[Core] Ignore empty groups

Lookup groups are removed once empty, we shouldn't be adding them if
they're empty. This happens when all of a category's properties are
removed to form a grouped category, the original category lingered.

Xamarin.PropertyEditing.Tests/ObservableLookupTests.cs

@@ -1,4 +1,5 @@
-using System.Collections.Specialized;
+using System;
+using System.Collections.Specialized;
 using System.Linq;
 using NUnit.Framework;
 
@@ -67,5 +68,53 @@ namespace Xamarin.PropertyEditing.Tests
 			Assert.That (grouping, Does.Not.Contains (value));
 			Assert.That (lookup, Does.Not.Contain (grouping));
 		}
+
+		[TestCase ("group")]
+		[TestCase (null)]
+		[Description ("If removing the last item from a group removes the group, adding a group with no items shouldn't add it")]
+		public void AddingEmptyElementsIsIgnored (string groupName)
+		{
+			var lookup = new ObservableLookup<string, string> ();
+			bool changed = false;
+			lookup.CollectionChanged += (sender, args) => {
+				changed = true;
+			};
+			lookup.Add (groupName, Enumerable.Empty<string> ());
+
+			Assert.That (lookup, Is.Empty);
+			Assert.That (changed, Is.False);
+		}
+
+		[TestCase ("group")]
+		[TestCase (null)]
+		[Description ("If removing the last item from a group removes the group, adding a group with no items shouldn't add it")]
+		public void AddingEmptyGroupingIsIgnored (string groupName)
+		{
+			var lookup = new ObservableLookup<string, string> ();
+			bool changed = false;
+			lookup.CollectionChanged += (sender, args) => {
+				changed = true;
+			};
+			lookup.Add (new ObservableGrouping<string, string> (groupName));
+
+			Assert.That (lookup, Is.Empty);
+			Assert.That (changed, Is.False);
+		}
+
+		[TestCase ("group")]
+		[TestCase (null)]
+		[Description ("If removing the last item from a group removes the group, adding a group with no items shouldn't add it")]
+		public void InsertingEmptyIsIgnored (string groupName)
+		{
+			var lookup = new ObservableLookup<string, string> ();
+			bool changed = false;
+			lookup.CollectionChanged += (sender, args) => {
+				changed = true;
+			};
+			lookup.Insert (0, new ObservableGrouping<string, string> (groupName));
+
+			Assert.That (lookup, Is.Empty);
+			Assert.That (changed, Is.False);
+		}
 	}
 }

Xamarin.PropertyEditing/ObservableLookup.cs

@@ -102,20 +102,26 @@ namespace Xamarin.PropertyEditing
 		public void Add (TKey key, IEnumerable<TElement> elements)
 		{
 			if (elements == null)
-				throw new ArgumentNullException ("elements");
+				throw new ArgumentNullException (nameof(elements));
 
 			ObservableGrouping<TKey, TElement> grouping;
 			if (key == null) {
 				bool wasEmpty = this.nullGrouping.Count == 0;
 				grouping = nullGrouping;
 				grouping.AddRange (elements);
-				if (wasEmpty)
+				if (wasEmpty && this.nullGrouping.Count > 0)
 					OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Add, (object)grouping, this.groupings.Count));
 			} else if (!this.groupings.TryGetValue (key, out grouping)) {
 				if (!ReuseGroups || !this.oldGroups.TryRemove (key, out grouping))
 					grouping = new ObservableGrouping<TKey, TElement> (key);
 
 				grouping.AddRange (elements);
+				if (grouping.Count == 0) {
+					if (ReuseGroups)
+						this.oldGroups.Add (grouping.Key, grouping);
+
+					return;
+				}
 
 				this.groupings.Add (key, grouping);
 				OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Add, (object)grouping, this.groupings.Count - 1));
@@ -124,10 +130,13 @@ namespace Xamarin.PropertyEditing
 
 		public void Add (IGrouping<TKey, TElement> grouping)
 		{
+			if (grouping == null)
+				throw new ArgumentNullException (nameof(grouping));
+
 			if (grouping.Key == null) {
 				bool wasEmpty = this.nullGrouping.Count == 0;
 				this.nullGrouping.AddRange (grouping);
-				if (wasEmpty)
+				if (wasEmpty && this.nullGrouping.Count > 0)
 					OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Add, (object)grouping, this.groupings.Count));
 
 				return;
@@ -139,6 +148,12 @@ namespace Xamarin.PropertyEditing
 			}
 
 			og.AddRange (grouping);
+			if (og.Count == 0) {
+				if (ReuseGroups)
+					this.oldGroups.Add (grouping.Key, og);
+
+				return;
+			}
 
 			this.groupings.Add (grouping.Key, og);
 			OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Add, (object)og, this.groupings.Count - 1));
@@ -152,6 +167,13 @@ namespace Xamarin.PropertyEditing
 			}
 
 			og.AddRange (grouping);
+			if (og.Count == 0) {
+				if (ReuseGroups)
+					this.oldGroups.Add (grouping.Key, og);
+
+				return;
+			}
+
 			this.groupings.Insert (index, og.Key, og);
 			OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Add, (object)og, index));
 		}