Ensure comparer changed is subscribed to before data changed for list. Fixes reactivemarbles#473 (reactivemarbles#476)

Author: RolandPheasant

src/DynamicData.Tests/List/SortFixture.cs

@@ -1,7 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-
+using System.Reactive.Subjects;
 using DynamicData.Binding;
 using DynamicData.Tests.Domain;
 
@@ -11,6 +12,50 @@
 
 namespace DynamicData.Tests.List
 {
+    public class SortChangedFixture
+    {
+        private static readonly IComparer<ListItem> DefaultComparer = SortExpressionComparer<ListItem>.Ascending(x => x.Number);
+
+
+        /// <summary>
+        /// See https://github.com/reactivemarbles/DynamicData/issues/473
+        /// </summary>
+        [Fact]
+        public void SortsWithoutError()
+        {
+            var source = new SourceList<ListItem>();
+            var sorter = new Subject<IComparer<ListItem>>();
+
+            source.AddRange(Enumerable.Range(1, 10).Select(i => new ListItem(i)));
+
+            source.Connect()
+                .Sort(sorter)
+                .Bind(out var bound)
+                .Subscribe();
+
+            bound.Select(x => x.Number).Should().BeInAscendingOrder();
+
+            sorter.OnNext(SortExpressionComparer<ListItem>.Descending(x => x.Number));
+
+
+            bound.Select(x => x.Number).Should().BeInDescendingOrder();
+        }
+
+
+        private class ListItem : IComparable<ListItem>
+        {
+            public int Number { get; }
+
+            public ListItem(int number)
+            {
+                Number = number;
+            }
+
+            public int CompareTo([AllowNull] ListItem other) => DefaultComparer.Compare(this, other);
+            
+        }
+    }
+
     public class SortFixture : IDisposable
     {
         private readonly IComparer<Person> _comparer = SortExpressionComparer<Person>.Ascending(p => p.Name).ThenByAscending(p => p.Age);

src/DynamicData/List/Internal/Sort.cs

@@ -45,7 +45,7 @@ public IObservable<IChangeSet<T>> Run()
                         var original = new List<T>();
                         var target = new ChangeAwareList<T>();
 
-                        var changed = _source.Synchronize(locker).Select(
+                        var dataChanged = _source.Synchronize(locker).Select(
                             changes =>
                                 {
                                     if (_resetThreshold > 1)
@@ -58,7 +58,7 @@ public IObservable<IChangeSet<T>> Run()
                         var resort = _resort.Synchronize(locker).Select(_ => Reorder(target));
                         var changeComparer = _comparerObservable.Synchronize(locker).Select(comparer => ChangeComparer(target, comparer));
 
-                        return changed.Merge(resort).Merge(changeComparer).Where(changes => changes.Count != 0).SubscribeSafe(observer);
+                        return changeComparer.Merge(resort).Merge(dataChanged).Where(changes => changes.Count != 0).SubscribeSafe(observer);
                     });
         }