From 0ddf4caa95c125f31515f718d30afaa57d8d746c Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@github.com>
Date: Fri, 7 Oct 2016 16:32:14 +0100
Subject: [PATCH 1/8] Added a "stream" binding expression operator.

Which is now required when wanting to get the value produced by a
Task/IObservable rather than the Task/IObservable itself. Fixes #711.
---
 .../Avalonia.Markup/Avalonia.Markup.csproj    |  1 +
 .../Avalonia.Markup/Data/ExpressionNode.cs    | 53 +++++--------------
 .../Data/Parsers/ExpressionParser.cs          | 10 ++++
 src/Markup/Avalonia.Markup/Data/StreamNode.cs | 31 +++++++++++
 .../ExpressionObserverTests_Observable.cs     | 26 +++++++--
 .../Data/ExpressionObserverTests_Task.cs      | 15 +++---
 6 files changed, 84 insertions(+), 52 deletions(-)
 create mode 100644 src/Markup/Avalonia.Markup/Data/StreamNode.cs

diff --git a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
index 88c4a6ab18..616891c0ea 100644
--- a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
+++ b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
@@ -42,6 +42,7 @@
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
+    <Compile Include="Data\StreamNode.cs" />
     <Compile Include="Data\MarkupBindingChainNullException.cs" />
     <Compile Include="Data\CommonPropertyNames.cs" />
     <Compile Include="Data\EmptyExpressionNode.cs" />
diff --git a/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs b/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
index b0957c7187..9d590b0e19 100644
--- a/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
+++ b/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
@@ -17,7 +17,6 @@ namespace Avalonia.Markup.Data
         private WeakReference _target = UnsetReference;
         private IDisposable _valueSubscription;
         private IObserver<object> _observer;
-        private IDisposable _valuePluginSubscription;
 
         public abstract string Description { get; }
         public ExpressionNode Next { get; set; }
@@ -37,7 +36,6 @@ namespace Avalonia.Markup.Data
                 {
                     _valueSubscription?.Dispose();
                     _valueSubscription = null;
-                    _valuePluginSubscription?.Dispose();
                     _target = value;
 
                     if (running)
@@ -63,8 +61,6 @@ namespace Avalonia.Markup.Data
             {
                 _valueSubscription?.Dispose();
                 _valueSubscription = null;
-                _valuePluginSubscription?.Dispose();
-                _valuePluginSubscription = null;
                 nextSubscription?.Dispose();
                 _observer = null;
             });
@@ -115,25 +111,22 @@ namespace Avalonia.Markup.Data
                 source = StartListeningCore(_target);
             }
 
-            return source.Subscribe(TargetValueChanged);
+            return source.Subscribe(ValueChanged);
         }
 
-        private void TargetValueChanged(object value)
+        private void ValueChanged(object value)
         {
             var notification = value as BindingNotification;
 
             if (notification == null)
             {
-                if (!HandleSpecialValue(value))
+                if (Next != null)
                 {
-                    if (Next != null)
-                    {
-                        Next.Target = new WeakReference(value);
-                    }
-                    else
-                    {
-                        _observer.OnNext(value);
-                    }
+                    Next.Target = new WeakReference(value);
+                }
+                else
+                {
+                    _observer.OnNext(value);
                 }
             }
             else
@@ -144,38 +137,16 @@ namespace Avalonia.Markup.Data
                 }
                 else if (notification.HasValue)
                 {
-                    if (!HandleSpecialValue(notification.Value))
+                    if (Next != null)
                     {
-                        if (Next != null)
-                        {
-                            Next.Target = new WeakReference(notification.Value);
-                        }
-                        else
-                        {
-                            _observer.OnNext(value);
-                        }
+                        Next.Target = new WeakReference(notification.Value);
                     }
-                }
-            }
-        }
-
-        private bool HandleSpecialValue(object value)
-        {
-            if (_valuePluginSubscription == null)
-            {
-                var reference = new WeakReference(value);
-
-                foreach (var plugin in ExpressionObserver.ValueHandlers)
-                {
-                    if (plugin.Match(reference))
+                    else
                     {
-                        _valuePluginSubscription = plugin.Start(reference)?.Subscribe(TargetValueChanged);
-                        return true;
+                        _observer.OnNext(value);
                     }
                 }
             }
-
-            return false;
         }
 
         private BindingNotification TargetNullNotification()
diff --git a/src/Markup/Avalonia.Markup/Data/Parsers/ExpressionParser.cs b/src/Markup/Avalonia.Markup/Data/Parsers/ExpressionParser.cs
index 6f9f2925bc..93ddf77376 100644
--- a/src/Markup/Avalonia.Markup/Data/Parsers/ExpressionParser.cs
+++ b/src/Markup/Avalonia.Markup/Data/Parsers/ExpressionParser.cs
@@ -87,6 +87,11 @@ namespace Avalonia.Markup.Data.Parsers
             {
                 return State.BeforeMember;
             }
+            else if (ParseStreamOperator(r))
+            {
+                nodes.Add(new StreamNode());
+                return State.AfterMember;
+            }
             else
             {
                 var args = ArgumentListParser.Parse(r, '[', ']');
@@ -161,6 +166,11 @@ namespace Avalonia.Markup.Data.Parsers
             return !r.End && r.TakeIf('(');
         }
 
+        private static bool ParseStreamOperator(Reader r)
+        {
+            return !r.End && r.TakeIf('^');
+        }
+
         private enum State
         {
             Start,
diff --git a/src/Markup/Avalonia.Markup/Data/StreamNode.cs b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
new file mode 100644
index 0000000000..7513b4323f
--- /dev/null
+++ b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
@@ -0,0 +1,31 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Globalization;
+using Avalonia.Data;
+using System.Reactive.Linq;
+
+namespace Avalonia.Markup.Data
+{
+    internal class StreamNode : ExpressionNode
+    {
+        public override string Description => "^";
+
+        protected override IObservable<object> StartListeningCore(WeakReference reference)
+        {
+            foreach (var plugin in ExpressionObserver.ValueHandlers)
+            {
+                if (plugin.Match(reference))
+                {
+                    return plugin.Start(reference);
+                }
+            }
+
+            // TODO: Improve error.
+            return Observable.Return(new BindingNotification(
+                new InvalidCastException("Value could not be streamed."),
+                BindingErrorType.Error));
+        }
+    }
+}
diff --git a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
index 3263aaace2..5e1a392f96 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
@@ -15,7 +15,7 @@ namespace Avalonia.Markup.UnitTests.Data
     public class ExpressionObserverTests_Observable
     {
         [Fact]
-        public void Should_Get_Simple_Observable_Value()
+        public void Should_Not_Get_Observable_Value_Without_Modifier_Char()
         {
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
@@ -28,6 +28,24 @@ namespace Avalonia.Markup.UnitTests.Data
                 source.OnNext("bar");
                 sync.ExecutePostedCallbacks();
 
+                Assert.Equal(new[] { source }, result);
+            }
+        }
+
+        [Fact]
+        public void Should_Get_Simple_Observable_Value()
+        {
+            using (var sync = UnitTestSynchronizationContext.Begin())
+            {
+                var source = new BehaviorSubject<string>("foo");
+                var data = new { Foo = source };
+                var target = new ExpressionObserver(data, "Foo^");
+                var result = new List<object>();
+
+                var sub = target.Subscribe(x => result.Add(x));
+                source.OnNext("bar");
+                sync.ExecutePostedCallbacks();
+
                 Assert.Equal(new[] { "foo", "bar" }, result);
             }
         }
@@ -38,7 +56,7 @@ namespace Avalonia.Markup.UnitTests.Data
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new Class1();
-                var target = new ExpressionObserver(data, "Next.Foo");
+                var target = new ExpressionObserver(data, "Next^.Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -59,7 +77,7 @@ namespace Avalonia.Markup.UnitTests.Data
             {
                 var source = new BehaviorSubject<string>("foo");
                 var data = new { Foo = source };
-                var target = new ExpressionObserver(data, "Foo", true);
+                var target = new ExpressionObserver(data, "Foo^", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -78,7 +96,7 @@ namespace Avalonia.Markup.UnitTests.Data
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new Class1();
-                var target = new ExpressionObserver(data, "Next.Foo", true);
+                var target = new ExpressionObserver(data, "Next^.Foo", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
diff --git a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Task.cs b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Task.cs
index 3dcd8a4fbc..61e6dcb833 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Task.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Task.cs
@@ -15,7 +15,7 @@ namespace Avalonia.Markup.UnitTests.Data
     public class ExpressionObserverTests_Task
     {
         [Fact]
-        public void Should_Get_Simple_Task_Value()
+        public void Should_Not_Get_Task_Result_Without_Modifier_Char()
         {
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
@@ -28,7 +28,8 @@ namespace Avalonia.Markup.UnitTests.Data
                 tcs.SetResult("foo");
                 sync.ExecutePostedCallbacks();
 
-                Assert.Equal(new[] { "foo" }, result);
+                Assert.Equal(1, result.Count);
+                Assert.IsType<Task<string>>(result[0]);
             }
         }
 
@@ -38,7 +39,7 @@ namespace Avalonia.Markup.UnitTests.Data
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new { Foo = Task.FromResult("foo") };
-                var target = new ExpressionObserver(data, "Foo");
+                var target = new ExpressionObserver(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -54,7 +55,7 @@ namespace Avalonia.Markup.UnitTests.Data
             {
                 var tcs = new TaskCompletionSource<Class2>();
                 var data = new Class1(tcs.Task);
-                var target = new ExpressionObserver(data, "Next.Foo");
+                var target = new ExpressionObserver(data, "Next^.Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -72,7 +73,7 @@ namespace Avalonia.Markup.UnitTests.Data
             {
                 var tcs = new TaskCompletionSource<string>();
                 var data = new { Foo = tcs.Task };
-                var target = new ExpressionObserver(data, "Foo");
+                var target = new ExpressionObserver(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -96,7 +97,7 @@ namespace Avalonia.Markup.UnitTests.Data
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new { Foo = TaskFromException(new NotSupportedException()) };
-                var target = new ExpressionObserver(data, "Foo");
+                var target = new ExpressionObserver(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -119,7 +120,7 @@ namespace Avalonia.Markup.UnitTests.Data
             {
                 var tcs = new TaskCompletionSource<string>();
                 var data = new { Foo = tcs.Task };
-                var target = new ExpressionObserver(data, "Foo", true);
+                var target = new ExpressionObserver(data, "Foo^", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));

From 715149b1f5b5b11696b00adbd0b4ed71abeb4f29 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 10 Oct 2016 22:35:44 +0200
Subject: [PATCH 2/8] Improve stream operator error message.

When a stream operator is applied to an unsupported type.
---
 src/Avalonia.Base/Avalonia.Base.csproj        |  2 +-
 ...lException.cs => BindingChainException.cs} | 57 ++++++++-----------
 src/Avalonia.Controls/TextBox.cs              |  2 +-
 .../Avalonia.Markup/Avalonia.Markup.csproj    |  2 +-
 .../Avalonia.Markup/Data/ExpressionNode.cs    |  4 +-
 .../Data/ExpressionObserver.cs                |  2 +-
 .../Data/MarkupBindingChainException.cs       | 42 ++++++++++++++
 .../Data/MarkupBindingChainNullException.cs   | 33 -----------
 src/Markup/Avalonia.Markup/Data/StreamNode.cs |  2 +-
 .../ExpressionObserverTests_DataValidation.cs |  2 +-
 .../ExpressionObserverTests_Observable.cs     | 25 ++++++++
 .../Data/ExpressionObserverTests_Property.cs  |  4 +-
 12 files changed, 102 insertions(+), 75 deletions(-)
 rename src/Avalonia.Base/Data/{BindingChainNullException.cs => BindingChainException.cs} (52%)
 create mode 100644 src/Markup/Avalonia.Markup/Data/MarkupBindingChainException.cs
 delete mode 100644 src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs

diff --git a/src/Avalonia.Base/Avalonia.Base.csproj b/src/Avalonia.Base/Avalonia.Base.csproj
index bc52e31d2c..63f500270d 100644
--- a/src/Avalonia.Base/Avalonia.Base.csproj
+++ b/src/Avalonia.Base/Avalonia.Base.csproj
@@ -44,7 +44,7 @@
     <Compile Include="..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
-    <Compile Include="Data\BindingChainNullException.cs" />
+    <Compile Include="Data\BindingChainException.cs" />
     <Compile Include="Data\BindingNotification.cs" />
     <Compile Include="Data\IndexerBinding.cs" />
     <Compile Include="Diagnostics\INotifyCollectionChangedDebug.cs" />
diff --git a/src/Avalonia.Base/Data/BindingChainNullException.cs b/src/Avalonia.Base/Data/BindingChainException.cs
similarity index 52%
rename from src/Avalonia.Base/Data/BindingChainNullException.cs
rename to src/Avalonia.Base/Data/BindingChainException.cs
index 0e50a36d8a..97b0d3ba8b 100644
--- a/src/Avalonia.Base/Data/BindingChainNullException.cs
+++ b/src/Avalonia.Base/Data/BindingChainException.cs
@@ -10,36 +10,39 @@ namespace Avalonia.Data
     /// requested binding expression could not be evaluated because of a null in one of the links
     /// of the binding chain.
     /// </summary>
-    public class BindingChainNullException : Exception
+    public class BindingChainException : Exception
     {
         private string _message;
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
-        public BindingChainNullException()
+        public BindingChainException()
         {
         }
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
-        public BindingChainNullException(string message)
+        /// <param name="message">The error message.</param>
+        public BindingChainException(string message)
         {
             _message = message;
         }
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
+        /// <param name="message">The error message.</param>
         /// <param name="expression">The expression.</param>
-        /// <param name="expressionNullPoint">
-        /// The point in the expression at which the null was encountered.
+        /// <param name="errorPoint">
+        /// The point in the expression at which the error was encountered.
         /// </param>
-        public BindingChainNullException(string expression, string expressionNullPoint)
+        public BindingChainException(string message, string expression, string errorPoint)
         {
+            _message = message;
             Expression = expression;
-            ExpressionNullPoint = expressionNullPoint;
+            ExpressionErrorPoint = errorPoint;
         }
 
         /// <summary>
@@ -48,37 +51,27 @@ namespace Avalonia.Data
         public string Expression { get; protected set; }
 
         /// <summary>
-        /// Gets the point in the expression at which the null was encountered.
+        /// Gets the point in the expression at which the error occured.
         /// </summary>
-        public string ExpressionNullPoint { get; protected set; }
+        public string ExpressionErrorPoint { get; protected set; }
 
         /// <inheritdoc/>
         public override string Message
         {
             get
             {
-                if (_message == null)
+                if (Expression != null && ExpressionErrorPoint != null)
                 {
-                    _message = BuildMessage();
+                    return $"{_message} in expression '{Expression}' at '{ExpressionErrorPoint}'.";
+                }
+                else if (ExpressionErrorPoint != null)
+                {
+                    return $"{_message} in expression '{ExpressionErrorPoint}'.";
+                }
+                else
+                {
+                    return $"{_message} in expression.";
                 }
-
-                return _message;
-            }
-        }
-
-        private string BuildMessage()
-        {
-            if (Expression != null && ExpressionNullPoint != null)
-            {
-                return $"'{ExpressionNullPoint}' is null in expression '{Expression}'.";
-            }
-            else if (ExpressionNullPoint != null)
-            {
-                return $"'{ExpressionNullPoint}' is null in expression.";
-            }
-            else
-            {
-                return "Null encountered in binding expression.";
             }
         }
     }
diff --git a/src/Avalonia.Controls/TextBox.cs b/src/Avalonia.Controls/TextBox.cs
index ed92899cc6..bc8da38724 100644
--- a/src/Avalonia.Controls/TextBox.cs
+++ b/src/Avalonia.Controls/TextBox.cs
@@ -547,7 +547,7 @@ namespace Avalonia.Controls
                 var exceptions = aggregate == null ?
                     (IEnumerable<Exception>)new[] { exception } :
                     aggregate.InnerExceptions;
-                var filtered = exceptions.Where(x => !(x is BindingChainNullException)).ToList();
+                var filtered = exceptions.Where(x => !(x is BindingChainException)).ToList();
 
                 if (filtered.Count > 0)
                 {
diff --git a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
index 616891c0ea..d97ab89a09 100644
--- a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
+++ b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
@@ -43,7 +43,7 @@
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
     <Compile Include="Data\StreamNode.cs" />
-    <Compile Include="Data\MarkupBindingChainNullException.cs" />
+    <Compile Include="Data\MarkupBindingChainException.cs" />
     <Compile Include="Data\CommonPropertyNames.cs" />
     <Compile Include="Data\EmptyExpressionNode.cs" />
     <Compile Include="Data\ExpressionNodeBuilder.cs" />
diff --git a/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs b/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
index 9d590b0e19..93f20e4c77 100644
--- a/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
+++ b/src/Markup/Avalonia.Markup/Data/ExpressionNode.cs
@@ -88,7 +88,7 @@ namespace Avalonia.Markup.Data
 
         protected virtual void NextValueChanged(object value)
         {
-            var bindingBroken = BindingNotification.ExtractError(value) as MarkupBindingChainNullException;
+            var bindingBroken = BindingNotification.ExtractError(value) as MarkupBindingChainException;
             bindingBroken?.AddNode(Description);
             _observer.OnNext(value);
         }
@@ -152,7 +152,7 @@ namespace Avalonia.Markup.Data
         private BindingNotification TargetNullNotification()
         {
             return new BindingNotification(
-                new MarkupBindingChainNullException(),
+                new MarkupBindingChainException("Null value"),
                 BindingErrorType.Error,
                 AvaloniaProperty.UnsetValue);
         }
diff --git a/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs b/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
index 819949b7b9..90c1aa2894 100644
--- a/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
+++ b/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
@@ -235,7 +235,7 @@ namespace Avalonia.Markup.Data
             }
             else
             {
-                var broken = BindingNotification.ExtractError(o) as MarkupBindingChainNullException;
+                var broken = BindingNotification.ExtractError(o) as MarkupBindingChainException;
 
                 if (broken != null)
                 {
diff --git a/src/Markup/Avalonia.Markup/Data/MarkupBindingChainException.cs b/src/Markup/Avalonia.Markup/Data/MarkupBindingChainException.cs
new file mode 100644
index 0000000000..dab5756976
--- /dev/null
+++ b/src/Markup/Avalonia.Markup/Data/MarkupBindingChainException.cs
@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Avalonia.Data;
+
+namespace Avalonia.Markup.Data
+{
+    internal class MarkupBindingChainException : BindingChainException
+    {
+        private IList<string> _nodes = new List<string>();
+
+        public MarkupBindingChainException(string message)
+            : base(message)
+        {
+        }
+
+        public MarkupBindingChainException(string message, string node)
+            : base(message)
+        {
+            AddNode(node);
+        }
+
+        public MarkupBindingChainException(string message, string expression, string expressionNullPoint)
+            : base(message, expression, expressionNullPoint)
+        {
+            _nodes = null;
+        }
+
+        public bool HasNodes => _nodes.Count > 0;
+        public void AddNode(string node) => _nodes.Add(node);
+
+        public void Commit(string expression)
+        {
+            Expression = expression;
+            ExpressionErrorPoint = string.Join(".", _nodes.Reverse())
+                .Replace(".!", "!")
+                .Replace(".[", "[")
+                .Replace(".^", "^");
+            _nodes = null;
+        }
+    }
+}
diff --git a/src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs b/src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs
deleted file mode 100644
index a549d6ebb6..0000000000
--- a/src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs
+++ /dev/null
@@ -1,33 +0,0 @@
-using System.Collections.Generic;
-using System.Linq;
-using Avalonia.Data;
-
-namespace Avalonia.Markup.Data
-{
-    internal class MarkupBindingChainNullException : BindingChainNullException
-    {
-        private IList<string> _nodes = new List<string>();
-
-        public MarkupBindingChainNullException()
-        {
-        }
-
-        public MarkupBindingChainNullException(string expression, string expressionNullPoint)
-            : base(expression, expressionNullPoint)
-        {
-            _nodes = null;
-        }
-
-        public bool HasNodes => _nodes.Count > 0;
-        public void AddNode(string node) => _nodes.Add(node);
-
-        public void Commit(string expression)
-        {
-            Expression = expression;
-            ExpressionNullPoint = string.Join(".", _nodes.Reverse())
-                .Replace(".!", "!")
-                .Replace(".[", "[");
-            _nodes = null;
-        }
-    }
-}
diff --git a/src/Markup/Avalonia.Markup/Data/StreamNode.cs b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
index 7513b4323f..7a5cfe5009 100644
--- a/src/Markup/Avalonia.Markup/Data/StreamNode.cs
+++ b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
@@ -24,7 +24,7 @@ namespace Avalonia.Markup.Data
 
             // TODO: Improve error.
             return Observable.Return(new BindingNotification(
-                new InvalidCastException("Value could not be streamed."),
+                new MarkupBindingChainException("Stream operator applied to unsupported type", Description),
                 BindingErrorType.Error));
         }
     }
diff --git a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs
index 546cfe015f..3b5ca26db1 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs
@@ -143,7 +143,7 @@ namespace Avalonia.Markup.UnitTests.Data
             Assert.Equal(new[]
             {
                 new BindingNotification(
-                    new MarkupBindingChainNullException("Inner.MustBePositive", "Inner"),
+                    new MarkupBindingChainException("Null value", "Inner.MustBePositive", "Inner"),
                     BindingErrorType.Error,
                     AvaloniaProperty.UnsetValue),
             }, result);
diff --git a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
index 5e1a392f96..640d82fa19 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs
@@ -110,6 +110,31 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
+        [Fact]
+        public void Should_Return_BindingNotification_If_Stream_Operator_Applied_To_Not_Supported_Type()
+        {
+            using (var sync = UnitTestSynchronizationContext.Begin())
+            {
+                var data = new Class2("foo");
+                var target = new ExpressionObserver(data, "Foo^", true);
+                var result = new List<object>();
+
+                var sub = target.Subscribe(x => result.Add(x));
+                sync.ExecutePostedCallbacks();
+
+                Assert.Equal(
+                    new[]
+                    {
+                        new BindingNotification(
+                            new MarkupBindingChainException("Stream operator applied to unsupported type", "Foo^", "Foo^"),
+                            BindingErrorType.Error)
+                    },
+                    result);
+
+                sub.Dispose();
+            }
+        }
+
         private class Class1 : NotifyingBase
         {
             public Subject<Class2> Next { get; } = new Subject<Class2>();
diff --git a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs
index aa9ee7d58b..bdcd39d997 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs
@@ -146,7 +146,7 @@ namespace Avalonia.Markup.UnitTests.Data
                 new[]
                 {
                     new BindingNotification(
-                    new MarkupBindingChainNullException("Foo.Bar.Baz", "Foo"),
+                        new MarkupBindingChainException("Null value", "Foo.Bar.Baz", "Foo"),
                         BindingErrorType.Error,
                         AvaloniaProperty.UnsetValue),
                 },
@@ -274,7 +274,7 @@ namespace Avalonia.Markup.UnitTests.Data
                 {
                     "bar",
                     new BindingNotification(
-                        new MarkupBindingChainNullException("Next.Next.Bar", "Next.Next"),
+                        new MarkupBindingChainException("Null value", "Next.Next.Bar", "Next.Next"),
                         BindingErrorType.Error,
                         AvaloniaProperty.UnsetValue),
                     "bar"

From b6c51b34effb17629e94df89f4b4b0a6902e0767 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 10 Oct 2016 22:48:21 +0200
Subject: [PATCH 3/8] Renamed ValueHandler -> StreamPlugin

---
 .../Avalonia.Markup/Avalonia.Markup.csproj     |  6 +++---
 .../Avalonia.Markup/Data/ExpressionObserver.cs | 12 ++++++------
 .../{IValuePlugin.cs => IStreamPlugin.cs}      |  4 ++--
 ...aluePlugin.cs => ObservableStreamPlugin.cs} | 18 +++---------------
 ...{TaskValuePlugin.cs => TaskStreamPlugin.cs} |  4 ++--
 src/Markup/Avalonia.Markup/Data/StreamNode.cs  |  2 +-
 6 files changed, 17 insertions(+), 29 deletions(-)
 rename src/Markup/Avalonia.Markup/Data/Plugins/{IValuePlugin.cs => IStreamPlugin.cs} (89%)
 rename src/Markup/Avalonia.Markup/Data/Plugins/{ObservableValuePlugin.cs => ObservableStreamPlugin.cs} (62%)
 rename src/Markup/Avalonia.Markup/Data/Plugins/{TaskValuePlugin.cs => TaskStreamPlugin.cs} (95%)

diff --git a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
index d97ab89a09..1c3f453280 100644
--- a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
+++ b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
@@ -64,9 +64,9 @@
     <Compile Include="Data\Parsers\IdentifierParser.cs" />
     <Compile Include="Data\Parsers\ExpressionParser.cs" />
     <Compile Include="Data\Parsers\Reader.cs" />
-    <Compile Include="Data\Plugins\ObservableValuePlugin.cs" />
-    <Compile Include="Data\Plugins\TaskValuePlugin.cs" />
-    <Compile Include="Data\Plugins\IValuePlugin.cs" />
+    <Compile Include="Data\Plugins\ObservableStreamPlugin.cs" />
+    <Compile Include="Data\Plugins\TaskStreamPlugin.cs" />
+    <Compile Include="Data\Plugins\IStreamPlugin.cs" />
     <Compile Include="Data\Plugins\PropertyAccessorBase.cs" />
     <Compile Include="Data\Plugins\PropertyError.cs" />
     <Compile Include="Data\Plugins\DataValidatiorBase.cs" />
diff --git a/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs b/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
index 90c1aa2894..37226ee74b 100644
--- a/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
+++ b/src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs
@@ -41,14 +41,14 @@ namespace Avalonia.Markup.Data
             };
 
         /// <summary>
-        /// An ordered collection of value handlers that can be used to customize the handling
-        /// of certain values.
+        /// An ordered collection of stream plugins that can be used to customize the behavior
+        /// of the '^' stream binding operator.
         /// </summary>
-        public static readonly IList<IValuePlugin> ValueHandlers =
-            new List<IValuePlugin>
+        public static readonly IList<IStreamPlugin> StreamHandlers =
+            new List<IStreamPlugin>
             {
-                new TaskValuePlugin(),
-                new ObservableValuePlugin(),
+                new TaskStreamPlugin(),
+                new ObservableStreamPlugin(),
             };
 
         private static readonly object UninitializedValue = new object();
diff --git a/src/Markup/Avalonia.Markup/Data/Plugins/IValuePlugin.cs b/src/Markup/Avalonia.Markup/Data/Plugins/IStreamPlugin.cs
similarity index 89%
rename from src/Markup/Avalonia.Markup/Data/Plugins/IValuePlugin.cs
rename to src/Markup/Avalonia.Markup/Data/Plugins/IStreamPlugin.cs
index fb285c6d73..efb2e2d93a 100644
--- a/src/Markup/Avalonia.Markup/Data/Plugins/IValuePlugin.cs
+++ b/src/Markup/Avalonia.Markup/Data/Plugins/IStreamPlugin.cs
@@ -6,9 +6,9 @@ using System;
 namespace Avalonia.Markup.Data.Plugins
 {
     /// <summary>
-    /// Defines how values are observed by an <see cref="ExpressionObserver"/>.
+    /// Defines a plugin that handles the '^' stream binding operator.
     /// </summary>
-    public interface IValuePlugin
+    public interface IStreamPlugin
     {
         /// <summary>
         /// Checks whether this plugin handles the specified value.
diff --git a/src/Markup/Avalonia.Markup/Data/Plugins/ObservableValuePlugin.cs b/src/Markup/Avalonia.Markup/Data/Plugins/ObservableStreamPlugin.cs
similarity index 62%
rename from src/Markup/Avalonia.Markup/Data/Plugins/ObservableValuePlugin.cs
rename to src/Markup/Avalonia.Markup/Data/Plugins/ObservableStreamPlugin.cs
index a406fc55b9..a1da42d28f 100644
--- a/src/Markup/Avalonia.Markup/Data/Plugins/ObservableValuePlugin.cs
+++ b/src/Markup/Avalonia.Markup/Data/Plugins/ObservableStreamPlugin.cs
@@ -2,32 +2,20 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Reactive.Linq;
-using System.Reactive.Subjects;
-using System.Reflection;
-using System.Threading.Tasks;
-using System.Windows.Input;
-using Avalonia.Data;
 
 namespace Avalonia.Markup.Data.Plugins
 {
     /// <summary>
-    /// Handles binding to <see cref="IObservable{T}"/>s in an <see cref="ExpressionObserver"/>.
+    /// Handles binding to <see cref="IObservable{T}"/>s for the '^' stream binding operator.
     /// </summary>
-    public class ObservableValuePlugin : IValuePlugin
+    public class ObservableStreamPlugin : IStreamPlugin
     {
         /// <summary>
         /// Checks whether this plugin handles the specified value.
         /// </summary>
         /// <param name="reference">A weak reference to the value.</param>
         /// <returns>True if the plugin can handle the value; otherwise false.</returns>
-        public virtual bool Match(WeakReference reference)
-        {
-            var target = reference.Target;
-
-            // ReactiveCommand is an IObservable but we want to bind to it, not its value.
-            return target is IObservable<object> && !(target is ICommand);
-        }
+        public virtual bool Match(WeakReference reference) => reference.Target is IObservable<object>;
 
         /// <summary>
         /// Starts producing output based on the specified value.
diff --git a/src/Markup/Avalonia.Markup/Data/Plugins/TaskValuePlugin.cs b/src/Markup/Avalonia.Markup/Data/Plugins/TaskStreamPlugin.cs
similarity index 95%
rename from src/Markup/Avalonia.Markup/Data/Plugins/TaskValuePlugin.cs
rename to src/Markup/Avalonia.Markup/Data/Plugins/TaskStreamPlugin.cs
index b6fda67503..d2c8c1b064 100644
--- a/src/Markup/Avalonia.Markup/Data/Plugins/TaskValuePlugin.cs
+++ b/src/Markup/Avalonia.Markup/Data/Plugins/TaskStreamPlugin.cs
@@ -12,9 +12,9 @@ using Avalonia.Data;
 namespace Avalonia.Markup.Data.Plugins
 {
     /// <summary>
-    /// Handles binding to <see cref="Task"/>s in an <see cref="ExpressionObserver"/>.
+    /// Handles binding to <see cref="Task"/>s for the '^' stream binding operator.
     /// </summary>
-    public class TaskValuePlugin : IValuePlugin
+    public class TaskStreamPlugin : IStreamPlugin
     {
         /// <summary>
         /// Checks whether this plugin handles the specified value.
diff --git a/src/Markup/Avalonia.Markup/Data/StreamNode.cs b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
index 7a5cfe5009..ebcbfc9598 100644
--- a/src/Markup/Avalonia.Markup/Data/StreamNode.cs
+++ b/src/Markup/Avalonia.Markup/Data/StreamNode.cs
@@ -14,7 +14,7 @@ namespace Avalonia.Markup.Data
 
         protected override IObservable<object> StartListeningCore(WeakReference reference)
         {
-            foreach (var plugin in ExpressionObserver.ValueHandlers)
+            foreach (var plugin in ExpressionObserver.StreamHandlers)
             {
                 if (plugin.Match(reference))
                 {

From b0e0596500daa237db46a95355657bb3d75c874e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 11 Oct 2016 00:29:07 +0200
Subject: [PATCH 4/8] Updated OmniXAML to allow '^' in markup extension.

---
 src/Markup/Avalonia.Markup.Xaml/OmniXAML | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Markup/Avalonia.Markup.Xaml/OmniXAML b/src/Markup/Avalonia.Markup.Xaml/OmniXAML
index b122549406..544af79d21 160000
--- a/src/Markup/Avalonia.Markup.Xaml/OmniXAML
+++ b/src/Markup/Avalonia.Markup.Xaml/OmniXAML
@@ -1 +1 @@
-Subproject commit b122549406107170bbe6e67c0d6a1a4252beef77
+Subproject commit 544af79d218127b4174da4be19896c5ca78eaa5d

From 4c5c099ec31d8e848742d8eff0db41cd1819c58c Mon Sep 17 00:00:00 2001
From: Dan Walmsley <dan@walms.co.uk>
Date: Tue, 11 Oct 2016 21:43:52 +0100
Subject: [PATCH 5/8] Dont focus menu then alt key down event is followed

by another keydown event.
---
 src/Avalonia.Input/AccessKeyHandler.cs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/Avalonia.Input/AccessKeyHandler.cs b/src/Avalonia.Input/AccessKeyHandler.cs
index 7acbc109fc..b23cdf550d 100644
--- a/src/Avalonia.Input/AccessKeyHandler.cs
+++ b/src/Avalonia.Input/AccessKeyHandler.cs
@@ -43,6 +43,11 @@ namespace Avalonia.Input
         /// </summary>
         private bool _ignoreAltUp;
 
+        /// <summary>
+        /// Whether the AltKey is down.
+        /// </summary>
+        private bool _altIsDown;
+
         /// <summary>
         /// Gets or sets the window's main menu.
         /// </summary>
@@ -110,6 +115,8 @@ namespace Avalonia.Input
         {
             if (e.Key == Key.LeftAlt)
             {
+                _altIsDown = true;
+
                 if (MainMenu == null || !MainMenu.IsOpen)
                 {
                     // When Alt is pressed without a main menu, or with a closed main menu, show
@@ -126,6 +133,10 @@ namespace Avalonia.Input
                 // We always handle the Alt key.
                 e.Handled = true;
             }
+            else if(_altIsDown)
+            {
+                _ignoreAltUp = true;
+            }
         }
 
         /// <summary>
@@ -179,6 +190,8 @@ namespace Avalonia.Input
             switch (e.Key)
             {
                 case Key.LeftAlt:
+                    _altIsDown = false;
+
                     if (_ignoreAltUp)
                     {
                         _ignoreAltUp = false;

From 3fea4d34f18a72a51f7691a6be523b9a934b353f Mon Sep 17 00:00:00 2001
From: Dan Walmsley <dan@walms.co.uk>
Date: Tue, 11 Oct 2016 21:48:36 +0100
Subject: [PATCH 6/8] Restore focus of input element after alt-key has stolen
 focus.

---
 src/Avalonia.Input/AccessKeyHandler.cs | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Input/AccessKeyHandler.cs b/src/Avalonia.Input/AccessKeyHandler.cs
index b23cdf550d..7b88fe18f3 100644
--- a/src/Avalonia.Input/AccessKeyHandler.cs
+++ b/src/Avalonia.Input/AccessKeyHandler.cs
@@ -48,6 +48,11 @@ namespace Avalonia.Input
         /// </summary>
         private bool _altIsDown;
 
+        /// <summary>
+        /// Element to restore folowing AltKey taking focus.
+        /// </summary>
+        private IInputElement _restoreFocusElement;
+
         /// <summary>
         /// Gets or sets the window's main menu.
         /// </summary>
@@ -119,15 +124,21 @@ namespace Avalonia.Input
 
                 if (MainMenu == null || !MainMenu.IsOpen)
                 {
+                    // Save currently focused input element.
+                    _restoreFocusElement = FocusManager.Instance.Current;
+
                     // When Alt is pressed without a main menu, or with a closed main menu, show
                     // access key markers in the window (i.e. "_File").
-                    _owner.ShowAccessKeys = _showingAccessKeys = true;
+                    _owner.ShowAccessKeys = _showingAccessKeys = true;                    
                 }
                 else
                 {
                     // If the Alt key is pressed and the main menu is open, close the main menu.
                     CloseMenu();
                     _ignoreAltUp = true;
+                    
+                    _restoreFocusElement?.Focus();
+                    _restoreFocusElement = null;
                 }
 
                 // We always handle the Alt key.

From 980a04b38f959927bbf496dca442d9d9015d6ba1 Mon Sep 17 00:00:00 2001
From: Dan Walmsley <dan@walms.co.uk>
Date: Tue, 11 Oct 2016 21:53:34 +0100
Subject: [PATCH 7/8] Whitespace changes.

---
 src/Avalonia.Input/AccessKeyHandler.cs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Input/AccessKeyHandler.cs b/src/Avalonia.Input/AccessKeyHandler.cs
index 7b88fe18f3..06e8e8cea5 100644
--- a/src/Avalonia.Input/AccessKeyHandler.cs
+++ b/src/Avalonia.Input/AccessKeyHandler.cs
@@ -129,14 +129,14 @@ namespace Avalonia.Input
 
                     // When Alt is pressed without a main menu, or with a closed main menu, show
                     // access key markers in the window (i.e. "_File").
-                    _owner.ShowAccessKeys = _showingAccessKeys = true;                    
+                    _owner.ShowAccessKeys = _showingAccessKeys = true;
                 }
                 else
                 {
                     // If the Alt key is pressed and the main menu is open, close the main menu.
                     CloseMenu();
                     _ignoreAltUp = true;
-                    
+
                     _restoreFocusElement?.Focus();
                     _restoreFocusElement = null;
                 }
@@ -144,7 +144,7 @@ namespace Avalonia.Input
                 // We always handle the Alt key.
                 e.Handled = true;
             }
-            else if(_altIsDown)
+            else if (_altIsDown)
             {
                 _ignoreAltUp = true;
             }

From c33a7da97b64165b2e3c077b7a776ea00c6335e7 Mon Sep 17 00:00:00 2001
From: Dan Walmsley <dan@walms.co.uk>
Date: Wed, 12 Oct 2016 21:20:52 +0100
Subject: [PATCH 8/8] Added todo comment for focus scopes.

---
 src/Avalonia.Input/AccessKeyHandler.cs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Input/AccessKeyHandler.cs b/src/Avalonia.Input/AccessKeyHandler.cs
index 06e8e8cea5..7baa4103d7 100644
--- a/src/Avalonia.Input/AccessKeyHandler.cs
+++ b/src/Avalonia.Input/AccessKeyHandler.cs
@@ -124,8 +124,9 @@ namespace Avalonia.Input
 
                 if (MainMenu == null || !MainMenu.IsOpen)
                 {
+                    // TODO: Use FocusScopes to store the current element and restore it when context menu is closed.
                     // Save currently focused input element.
-                    _restoreFocusElement = FocusManager.Instance.Current;
+                    _restoreFocusElement = FocusManager.Instance.Current;                    
 
                     // When Alt is pressed without a main menu, or with a closed main menu, show
                     // access key markers in the window (i.e. "_File").