Merge branch 'master' into binding-tweaks-and-docs

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" />

src/Avalonia.Base/Data/BindingChainNullException.cs → 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">
+        /// <param name="errorPoint">
-        /// The point in the expression at which the null was encountered.
+        /// 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.";
             }
         }
     }

src/Avalonia.Controls/TextBox.cs

@@ -495,10 +495,10 @@ namespace Avalonia.Controls
                         case 2:
                             if (!StringUtils.IsStartOfWord(text, index))
                             {
-                                SelectionStart = StringUtils.PreviousWord(text, index, false);
+                                SelectionStart = StringUtils.PreviousWord(text, index);
                             }
 
-                            SelectionEnd = StringUtils.NextWord(text, index, false);
+                            SelectionEnd = StringUtils.NextWord(text, index);
                             break;
                         case 3:
                             SelectionStart = 0;
@@ -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)
                 {
@@ -638,11 +638,11 @@ namespace Avalonia.Controls
             {
                 if (direction > 0)
                 {
-                    CaretIndex += StringUtils.NextWord(text, caretIndex, false) - caretIndex;
+                    CaretIndex += StringUtils.NextWord(text, caretIndex) - caretIndex;
                 }
                 else
                 {
-                    CaretIndex += StringUtils.PreviousWord(text, caretIndex, false) - caretIndex;
+                    CaretIndex += StringUtils.PreviousWord(text, caretIndex) - caretIndex;
                 }
             }
         }
@@ -718,6 +718,10 @@ namespace Avalonia.Controls
                         if (pos < text.Length)
                         {
                             --pos;
+                            if (pos > 0 && Text[pos - 1] == '\r' && Text[pos] == '\n')
+                            {
+                                --pos;
+                            }
                         }
 
                         break;
@@ -820,12 +824,6 @@ namespace Avalonia.Controls
             SelectionStart = CaretIndex;
             MoveHorizontal(1, modifiers);
             SelectionEnd = CaretIndex;
-
-            string selection = GetSelection();
-            if (selection != " " && selection.EndsWith(" "))
-            {
-                SelectionEnd = CaretIndex - 1;
-            }
         }
 
         UndoRedoState UndoRedoHelper<UndoRedoState>.IUndoRedoHost.UndoRedoState

src/Avalonia.Controls/Utils/StringUtils.cs

@@ -57,7 +57,7 @@ namespace Avalonia.Controls.Utils
             }
         }
 
-        public static int PreviousWord(string text, int cursor, bool gtkMode)
+        public static int PreviousWord(string text, int cursor)
         {
             int begin;
             int i;
@@ -81,60 +81,21 @@ namespace Avalonia.Controls.Utils
                 return (cr > 0) ? cr : 0;
             }
 
-            if (gtkMode)
+            CharClass cc = GetCharClass(text[cursor - 1]);
-            {
+            begin = lf + 1;
-                CharClass cc = GetCharClass(text[cursor - 1]);
+            i = cursor;
-                begin = lf + 1;
-                i = cursor;
-
-                // skip over the word, punctuation, or run of whitespace
-                while (i > begin && GetCharClass(text[i - 1]) == cc)
-                {
-                    i--;
-                }
 
-                // if the cursor was at whitespace, skip back a word too
+            // skip over the word, punctuation, or run of whitespace
-                if (cc == CharClass.CharClassWhitespace && i > begin)
+            while (i > begin && GetCharClass(text[i - 1]) == cc)
-                {
-                    cc = GetCharClass(text[i - 1]);
-                    while (i > begin && GetCharClass(text[i - 1]) == cc)
-                    {
-                        i--;
-                    }
-                }
-            }
-            else
             {
-                begin = lf + 1;
+                i--;
-                i = cursor;
+            }
-
-                if (cursor < text.Length)
-                {
-                    // skip to the beginning of this word
-                    while (i > begin && !char.IsWhiteSpace(text[i - 1]))
-                    {
-                        i--;
-                    }
-
-                    if (i < cursor && IsStartOfWord(text, i))
-                    {
-                        return i;
-                    }
-                }
-
-                // skip to the start of the lwsp
-                while (i > begin && char.IsWhiteSpace(text[i - 1]))
-                {
-                    i--;
-                }
-
-                if (i > begin)
-                {
-                    i--;
-                }
 
-                // skip to the beginning of the word
+            // if the cursor was at whitespace, skip back a word too
-                while (i > begin && !IsStartOfWord(text, i))
+            if (cc == CharClass.CharClassWhitespace && i > begin)
+            {
+                cc = GetCharClass(text[i - 1]);
+                while (i > begin && GetCharClass(text[i - 1]) == cc)
                 {
                     i--;
                 }
@@ -143,7 +104,7 @@ namespace Avalonia.Controls.Utils
             return i;
         }
 
-        public static int NextWord(string text, int cursor, bool gtkMode)
+        public static int NextWord(string text, int cursor)
         {
             int i, lf, cr;
 
@@ -169,50 +130,19 @@ namespace Avalonia.Controls.Utils
                 return cursor;
             }
 
-            if (gtkMode)
+            CharClass cc = GetCharClass(text[cursor]);
-            {
+            i = cursor;
-                CharClass cc = GetCharClass(text[cursor]);
-                i = cursor;
-
-                // skip over the word, punctuation, or run of whitespace
-                while (i < cr && GetCharClass(text[i]) == cc)
-                {
-                    i++;
-                }
 
-                // skip any whitespace after the word/punct
+            // skip over the word, punctuation, or run of whitespace
-                while (i < cr && char.IsWhiteSpace(text[i]))
+            while (i < cr && GetCharClass(text[i]) == cc)
-                {
-                    i++;
-                }
-            }
-            else
             {
-                i = cursor;
+                i++;
-
+            }
-                // skip any whitespace before the word
-                while (i < cr && char.IsWhiteSpace(text[i]))
-                {
-                    i++;
-                }
-
-                // skip to the end of the current word
-                while (i < cr && !char.IsWhiteSpace(text[i]))
-                {
-                    i++;
-                }
-
-                // skip any whitespace after the word
-                while (i < cr && char.IsWhiteSpace(text[i]))
-                {
-                    i++;
-                }
 
-                // find the start of the next word
+            // skip any whitespace after the word/punct
-                while (i < cr && !IsStartOfWord(text, i))
+            while (i < cr && char.IsWhiteSpace(text[i]))
-                {
+            {
-                    i++;
+                i++;
-                }
             }
 
             return i;

src/Avalonia.Input/AccessKeyHandler.cs

@@ -43,6 +43,16 @@ namespace Avalonia.Input
         /// </summary>
         private bool _ignoreAltUp;
 
+        /// <summary>
+        /// Whether the AltKey is down.
+        /// </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>
@@ -110,8 +120,14 @@ namespace Avalonia.Input
         {
             if (e.Key == Key.LeftAlt)
             {
+                _altIsDown = true;
+
                 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;                    
+
                     // 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;
@@ -121,11 +137,18 @@ namespace Avalonia.Input
                     // 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.
                 e.Handled = true;
             }
+            else if (_altIsDown)
+            {
+                _ignoreAltUp = true;
+            }
         }
 
         /// <summary>
@@ -179,6 +202,8 @@ namespace Avalonia.Input
             switch (e.Key)
             {
                 case Key.LeftAlt:
+                    _altIsDown = false;
+
                     if (_ignoreAltUp)
                     {
                         _ignoreAltUp = false;

src/Markup/Avalonia.Markup.Xaml/OmniXAML

@@ -1 +1 @@
-Subproject commit b122549406107170bbe6e67c0d6a1a4252beef77
+Subproject commit 544af79d218127b4174da4be19896c5ca78eaa5d

src/Markup/Avalonia.Markup/Avalonia.Markup.csproj

@@ -42,7 +42,8 @@
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
-    <Compile Include="Data\MarkupBindingChainNullException.cs" />
+    <Compile Include="Data\StreamNode.cs" />
+    <Compile Include="Data\MarkupBindingChainException.cs" />
     <Compile Include="Data\CommonPropertyNames.cs" />
     <Compile Include="Data\EmptyExpressionNode.cs" />
     <Compile Include="Data\ExpressionNodeBuilder.cs" />
@@ -63,9 +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\ObservableStreamPlugin.cs" />
-    <Compile Include="Data\Plugins\TaskValuePlugin.cs" />
+    <Compile Include="Data\Plugins\TaskStreamPlugin.cs" />
-    <Compile Include="Data\Plugins\IValuePlugin.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" />

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;
             });
@@ -92,7 +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);
         }
@@ -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);
-                    {
+                }
-                        Next.Target = new WeakReference(value);
+                else
-                    }
+                {
-                    else
+                    _observer.OnNext(value);
-                    {
-                        _observer.OnNext(value);
-                    }
                 }
             }
             else
@@ -144,44 +137,22 @@ namespace Avalonia.Markup.Data
                 }
                 else if (notification.HasValue)
                 {
-                    if (!HandleSpecialValue(notification.Value))
+                    if (Next != null)
                     {
-                        if (Next != null)
+                        Next.Target = new WeakReference(notification.Value);
-                        {
-                            Next.Target = new WeakReference(notification.Value);
-                        }
-                        else
-                        {
-                            _observer.OnNext(value);
-                        }
                     }
-                }
+                    else
-            }
-        }
-
-        private bool HandleSpecialValue(object value)
-        {
-            if (_valuePluginSubscription == null)
-            {
-                var reference = new WeakReference(value);
-
-                foreach (var plugin in ExpressionObserver.ValueHandlers)
-                {
-                    if (plugin.Match(reference))
                     {
-                        _valuePluginSubscription = plugin.Start(reference)?.Subscribe(TargetValueChanged);
+                        _observer.OnNext(value);
-                        return true;
                     }
                 }
             }
-
-            return false;
         }
 
         private BindingNotification TargetNullNotification()
         {
             return new BindingNotification(
-                new MarkupBindingChainNullException(),
+                new MarkupBindingChainException("Null value"),
                 BindingErrorType.Error,
                 AvaloniaProperty.UnsetValue);
         }

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
+        /// An ordered collection of stream plugins that can be used to customize the behavior
-        /// of certain values.
+        /// of the '^' stream binding operator.
         /// </summary>
-        public static readonly IList<IValuePlugin> ValueHandlers =
+        public static readonly IList<IStreamPlugin> StreamHandlers =
-            new List<IValuePlugin>
+            new List<IStreamPlugin>
             {
-                new TaskValuePlugin(),
+                new TaskStreamPlugin(),
-                new ObservableValuePlugin(),
+                new ObservableStreamPlugin(),
             };
 
         private static readonly object UninitializedValue = new object();
@@ -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)
                 {

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;
+        }
+    }
+}

src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs

@@ -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;
-        }
-    }
-}

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,

src/Markup/Avalonia.Markup/Data/Plugins/IValuePlugin.cs → 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.

src/Markup/Avalonia.Markup/Data/Plugins/ObservableValuePlugin.cs → 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)
+        public virtual bool Match(WeakReference reference) => reference.Target is IObservable<object>;
-        {
-            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);
-        }
 
         /// <summary>
         /// Starts producing output based on the specified value.

src/Markup/Avalonia.Markup/Data/Plugins/TaskValuePlugin.cs → 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.

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.StreamHandlers)
+            {
+                if (plugin.Match(reference))
+                {
+                    return plugin.Start(reference);
+                }
+            }
+
+            // TODO: Improve error.
+            return Observable.Return(new BindingNotification(
+                new MarkupBindingChainException("Stream operator applied to unsupported type", Description),
+                BindingErrorType.Error));
+        }
+    }
+}

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -175,24 +175,25 @@ namespace Avalonia.Controls.UnitTests
                 RaiseKeyEvent(textBox, Key.Delete, InputModifiers.Control);
                 Assert.Equal("First Second Third ", textBox.Text);
 
-                // (First Second| Third )
+                // (First Second |Third )
-                textBox.CaretIndex = 12;
+                textBox.CaretIndex = 13;
                 RaiseKeyEvent(textBox, Key.Delete, InputModifiers.Control);
                 Assert.Equal("First Second ", textBox.Text);
 
                 // (First Sec|ond )
                 textBox.CaretIndex = 9;
                 RaiseKeyEvent(textBox, Key.Delete, InputModifiers.Control);
-                Assert.Equal("First Sec ", textBox.Text);
+                Assert.Equal("First Sec", textBox.Text);
 
                 // (Fi[rs]t Sec )
                 textBox.SelectionStart = 2;
                 textBox.SelectionEnd = 4;
 
                 RaiseKeyEvent(textBox, Key.Delete, InputModifiers.Control);
-                Assert.Equal("Fit Sec ", textBox.Text);
+                Assert.Equal("Fit Sec", textBox.Text);
 
                 // (Fit Sec| )
+                textBox.Text += " ";
                 textBox.CaretIndex = 7;
                 RaiseKeyEvent(textBox, Key.Delete, InputModifiers.Control);
                 Assert.Equal("Fit Sec", textBox.Text);

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);

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));
@@ -92,6 +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>();

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"

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));