.asset);
+        _maybeFinishApplyController();
+      }
+    } else {
+      if (_forced) input.force();
+      if (_state == _State.APPLYING && !_applyController.addedId(input.id)) {
+        // If the input hasn't yet been added to the transform's input stream,
+        // there's no need to consider the transformation dirty.
+        return;
+      }
+      _dirty();
+    }
+  }
+
+  /// Run the entire transformation, including both `declareOutputs` (if
+  /// applicable) and `apply`.
+  void _run() {
+    assert(_state != _State.DECLARING);
+    assert(_state != _State.APPLYING);
+
+    _markOutputsDirty();
+    _declareOutputs(() {
+      if (_forced || _canRunDeclaringEagerly) {
+        _apply();
+      } else {
+        _state = _State.DECLARED;
+        _streams.changeStatus(NodeStatus.IDLE);
+      }
+    });
+  }
+
+  /// Restart the entire transformation, including `declareOutputs` if
+  /// applicable.
+  void _restartRun() {
+    if (_state == _State.DECLARED || _state == _State.APPLIED) {
+      // If we're currently idle, we can restart the transformation immediately.
+      _run();
+      return;
+    }
+
+    // If we're actively running `declareOutputs` or `apply`, cancel the
+    // transforms and transition to `NEEDS_DECLARE`. Once the transformer's
+    // method returns, we'll transition to `DECLARING`.
+    if (_declareController != null) _declareController.cancel();
+    if (_applyController != null) _applyController.cancel();
+    _state = _State.NEEDS_DECLARE;
+  }
+
   /// Runs [transform.declareOutputs] and emits the resulting assets as dirty
   /// assets.
-  Future _declareOutputs() {
-    if (transformer is! DeclaringTransformer) return new Future.value();
+  ///
+  /// Calls [callback] when it's finished. This doesn't return a future so that
+  /// [callback] is called synchronously if there are no outputs to declare. If
+  /// [this] is removed while inputs are being declared, [callback] will not be
+  /// called.
+  void _declareOutputs(void callback()) {
+    if (transformer is! DeclaringAggregateTransformer) {
+      callback();
+      return;
+    }
 
+    _state = _State.DECLARING;
     var controller = new DeclaringAggregateTransformController(this);
+    _declareController = controller;
     _streams.onLogPool.add(controller.onLog);
-    controller.idController.add(primary.id);
-    return newDeclaringTransform(controller.transform).then((transform) {
-      return (transformer as DeclaringTransformer).declareOutputs(transform);
+    for (var primary in _primaries) {
+      controller.addId(primary.id);
+    }
+
+    syncFuture(() {
+      return (transformer as DeclaringAggregateTransformer)
+          .declareOutputs(controller.transform);
+    }).whenComplete(() {
+      // Cancel the controller here even if `declareOutputs` wasn't interrupted.
+      // Since the declaration is finished, we want to close out the
+      // controller's streams.
+      controller.cancel();
+      _declareController = null;
     }).then((_) {
       if (_isRemoved) return;
-      if (controller.loggedError) return;
+      if (_state == _State.NEEDS_DECLARE) {
+        _declareOutputs(callback);
+        return;
+      }
 
-      _consumePrimary = controller.consumedPrimaries.contains(primary.id);
+      if (controller.loggedError) {
+        // If `declareOutputs` fails, fall back to treating a declaring
+        // transformer as though it were eager.
+        if (transformer is! LazyAggregateTransformer) _forced = true;
+        callback();
+        return;
+      }
+
+      _consumedPrimaries = controller.consumedPrimaries;
       _declaredOutputs = controller.outputIds;
       var invalidIds = _declaredOutputs
           .where((id) => id.package != phase.cascade.package).toSet();
@@ -257,12 +424,17 @@ class TransformNode {
         phase.cascade.reportError(new InvalidOutputException(info, id));
       }
 
-      if (!_declaredOutputs.contains(primary.id)) _emitPassThrough();
+      for (var primary in _primaries) {
+        if (_declaredOutputs.contains(primary.id)) continue;
+        _passThrough(primary.id);
+      }
       _emitDeclaredOutputs();
+      callback();
     }).catchError((error, stackTrace) {
       if (_isRemoved) return;
-      if (transformer is! LazyTransformer) _forced = true;
+      if (transformer is! LazyAggregateTransformer) _forced = true;
       phase.cascade.reportError(_wrapException(error, stackTrace));
+      callback();
     });
   }
 
@@ -284,13 +456,26 @@ class TransformNode {
     }
   }
 
+  //// Mark all emitted and passed-through outputs of this transform as dirty.
+  void _markOutputsDirty() {
+    for (var controller in _passThroughControllers.values) {
+      controller.setDirty();
+    }
+    for (var controller in _outputControllers.values) {
+      if (_forced) {
+        controller.setDirty();
+      } else {
+        controller.setLazy(force);
+      }
+    }
+  }
+
   /// Applies this transform.
   void _apply() {
     assert(!_isRemoved);
 
-    // Clear input subscriptions here as well as in [_process] because [_apply]
-    // may be restarted independently if only a secondary input changes.
-    _clearInputSubscriptions();
+    _markOutputsDirty();
+    _clearSecondarySubscriptions();
     _state = _State.APPLYING;
     _streams.changeStatus(status);
     _runApply().then((hadError) {
@@ -298,8 +483,19 @@ class TransformNode {
 
       if (_state == _State.DECLARED) return;
 
+      if (_state == _State.NEEDS_DECLARE) {
+        _run();
+        return;
+      }
+
+      // If an input's contents changed while running `apply`, retry unless the
+      // transformer is deferred and hasn't been forced.
       if (_state == _State.NEEDS_APPLY) {
-        _apply();
+        if (_forced || _canRunDeclaringEagerly) {
+          _apply();
+        } else {
+          _state = _State.DECLARED;
+        }
         return;
       }
 
@@ -309,14 +505,18 @@ class TransformNode {
       if (hadError) {
         _clearOutputs();
         // If the transformer threw an error, we don't want to emit the
-        // pass-through asset in case it will be overwritten by the transformer.
-        // However, if the transformer declared that it wouldn't overwrite or
-        // consume the pass-through asset, we can safely emit it.
-        if (_declaredOutputs != null && !_consumePrimary &&
-            !_declaredOutputs.contains(primary.id)) {
-          _emitPassThrough();
-        } else {
-          _dontEmitPassThrough();
+        // pass-through assets in case they'll be overwritten by the
+        // transformer. However, if the transformer declared that it wouldn't
+        // overwrite or consume a pass-through asset, we can safely emit it.
+        if (_declaredOutputs != null) {
+          for (var input in _primaries) {
+            if (_consumedPrimaries.contains(input.id) ||
+                _declaredOutputs.contains(input.id)) {
+              _consumePrimary(input.id);
+            } else {
+              _passThrough(input.id);
+            }
+          }
         }
       }
 
@@ -338,30 +538,39 @@ class TransformNode {
         throw new AssetNotFoundException(id);
       }
 
-      _inputSubscriptions.putIfAbsent(node.id, () {
-        return node.onStateChange.listen((state) => _dirty());
+      _secondarySubscriptions.putIfAbsent(node.id, () {
+        return node.onStateChange.listen((_) => _dirty());
       });
 
       return node.asset;
     });
   }
 
-  /// Run [Transformer.apply] as soon as [primary] is available.
+  /// Run [AggregateTransformer.apply].
   ///
   /// Returns whether or not an error occurred while running the transformer.
   Future<bool> _runApply() {
     var controller = new AggregateTransformController(this);
+    _applyController = controller;
     _streams.onLogPool.add(controller.onLog);
+    for (var primary in _primaries) {
+      if (!primary.state.isAvailable) continue;
+      controller.addInput(primary.asset);
+    }
 
-    return primary.whenAvailable((_) {
-      if (_isRemoved) return null;
-      _state = _State.APPLYING;
-      controller.inputController.add(primary.asset);
-      return newTransform(controller.transform).then((transform) {
-        return transformer.apply(transform);
-      });
+    return syncFuture(() {
+      return transformer.apply(controller.transform);
+    }).whenComplete(() {
+      // Cancel the controller here even if `apply` wasn't interrupted. Since
+      // the apply is finished, we want to close out the controller's streams.
+      controller.cancel();
+      _applyController = null;
     }).then((_) {
-      if (!_forced && !primary.state.isAvailable) {
+      assert(_state != _State.DECLARED);
+      assert(_state != _State.DECLARING);
+      assert(_state != _State.APPLIED);
+
+      if (!_forced && _primaries.any((node) => !node.state.isAvailable)) {
         _state = _State.DECLARED;
         _streams.changeStatus(NodeStatus.IDLE);
         return false;
@@ -369,7 +578,7 @@ class TransformNode {
 
       if (_isRemoved) return false;
       if (_state == _State.NEEDS_APPLY) return false;
-      if (_state == _State.DECLARING) return false;
+      if (_state == _State.NEEDS_DECLARE) return false;
       if (controller.loggedError) return true;
       _handleApplyResults(controller);
       return false;
@@ -390,7 +599,7 @@ class TransformNode {
   /// [controller] should be the controller for the [AggegateTransform] passed
   /// to [AggregateTransformer.apply].
   void _handleApplyResults(AggregateTransformController controller) {
-    _consumePrimary = controller.consumedPrimaries.contains(primary.id);
+    _consumedPrimaries = controller.consumedPrimaries;
 
     var newOutputs = controller.outputs;
     // Any ids that are for a different package are invalid.
@@ -410,12 +619,14 @@ class TransformNode {
       _outputControllers.remove(id).setRemoved();
     }
 
-    // Emit or stop emitting the pass-through asset between removing and
-    // adding outputs to ensure there are no collisions.
-    if (!_consumePrimary && !newOutputs.containsId(primary.id)) {
-      _emitPassThrough();
-    } else {
-      _dontEmitPassThrough();
+    // Emit or stop emitting pass-through assets between removing and adding
+    // outputs to ensure there are no collisions.
+    for (var id in _primaries.map((node) => node.id)) {
+      if (_consumedPrimaries.contains(id) || newOutputs.containsId(id)) {
+        _consumePrimary(id);
+      } else {
+        _passThrough(id);
+      }
     }
 
     // Store any new outputs or new contents for existing outputs.
@@ -432,12 +643,12 @@ class TransformNode {
   }
 
   /// Cancels all subscriptions to secondary input nodes.
-  void _clearInputSubscriptions() {
+  void _clearSecondarySubscriptions() {
     _missingInputs.clear();
-    for (var subscription in _inputSubscriptions.values) {
+    for (var subscription in _secondarySubscriptions.values) {
       subscription.cancel();
     }
-    _inputSubscriptions.clear();
+    _secondarySubscriptions.clear();
   }
 
   /// Removes all output assets.
@@ -449,26 +660,49 @@ class TransformNode {
     _outputControllers.clear();
   }
 
-  /// Emit the pass-through asset if it's not being emitted already.
-  void _emitPassThrough() {
-    assert(!_outputControllers.containsKey(primary.id));
-
-    if (_consumePrimary) return;
-    if (_passThroughController == null) {
-      _passThroughController = new AssetNodeController.from(primary);
-      _streams.onAssetController.add(_passThroughController.node);
+  /// Emit the pass-through node for the primary input [id] if it's not being
+  /// emitted already.
+  void _passThrough(AssetId id) {
+    assert(!_outputControllers.containsKey(id));
+
+    if (_consumedPrimaries.contains(id)) return;
+    var controller = _passThroughControllers[id];
+    var primary = _primaries[id];
+    if (controller == null) {
+      controller = new AssetNodeController.from(primary);
+      _passThroughControllers[id] = controller;
+      _streams.onAssetController.add(controller.node);
     } else if (primary.state.isDirty) {
-      _passThroughController.setDirty();
-    } else if (!_passThroughController.node.state.isAvailable) {
-      _passThroughController.setAvailable(primary.asset);
+      controller.setDirty();
+    } else if (!controller.node.state.isAvailable) {
+      controller.setAvailable(primary.asset);
     }
   }
 
-  /// Stop emitting the pass-through asset if it's being emitted already.
-  void _dontEmitPassThrough() {
-    if (_passThroughController == null) return;
-    _passThroughController.setRemoved();
-    _passThroughController = null;
+  /// Stops emitting the pass-through node for the primary input [id] if it's
+  /// being emitted.
+  void _consumePrimary(AssetId id) {
+    var controller = _passThroughControllers.remove(id);
+    if (controller == null) return;
+    controller.setRemoved();
+  }
+
+  /// If `declareOutputs` is running and all previous phases have declared their
+  /// outputs, mark [_declareController] as done.
+  void _maybeFinishDeclareController() {
+    if (_declareController == null) return;
+    if (phase.previous.status == NodeStatus.RUNNING) return;
+    _declareController.done();
+  }
+
+  /// If `apply` is running, all previous phases have declared their outputs,
+  /// and all primary inputs are available and thus have been passed to the
+  /// transformer, mark [_applyController] as done.
+  void _maybeFinishApplyController() {
+    if (_applyController == null) return;
+    if (_primaries.any((input) => !input.state.isAvailable)) return;
+    if (phase.previous.status == NodeStatus.RUNNING) return;
+    _applyController.done();
   }
 
   BarbackException _wrapException(error, StackTrace stackTrace) {
@@ -479,62 +713,67 @@ class TransformNode {
     }
   }
 
-  /// Emit a warning about the transformer on [id].
-  void _warn(String message) {
-    _streams.onLogController.add(
-        new LogEntry(info, primary.id, LogLevel.WARNING, message, null));
-  }
-
   String toString() =>
-      "transform node in $_location for $transformer on $primary ($_state, "
-      "$status, ${_forced ? '' : 'un'}forced)";
+      "transform node in $_location for $transformer on ${info.primaryId} "
+      "($_state, $status, ${_forced ? '' : 'un'}forced)";
 }
 
 /// The enum of states that [TransformNode] can be in.
 class _State {
-  /// The transform is running [DeclaringTransformer.declareOutputs].
+  /// The transform is running [DeclaringAggregateTransformer.declareOutputs].
   ///
-  /// This is the initial state of the transformer, and it will only occur once
-  /// since [DeclaringTransformer.declareOutputs] is independent of the contents
-  /// of the primary input. Once the method finishes running, this will
-  /// transition to [APPLYING] if the transform is non-lazy and the input is
+  /// If the set of primary inputs changes while in this state, it will
+  /// transition to [NEEDS_DECLARE]. If the [TransformNode] is still in this
+  /// state when `declareOutputs` finishes running, it will transition to
+  /// [APPLYING] if the transform is non-lazy and all of its primary inputs are
   /// available, and [DECLARED] otherwise.
   ///
   /// Non-declaring transformers will transition out of this state and into
   /// [APPLYING] immediately.
-  static final DECLARING = const _State._("declaring outputs");
+  static const DECLARING = const _State._("declaring outputs");
 
-  /// The transform is deferred and has run
-  /// [DeclaringTransformer.declareOutputs] but hasn't yet been forced.
+  /// The transform is running [AggregateTransformer.declareOutputs] or
+  /// [AggregateTransform.apply], but a primary input was added or removed after
+  /// it started, so it will need to re-run `declareOutputs`.
   ///
-  /// This will transition to [APPLYING] when one of the outputs has been
-  /// forced.
-  static final DECLARED = const _State._("declared");
+  /// The [TransformNode] will transition to [DECLARING] once `declareOutputs`
+  /// or `apply` finishes running.
+  static const NEEDS_DECLARE = const _State._("needs declare");
 
-  /// The transform is running [Transformer.apply].
+  /// The transform is deferred and has run
+  /// [DeclaringAggregateTransformer.declareOutputs] but hasn't yet been forced.
   ///
-  /// If an input changes while in this state, it will transition to
-  /// [NEEDS_APPLY]. If the [TransformNode] is still in this state when
-  /// [Transformer.apply] finishes running, it will transition to [APPLIED].
-  static final APPLYING = const _State._("applying");
+  /// The [TransformNode] will transition to [APPLYING] when one of the outputs
+  /// has been forced or if the transformer is non-lazy and all of its primary
+  /// inputs become available.
+  static const DECLARED = const _State._("declared");
 
-  /// The transform is running [Transformer.apply], but an input changed after
-  /// it started, so it will need to re-run [Transformer.apply].
+  /// The transform is running [AggregateTransformer.apply].
   ///
-  /// This will transition to [APPLYING] once [Transformer.apply] finishes
-  /// running.
-  static final NEEDS_APPLY = const _State._("needs apply");
-
-  /// The transform has finished running [Transformer.apply], whether or not it
-  /// emitted an error.
+  /// If an input's contents change or a secondary input is added or removed
+  /// while in this state, the [TransformNode] will transition to [NEEDS_APPLY].
+  /// If a primary input is added or removed, it will transition to
+  /// [NEEDS_DECLARE]. If it's still in this state when `apply` finishes
+  /// running, it will transition to [APPLIED].
+  static const APPLYING = const _State._("applying");
+
+  /// The transform is running [AggregateTransformer.apply], but an input's
+  /// contents changed or a secondary input was added or removed after it
+  /// started, so it will need to re-run `apply`.
   ///
-  /// If the transformer is deferred, the [TransformNode] can also be in this
-  /// state when [Transformer.declareOutputs] has been run but
-  /// [Transformer.apply] has not.
+  /// If a primary input is added or removed while in this state, the
+  /// [TranformNode] will transition to [NEEDS_DECLARE]. If it's still in this
+  /// state when `apply` finishes running, it will transition to [APPLYING].
+  static const NEEDS_APPLY = const _State._("needs apply");
+
+  /// The transform has finished running [AggregateTransformer.apply], whether
+  /// or not it emitted an error.
   ///
-  /// If an input changes, this will transition to [DECLARED] if the transform
-  /// is deferred and [APPLYING] otherwise.
-  static final APPLIED = const _State._("applied");
+  /// If an input's contents change or a secondary input is added or removed,
+  /// the [TransformNode] will transition to [DECLARED] if the transform is
+  /// declaring and [APPLYING] otherwise. If a primary input is added or
+  /// removed, this will transition to [DECLARING].
+  static const APPLIED = const _State._("applied");
 
   final String name;