Move Barback to a more thoroughly push-based model.

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 17d32d8e2c2fdc110572fb9deaa1226ea93b1d87..b6aafc9bade58459e17ac49ce3beb6f61684085a 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -45,7 +45,7 @@ class Phase {
   /// The transformers that can access [inputs].
   ///
   /// Their outputs will be available to the next phase.
-  final Set<Transformer> _transformers;
+  final _transformers = new Set<Transformer>();
 
   /// The groups for this phase.
   final _groups = new Map<TransformerGroup, GroupRunner>();
@@ -77,21 +77,22 @@ class Phase {
   /// so, it's been forwarded unmodified.
   final _inputOrigins = new Multiset<AssetNode>();
 
-  /// A stream that emits an event whenever this phase becomes dirty and needs
-  /// to be run.
+  /// A stream that emits an event whenever [this] is no longer dirty.
   ///
-  /// This may emit events when the phase was already dirty or while processing
-  /// transforms. Events are emitted synchronously to ensure that the dirty
-  /// state is thoroughly propagated as soon as any assets are changed.
-  Stream get onDirty => _onDirtyPool.stream;
-  final _onDirtyPool = new StreamPool.broadcast();
+  /// This is synchronous in order to guarantee that it will emit an event as
+  /// soon as [isDirty] flips from `true` to `false`.
+  Stream get onDone => _onDoneController.stream;
+  final _onDoneController = new StreamController.broadcast(sync: true);
 
-  /// A controller whose stream feeds into [_onDirtyPool].
+  /// A stream that emits any new assets emitted by [this].
   ///
-  /// This is used whenever an input is added or transforms are changed.
-  final _onDirtyController = new StreamController.broadcast(sync: true);
+  /// Assets are emitted synchronously to ensure that any changes are thoroughly
+  /// propagated as soon as they occur. Only a phase with no [next] phase will
+  /// emit assets.
+  Stream<AssetNode> get onAsset => _onAssetController.stream;
+  final _onAssetController = new StreamController<AssetNode>(sync: true);
 
-  /// Whether this phase is dirty and needs to be run.
+  /// Whether [this] is dirty and still has more processing to do.
   bool get isDirty => _inputs.values.any((input) => input.isDirty) ||
       _groups.values.any((group) => group.isDirty);
 
@@ -117,20 +118,10 @@ class Phase {
   // TODO(nweiz): Rather than passing the cascade and the phase everywhere,
   // create an interface that just exposes [getInput]. Emit errors via
   // [AssetNode]s.
-  Phase(AssetCascade cascade, Iterable transformers, String location)
-      : this._(cascade, transformers, location, 0);
+  Phase(AssetCascade cascade, String location)
+      : this._(cascade, location, 0);
 
-  Phase._(this.cascade, Iterable transformers, this._location, this._index)
-      : _transformers = transformers.where((op) => op is Transformer).toSet() {
-    _onDirtyPool.add(_onDirtyController.stream);
-
-    for (var group in transformers.where((op) => op is TransformerGroup)) {
-      var runner = new GroupRunner(cascade, group, "$_location.$_index");
-      _groups[group] = runner;
-      _onDirtyPool.add(runner.onDirty);
-      _onLogPool.add(runner.onLog);
-    }
-  }
+  Phase._(this.cascade, this._location, this._index);
 
   /// Adds a new asset as an input for this phase.
   ///
@@ -151,12 +142,7 @@ class Phase {
     // there's one additional channel for the non-grouped transformers.
     var forwarder = new PhaseForwarder(_groups.length + 1);
     _forwarders[node.id] = forwarder;
-    forwarder.onForwarding.listen((asset) {
-      _addOutput(asset);
-
-      var exception = _outputs[asset.id].collisionException;
-      if (exception != null) cascade.reportError(exception);
-    });
+    forwarder.onAsset.listen(_handleOutputWithoutForwarder);
 
     _inputOrigins.add(node.origin);
     var input = new PhaseInput(this, node, _transformers, "$_location.$_index");
@@ -165,10 +151,13 @@ class Phase {
       _inputOrigins.remove(node.origin);
       _inputs.remove(node.id);
       _forwarders.remove(node.id).remove();
+      if (!isDirty) _onDoneController.add(null);
     });
-    _onDirtyPool.add(input.onDirty);
-    _onDirtyController.add(null);
+    input.onAsset.listen(_handleOutput);
     _onLogPool.add(input.onLog);
+    input.onDone.listen((_) {
+      if (!isDirty) _onDoneController.add(null);
+    });
 
     for (var group in _groups.values) {
       group.addInput(node);
@@ -201,8 +190,6 @@ class Phase {
 
   /// Set this phase's transformers to [transformers].
   void updateTransformers(Iterable transformers) {
-    _onDirtyController.add(null);
-
     var actualTransformers = transformers.where((op) => op is Transformer);
     _transformers.clear();
     _transformers.addAll(actualTransformers);
@@ -220,8 +207,11 @@ class Phase {
     for (var added in newGroups.difference(oldGroups)) {
       var runner = new GroupRunner(cascade, added, "$_location.$_index");
       _groups[added] = runner;
-      _onDirtyPool.add(runner.onDirty);
+      runner.onAsset.listen(_handleOutput);
       _onLogPool.add(runner.onLog);
+      runner.onDone.listen((_) {
+        if (!isDirty) _onDoneController.add(null);
+      });
       for (var input in _inputs.values) {
         runner.addInput(input.input);
       }
@@ -244,12 +234,12 @@ class Phase {
     }
   }
 
-  /// Add a new phase after this one with [transformers].
+  /// Add a new phase after this one.
   ///
   /// This may only be called on a phase with no phase following it.
-  Phase addPhase(Iterable transformers) {
+  Phase addPhase() {
     assert(_next == null);
-    _next = new Phase._(cascade, transformers, _location, _index + 1);
+    _next = new Phase._(cascade, _location, _index + 1);
     for (var output in _outputs.values.toList()) {
       // Remove [output]'s listeners because now they should get the asset from
       // [_next], rather than this phase. Any transforms consuming [output] will
@@ -270,7 +260,7 @@ class Phase {
     for (var group in _groups.values) {
       group.remove();
     }
-    _onDirtyPool.close();
+    _onAssetController.close();
     _onLogPool.close();
   }
 
@@ -281,60 +271,40 @@ class Phase {
     _next = null;
   }
 
-  /// Processes this phase.
-  ///
-  /// Returns a future that completes when processing is done. If there is
-  /// nothing to process, returns `null`.
-  Future process() {
-    if (!isDirty) return null;
-
-    var outputIds = new Set<AssetId>();
-    void _handleOutputs(Set<AssetNode> outputs) {
-      for (var asset in outputs) {
-        if (_inputOrigins.contains(asset.origin)) {
-          _forwarders[asset.id].addIntermediateAsset(asset);
-          continue;
-        }
-
-        outputIds.add(asset.id);
-        _addOutput(asset);
-      }
+  /// Add [asset] as an output of this phase.
+  void _handleOutput(AssetNode asset) {
+    if (_inputOrigins.contains(asset.origin)) {
+      _forwarders[asset.id].addIntermediateAsset(asset);
+    } else {
+      _handleOutputWithoutForwarder(asset);
     }
-
-    var outputFutures = [];
-    outputFutures.addAll(_inputs.values.map((input) {
-      if (!input.isDirty) return new Future.value(new Set());
-      return input.process().then(_handleOutputs);
-    }));
-    outputFutures.addAll(_groups.values.map((group) {
-      if (!group.isDirty) return new Future.value(new Set());
-      return group.process().then(_handleOutputs);
-    }));
-
-    return Future.wait(outputFutures).then((_) {
-      // Report collisions in a deterministic order.
-      outputIds = outputIds.toList();
-      outputIds.sort((a, b) => a.compareTo(b));
-      for (var id in outputIds) {
-        // It's possible the output was removed before other transforms in this
-        // phase finished.
-        if (!_outputs.containsKey(id)) continue;
-        var exception = _outputs[id].collisionException;
-        if (exception != null) cascade.reportError(exception);
-      }
-    });
   }
 
-  /// Add [asset] as an output of this phase.
-  void _addOutput(AssetNode asset) {
+  /// Add [asset] as an output of this phase without checking if it's a
+  /// forwarded asset.
+  void _handleOutputWithoutForwarder(AssetNode asset) {
     if (_outputs.containsKey(asset.id)) {
       _outputs[asset.id].add(asset);
     } else {
       _outputs[asset.id] = new PhaseOutput(this, asset, "$_location.$_index");
-      _outputs[asset.id].onAsset.listen((output) {
-        if (_next != null) _next.addInput(output);
-      }, onDone: () => _outputs.remove(asset.id));
-      if (_next != null) _next.addInput(_outputs[asset.id].output);
+      _outputs[asset.id].onAsset.listen(_emit,
+          onDone: () => _outputs.remove(asset.id));
+      _emit(_outputs[asset.id].output);
+    }
+
+    var exception = _outputs[asset.id].collisionException;
+    if (exception != null) cascade.reportError(exception);
+  }
+
+  /// Emit [asset] as an output of this phase.
+  ///
+  /// This should be called after [_handleOutput], so that collisions are
+  /// resolved.
+  void _emit(AssetNode asset) {
+    if (_next != null) {
+      _next.addInput(asset);
+    } else {
+      _onAssetController.add(asset);
     }
   }