Added conditional sticky items.

Added a method that creates an observable collection that tracks an
ITrackingCollection and adds a sticky item to the top of the collection
when a related selection is null.

Author: grokys

src/GitHub.Exports.Reactive/Collections/TrackingCollection.cs

@@ -100,6 +100,46 @@ public static ObservableCollection<T> CreateListenerCollection<T>(this ITracking
             };
             return col;
         }
+
+        /// <summary>
+        /// Creates an observable collection that tracks an <see cref="ITrackingCollection{T}"/>
+        /// and adds a sticky item to the top of the collection when a related selection is null.
+        /// </summary>
+        /// <typeparam name="T">The type of items in the collection.</typeparam>
+        /// <param name="tcol">The source tracking collection</param>
+        /// <param name="stickieItemOnTop">The sticky item to add to the top of the collection.</param>
+        /// <param name="selection">
+        /// The current selection. If null or equal to the sticky item then the sticky item will be
+        /// added to the collection.
+        /// </param>
+        /// <returns>An <see cref="ObservableCollection{T}"/>.</returns>
+        public static ObservableCollection<T> CreateListenerCollection<T>(this ITrackingCollection<T> tcol,
+            T stickieItemOnTop,
+            IObservable<T> selection)
+            where T : class, ICopyable<T>
+        {
+            Debug.Assert(stickieItemOnTop != null, "stickieItemOnTop may not be null in CreateListenerCollection");
+            Debug.Assert(selection != null, "selection may not be null in CreateListenerCollection");
+
+            var result = tcol.CreateListenerCollection(new[] { stickieItemOnTop });
+            var addedStickieItem = true;
+
+            selection.Subscribe(x =>
+            {
+                if ((x == null || object.Equals(x, stickieItemOnTop)) && addedStickieItem)
+                {
+                    result.Remove(stickieItemOnTop);
+                    addedStickieItem = false;
+                }
+                else if (x != null && !addedStickieItem)
+                {
+                    result.Insert(0, stickieItemOnTop);
+                    addedStickieItem = true;
+                }
+            });
+
+            return result;
+        }
     }
 
     /// <summary>

src/TrackingCollectionTests/ListenerCollectionTests.cs

@@ -0,0 +1,121 @@
+using System;
+using System.Linq;
+using System.Reactive.Linq;
+using System.Reactive.Subjects;
+using GitHub.Collections;
+using NUnit.Framework;
+
+[TestFixture]
+public class ListenerCollectionTests : TestBase
+{
+#if !DISABLE_REACTIVE_UI
+    [TestFixtureSetUp]
+    public void Setup()
+    {
+        Splat.ModeDetector.Current.SetInUnitTestRunner(true);
+    }
+#endif
+
+    [Test]
+    public void StickyItemShouldNotBePresentAfterCreationWhenSelectionNull()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = Observable.Return<Thing>(null);
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        CollectionAssert.AreEqual(source, target);
+    }
+
+    [Test]
+    public void StickyItemShouldBePresentAfterCreationWhenSelectionNotNull()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = Observable.Return(source[0]);
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        var expected = new[] { stickie }.Concat(source);
+        CollectionAssert.AreEqual(expected, target);
+    }
+
+    [Test]
+    public void StickyItemShouldNotBePresentAfterCreationWhenSelectionIsStickyItem()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = Observable.Return(stickie);
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        CollectionAssert.AreEqual(source, target);
+    }
+
+    [Test]
+    public void StickyItemShouldNotBePresentAfterCreationWhenSelectionEqualsStickyItem()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = Observable.Return(new Thing());
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        CollectionAssert.AreEqual(source, target);
+    }
+
+    [Test]
+    public void StickyItemShouldBeAddedWhenSelectionChangesFromNull()
+    {
+        var source = CreateSource();
+        var selection = new BehaviorSubject<Thing>(null);
+        var stickie = new Thing();
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        CollectionAssert.AreEqual(source, target);
+
+        selection.OnNext(source[0]);
+
+        var expected = new[] { stickie }.Concat(source);
+        CollectionAssert.AreEqual(expected, target);
+    }
+
+    [Test]
+    public void StickyItemShouldBeRemovedWhenSelectionChangesToNull()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = new BehaviorSubject<Thing>(source[0]);
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        var expected = new[] { stickie }.Concat(source);
+        CollectionAssert.AreEqual(expected, target);
+
+        selection.OnNext(null);
+
+        CollectionAssert.AreEqual(source, target);
+    }
+
+    [Test]
+    public void StickyItemShouldBeRemovedWhenSelectionChangesToStickyItem()
+    {
+        var source = CreateSource();
+        var stickie = new Thing();
+        var selection = new BehaviorSubject<Thing>(source[0]);
+        var target = source.CreateListenerCollection(stickie, selection);
+
+        var expected = new[] { stickie }.Concat(source);
+        CollectionAssert.AreEqual(expected, target);
+
+        selection.OnNext(stickie);
+
+        CollectionAssert.AreEqual(source, target);
+    }
+
+    TrackingCollection<Thing> CreateSource()
+    {
+        var result = new TrackingCollection<Thing>(Observable.Empty<Thing>());
+        result.Subscribe();
+        result.AddItem(new Thing(1, "item1", DateTimeOffset.MinValue));
+        result.AddItem(new Thing(2, "item2", DateTimeOffset.MinValue));
+        result.AddItem(new Thing(3, "item3", DateTimeOffset.MinValue));
+        return result;
+    }
+}

src/TrackingCollectionTests/TrackingCollectionTests.csproj

@@ -100,6 +100,7 @@
     <Compile Include="ITestOutputHelper.cs" />
     <Compile Include="TestBase.cs" />
     <Compile Include="Thing.cs" />
+    <Compile Include="ListenerCollectionTests.cs" />
     <Compile Include="TrackingCollectionTests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>