Solver.flush update returns the set of context associated with parameters in play

lib/param.dart

diff --git a/lib/param.dart b/lib/param.dart
index 0b2b5c1..f296c9d 100644
--- a/lib/param.dart
+++ b/lib/param.dart
@@ -6,12 +6,17 @@
 
 class Param extends _EquationMember {
   final Variable variable;
+  final dynamic context;
 
-  Param.withVariable(this.variable) {
+  Param([double value = 0.0])
+      : variable = new Variable(value),
+        context = null {
     variable._owner = this;
   }
 
-  Param([double value = 0.0]) : this.variable = new Variable(value) {
+  Param.withContext(ctx)
+      : variable = new Variable(0.0),
+        context = ctx {
     variable._owner = this;
   }
 

lib/solver.dart

diff --git a/lib/solver.dart b/lib/solver.dart
index 59ee312..f76ea23 100644
--- a/lib/solver.dart
+++ b/lib/solver.dart
@@ -168,8 +168,8 @@
     return _dualOptimize();
   }
 
-  List<Param> flushParameterUpdates() {
-    List<Param> updates = new List<Param>();
+  Set flushUpdates() {
+    Set updates = new Set();
 
     for (Variable variable in _vars.keys) {
       _Symbol symbol = _vars[variable];
@@ -178,9 +178,14 @@
       double updatedValue = row == null ? 0.0 : row.constant;
 
       if (variable._applyUpdate(updatedValue) && variable._owner != null) {
-        updates.add(variable._owner);
+        dynamic context = variable._owner.context;
+
+        if (context != null) {
+          updates.add(context);
+        }
       }
     }
+
     return updates;
   }
 

test/cassowary_test.dart

diff --git a/test/cassowary_test.dart b/test/cassowary_test.dart
index 7c2e60d..1ea67e1 100644
--- a/test/cassowary_test.dart
+++ b/test/cassowary_test.dart
@@ -427,7 +427,7 @@
     var left = new Param(-20.0);
     Solver s = new Solver();
     s.addConstraint(left >= CM(0.0));
-    s.flushParameterUpdates();
+    s.flushUpdates();
     expect(left.value, 0.0);
   });
 
@@ -443,7 +443,7 @@
     expect(s.addConstraint(right - left >= CM(100.0)), Result.success);
     expect(s.addConstraint(left >= CM(0.0)), Result.success);
 
-    s.flushParameterUpdates();
+    s.flushUpdates();
 
     expect(left.value, 0.0);
     expect(mid.value, 50.0);
@@ -484,7 +484,7 @@
     expect(s.addEditVariable(mid.variable, Priority.strong), Result.success);
     expect(s.suggestValueForVariable(mid.variable, 300.0), Result.success);
 
-    s.flushParameterUpdates();
+    s.flushUpdates();
 
     expect(left.value, 0.0);
     expect(mid.value, 300.0);
@@ -523,7 +523,7 @@
     solver.addConstraint((p2 == CM(2.0) * p1) as Constraint);
     solver.addConstraint((container == (p1 + p2 + p3)) as Constraint);
 
-    solver.flushParameterUpdates();
+    solver.flushUpdates();
 
     expect(container.value, 100.0);
 
@@ -533,9 +533,9 @@
   });
 
   test('test_updates_collection', () {
-    Param left = new Param();
-    Param mid = new Param();
-    Param right = new Param();
+    Param left = new Param.withContext("left");
+    Param mid = new Param.withContext("mid");
+    Param right = new Param.withContext("right");
 
     Solver s = new Solver();
 
@@ -547,13 +547,36 @@
 
     expect(s.suggestValueForVariable(mid.variable, 50.0), Result.success);
 
-    var updates = s.flushParameterUpdates();
+    var updates = s.flushUpdates();
 
     expect(updates.length, 2);
-    expect(updates[0] is Param, true);
 
     expect(left.value, 0.0);
     expect(mid.value, 50.0);
     expect(right.value, 100.0);
   });
+
+  test('test_updates_collection_is_set', () {
+    Param left = new Param.withContext("a");
+    Param mid = new Param.withContext("a");
+    Param right = new Param.withContext("a");
+
+    Solver s = new Solver();
+
+    expect(s.addEditVariable(mid.variable, Priority.strong), Result.success);
+
+    expect(s.addConstraint((mid * CM(2.0) == left + right) as Constraint),
+        Result.success);
+    expect(s.addConstraint(left >= CM(10.0)), Result.success);
+
+    expect(s.suggestValueForVariable(mid.variable, 50.0), Result.success);
+
+    var updates = s.flushUpdates();
+
+    expect(updates.length, 1);
+
+    expect(left.value, 10.0);
+    expect(mid.value, 50.0);
+    expect(right.value, 90.0);
+  });
 }