Added code for listening for indexer updates via INotifyPropertyChanged.

src/Markup/Perspex.Markup/Data/CommonPropertyNames.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Perspex.Markup.Data
+{
+    public static class CommonPropertyNames
+    {
+        public const string IndexerName = "Item";
+    }
+}

src/Markup/Perspex.Markup/Data/IndexerNode.cs

@@ -6,6 +6,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.ComponentModel;
 using System.Globalization;
 using System.Linq;
 using System.Reflection;
@@ -40,6 +41,22 @@ namespace Perspex.Markup.Data
             {
                 incc.CollectionChanged += CollectionChanged;
             }
+
+            var inpc = target as INotifyPropertyChanged;
+
+            if(inpc != null)
+            {
+                inpc.PropertyChanged += IndexerPropertyChanged;
+            }
+        }
+
+        private void IndexerPropertyChanged(object sender, PropertyChangedEventArgs e)
+        {
+            var typeInfo = sender.GetType().GetTypeInfo();
+            if (typeInfo.GetDeclaredProperty(e.PropertyName).GetIndexParameters().Any())
+            {
+                CurrentValue = GetValue(sender);
+            }
         }
 
         protected override void Unsubscribe(object target)
@@ -108,17 +125,8 @@ namespace Perspex.Markup.Data
             }
             else
             {
-                PropertyInfo indexerProperty = null;
-                ParameterInfo[] indexerParameters = null;
-                foreach (var property in typeInfo.DeclaredProperties)
-                {
-                    var indexParams = property.GetIndexParameters();
-                    if (indexParams.Length > 0)
-                    {
-                        indexerProperty = property;
-                        indexerParameters = indexParams;
-                    }
-                }
+                var indexerProperty = GetIndexer(typeInfo);
+                var indexerParameters = indexerProperty?.GetIndexParameters();
                 if (indexerProperty != null && indexerParameters.Length == Arguments.Count)
                 {
                     var convertedObjectArray = new object[indexerParameters.Length];
@@ -138,6 +146,25 @@ namespace Perspex.Markup.Data
             return PerspexProperty.UnsetValue;
         }
 
+        private static PropertyInfo GetIndexer(TypeInfo typeInfo)
+        {
+            PropertyInfo indexer;
+            // Check for the default indexer name first to make this faster.
+            // This will only be false when a class in VB has a custom indexer name.
+            if ((indexer = typeInfo.GetDeclaredProperty(CommonPropertyNames.IndexerName)) != null)
+            {
+                return indexer;
+            }
+            foreach (var property in typeInfo.DeclaredProperties)
+            {
+                if (property.GetIndexParameters().Any())
+                {
+                    return property;
+                }
+            }
+            return null;
+        }
+
         private bool InBounds(int[] args, Array array)
         {
             if (args.Length == array.Rank)

src/Markup/Perspex.Markup/Perspex.Markup.csproj

@@ -41,6 +41,7 @@
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
+    <Compile Include="Data\CommonPropertyNames.cs" />
     <Compile Include="Data\ExpressionNodeBuilder.cs" />
     <Compile Include="Data\ExpressionParseException.cs" />
     <Compile Include="Data\ExpressionSubject.cs" />

tests/Perspex.Markup.UnitTests/Data/ExpressionObserverTests_Indexer.cs

@@ -146,5 +146,40 @@ namespace Perspex.Markup.UnitTests.Data
 
             Assert.Equal(new[] { "bar", PerspexProperty.UnsetValue }, result);
         }
+
+        [Fact]
+        public void Should_Track_NonIntegerIndexer()
+        {
+            var data = new { Foo = new NonIntegerIndexer() };
+            data.Foo["foo"] = "bar";
+            data.Foo["baz"] = "qux";
+
+            var target = new ExpressionObserver(data, "Foo[foo]");
+            var result = new List<object>();
+
+            var sub = target.Subscribe(x => result.Add(x));
+            data.Foo["foo"] = "bar2";
+
+            var expected = new[] { "bar", "bar2" };
+            Assert.Equal(expected, result);
+        }
+
+        private class NonIntegerIndexer : NotifyingBase
+        {
+            private Dictionary<string, string> storage = new Dictionary<string, string>();
+
+            public string this[string key]
+            {
+                get
+                {
+                    return storage[key];
+                }
+                set
+                {
+                    storage[key] = value;
+                    RaisePropertyChanged(CommonPropertyNames.IndexerName);
+                }
+            }
+        }
     }
 }