Added BindingError class.

src/Perspex.Base/Data/BindingError.cs

@@ -0,0 +1,32 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Perspex.Data
+{
+    /// <summary>
+    /// Represents a recoverable binding error.
+    /// </summary>
+    /// <remarks>
+    /// When produced by a binding source observable, informs the binding system that an error
+    /// occurred. It causes a binding error to be logged: the value of the bound property will not
+    /// change.
+    /// </remarks>
+    public class BindingError
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BindingError"/> class.
+        /// </summary>
+        /// <param name="exception">An exception describing the binding error.</param>
+        public BindingError(Exception exception)
+        {
+            Exception = exception;
+        }
+
+        /// <summary>
+        /// Gets the exception describing the binding error.
+        /// </summary>
+        public Exception Exception { get; }
+    }
+}

src/Perspex.Base/Perspex.Base.csproj

@@ -43,6 +43,7 @@
     <Compile Include="..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
+    <Compile Include="Data\BindingError.cs" />
     <Compile Include="Diagnostics\INotifyCollectionChangedDebug.cs" />
     <Compile Include="Data\AssignBindingAttribute.cs" />
     <Compile Include="Data\BindingOperations.cs" />

src/Perspex.Base/PerspexObject.cs

@@ -403,9 +403,9 @@ namespace Perspex
                 IDisposable subscription = null;
 
                 subscription = source
-                    .Select(x => TypeUtilities.CastOrDefault(x, property.PropertyType))
+                    .Select(x => CastOrDefault(x, property.PropertyType))
                     .Do(_ => { }, () => s_directBindings.Remove(subscription))
-                    .Subscribe(x => SetValue(property, x));
+                    .Subscribe(x => DirectBindingSet(property, x));
 
                 s_directBindings.Add(subscription);
 
@@ -587,6 +587,27 @@ namespace Perspex
             }
         }
 
+        /// <summary>
+        /// Tries to cast a value to a type, taking into account that the value may be a
+        /// <see cref="BindingError"/>.
+        /// </summary>
+        /// <param name="value">The value.</param>
+        /// <param name="type">The type.</param>
+        /// <returns>The cast value, or a <see cref="BindingError"/>.</returns>
+        private static object CastOrDefault(object value, Type type)
+        {
+            var error = value as BindingError;
+
+            if (error == null)
+            {
+                return TypeUtilities.CastOrDefault(value, type);
+            }
+            else
+            {
+                return error;
+            }
+        }
+
         /// <summary>
         /// Creates a <see cref="PriorityValue"/> for a <see cref="PerspexProperty"/>.
         /// </summary>
@@ -635,6 +656,33 @@ namespace Perspex
             return result;
         }
 
+        /// <summary>
+        /// Sets a property value for a direct property binding.
+        /// </summary>
+        /// <param name="property">The property.</param>
+        /// <param name="value">The value.</param>
+        /// <returns></returns>
+        private void DirectBindingSet(PerspexProperty property, object value)
+        {
+            var error = value as BindingError;
+
+            if (error == null)
+            {
+                SetValue(property, value);
+            }
+            else
+            {
+                Logger.Error(
+                    LogArea.Binding,
+                    this,
+                    "Error binding to {Target}.{PropertyName}: {Message}",
+                    property.Name,
+                    property.PropertyType,
+                    value,
+                    value.GetType());
+            }
+        }
+
         /// <summary>
         /// Converts an unset value to the default value for a direct property.
         /// </summary>

src/Perspex.Base/PriorityBindingEntry.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using Perspex.Data;
 
 namespace Perspex
 {
@@ -63,10 +64,12 @@ namespace Perspex
         /// <param name="binding">The binding.</param>
         /// <param name="changed">Called when the binding changes.</param>
         /// <param name="completed">Called when the binding completes.</param>
+        /// <param name="error">Called when a binding error occurs.</param>
         public void Start(
             IObservable<object> binding,
             Action<PriorityBindingEntry> changed,
-            Action<PriorityBindingEntry> completed)
+            Action<PriorityBindingEntry> completed,
+            Action<PriorityBindingEntry, BindingError> error)
         {
             Contract.Requires<ArgumentNullException>(binding != null);
             Contract.Requires<ArgumentNullException>(changed != null);
@@ -88,8 +91,17 @@ namespace Perspex
             _subscription = binding.Subscribe(
                 value =>
                 {
-                    Value = value;
+                    var bindingError = value as BindingError;
-                    changed(this);
+
+                    if (bindingError == null)
+                    {
+                        Value = value;
+                        changed(this);
+                    }
+                    else if (error != null)
+                    {
+                        error(this, bindingError);
+                    }
                 },
                 () => completed(this));
         }

src/Perspex.Base/PriorityLevel.cs

@@ -4,6 +4,8 @@
 using System;
 using System.Collections.Generic;
 using System.Reactive.Disposables;
+using Perspex.Data;
+using Perspex.Logging;
 
 namespace Perspex
 {
@@ -52,6 +54,11 @@ namespace Perspex
         /// </summary>
         private readonly Action<PriorityLevel> _changed;
 
+        /// <summary>
+        /// Method called when a binding error occurs.
+        /// </summary>
+        private readonly Action<PriorityLevel, BindingError> _error;
+
         /// <summary>
         /// The current direct value.
         /// </summary>
@@ -70,15 +77,18 @@ namespace Perspex
         /// <param name="priority">The priority.</param>
         /// <param name="mode">The precedence mode.</param>
         /// <param name="changed">A method to be called when the current value changes.</param>
+        /// <param name="error">A method to be called when a binding error occurs.</param>
         public PriorityLevel(
             int priority,
             LevelPrecedenceMode mode,
-            Action<PriorityLevel> changed)
+            Action<PriorityLevel> changed,
+            Action<PriorityLevel, BindingError> error)
         {
             Contract.Requires<ArgumentNullException>(changed != null);
 
             _mode = mode;
             _changed = changed;
+            _error = error;
             Priority = priority;
             Value = _directValue = PerspexProperty.UnsetValue;
             ActiveBindingIndex = -1;
@@ -135,7 +145,7 @@ namespace Perspex
             var entry = new PriorityBindingEntry(_nextIndex++);
             var node = Bindings.AddFirst(entry);
 
-            entry.Start(binding, Changed, Completed);
+            entry.Start(binding, Changed, Completed, Error);
 
             return Disposable.Create(() =>
             {
@@ -184,6 +194,16 @@ namespace Perspex
             }
         }
 
+        /// <summary>
+        /// Invoked when an entry in <see cref="Bindings"/> encounters a recoverable error.
+        /// </summary>
+        /// <param name="entry">The entry that completed.</param>
+        /// <param name="error">The error.</param>
+        private void Error(PriorityBindingEntry entry, BindingError error)
+        {
+            _error(this, error);
+        }
+
         /// <summary>
         /// Activates the first binding that has a value.
         /// </summary>

src/Perspex.Base/PriorityValue.cs

@@ -6,6 +6,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Subjects;
 using System.Text;
+using Perspex.Data;
 using Perspex.Logging;
 using Perspex.Utilities;
 
@@ -210,7 +211,7 @@ namespace Perspex
             if (!_levels.TryGetValue(priority, out result))
             {
                 var mode = (LevelPrecedenceMode)(priority % 2);
-                result = new PriorityLevel(priority, mode, ValueChanged);
+                result = new PriorityLevel(priority, mode, ValueChanged, Error);
                 _levels.Add(priority, result);
             }
 
@@ -242,7 +243,7 @@ namespace Perspex
             else
             {
                 Logger.Error(
-                    LogArea.Property, 
+                    LogArea.Binding, 
                     _owner,
                     "Binding produced invalid value for {$Property} ({$PropertyType}): {$Value} ({$ValueType})",
                     _name, 
@@ -279,5 +280,22 @@ namespace Perspex
                 }
             }
         }
+
+        /// <summary>
+        /// Called when a priority level encounters an error.
+        /// </summary>
+        /// <param name="level">The priority level of the changed entry.</param>
+        /// <param name="error">The binding error.</param>
+        private void Error(PriorityLevel level, BindingError error)
+        {
+            Logger.Log(
+                LogEventLevel.Error,
+                LogArea.Binding,
+                _owner,
+                "Error binding to {Target}.{PropertyName}: {Message}",
+                _owner,
+                _name,
+                error.Exception.Message);
+        }
     }
 }

tests/Perspex.Base.UnitTests/Perspex.Base.UnitTests.csproj

@@ -61,10 +61,6 @@
       <HintPath>..\..\packages\Rx-PlatformServices.2.2.5\lib\net45\System.Reactive.PlatformServices.dll</HintPath>
       <Private>True</Private>
     </Reference>
-    <Reference Include="System.Reactive.PlatformServices, Version=2.2.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Rx-PlatformServices.2.2.5\lib\net45\System.Reactive.PlatformServices.dll</HintPath>
-      <Private>True</Private>
-    </Reference>
     <Reference Include="xunit.abstractions, Version=2.0.0.0, Culture=neutral, PublicKeyToken=8d05b1bb7a6fdb6c">
       <HintPath>..\..\packages\xunit.abstractions.2.0.0\lib\net35\xunit.abstractions.dll</HintPath>
     </Reference>
@@ -124,6 +120,10 @@
       <Project>{B09B78D8-9B26-48B0-9149-D64A2F120F3F}</Project>
       <Name>Perspex.Base</Name>
     </ProjectReference>
+    <ProjectReference Include="..\Perspex.UnitTests\Perspex.UnitTests.csproj">
+      <Project>{88060192-33d5-4932-b0f9-8bd2763e857d}</Project>
+      <Name>Perspex.UnitTests</Name>
+    </ProjectReference>
   </ItemGroup>
   <Choose>
     <When Condition="'$(VisualStudioVersion)' == '10.0' And '$(IsCodedUITest)' == 'True'">

tests/Perspex.Base.UnitTests/PerspexObjectTests_Binding.cs

@@ -2,10 +2,14 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using Microsoft.Reactive.Testing;
 using Perspex.Data;
+using Perspex.Logging;
+using Perspex.UnitTests;
 using Xunit;
 
 namespace Perspex.Base.UnitTests
@@ -263,6 +267,35 @@ namespace Perspex.Base.UnitTests
             Assert.Equal("first", target2.GetValue(Class1.FooProperty));
         }
 
+        [Fact]
+        public void Bind_Logs_BindingError()
+        {
+            var target = new Class1();
+            var source = new Subject<object>();
+            var called = false;
+            var expectedMessageTemplate = "Error binding to {Target}.{PropertyName}: {Message}";
+
+            LogCallback checkLogMessage = (level, area, src, mt, pv) =>
+            {
+                if (level == LogEventLevel.Error &&
+                    area == LogArea.Binding &&
+                    mt == expectedMessageTemplate)
+                {
+                    called = true;
+                }
+            };
+
+            using (TestLogSink.Start(checkLogMessage))
+            {
+                target.Bind(Class1.QuxProperty, source);
+                source.OnNext(6.7);
+                source.OnNext(new BindingError(new InvalidOperationException("Foo")));
+
+                Assert.Equal(6.7, target.GetValue(Class1.QuxProperty));
+                Assert.True(called);
+            }
+        }
+
         /// <summary>
         /// Returns an observable that returns a single value but does not complete.
         /// </summary>

tests/Perspex.Base.UnitTests/PerspexObjectTests_Direct.cs

@@ -5,6 +5,8 @@ using System;
 using System.Collections.Generic;
 using System.Reactive.Subjects;
 using Perspex.Data;
+using Perspex.Logging;
+using Perspex.UnitTests;
 using Xunit;
 
 namespace Perspex.Base.UnitTests
@@ -394,6 +396,34 @@ namespace Perspex.Base.UnitTests
             Assert.True(raised);
         }
 
+        [Fact]
+        public void Binding_To_Direct_Property_Logs_BindingError()
+        {
+            var target = new Class1();
+            var source = new Subject<object>();
+            var called = false;
+            var expectedMessageTemplate = "Error binding to {Target}.{PropertyName}: {Message}";
+
+            LogCallback checkLogMessage = (level, area, src, mt, pv) =>
+            {
+                if (level == LogEventLevel.Error &&
+                    area == LogArea.Binding &&
+                    mt == expectedMessageTemplate)
+                {
+                    called = true;
+                }
+            };
+
+            using (TestLogSink.Start(checkLogMessage))
+            {
+                target.Bind(Class1.FooProperty, source);
+                source.OnNext("baz");
+                source.OnNext(new BindingError(new InvalidOperationException("Foo")));
+            }
+
+            Assert.True(called);
+        }
+
         private class Class1 : PerspexObject
         {
             public static readonly DirectProperty<Class1, string> FooProperty =

tests/Perspex.UnitTests/Perspex.UnitTests.csproj

@@ -63,6 +63,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="NotifyingBase.cs" />
+    <Compile Include="TestLogSink.cs" />
     <Compile Include="TestTemplatedRoot.cs" />
     <Compile Include="TestRoot.cs" />
     <Compile Include="TestServices.cs" />

tests/Perspex.UnitTests/TestLogSink.cs

@@ -0,0 +1,38 @@
+// Copyright (c) The Perspex 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.Reactive.Disposables;
+using Perspex.Logging;
+
+namespace Perspex.UnitTests
+{
+    public delegate void LogCallback(
+        LogEventLevel level,
+        string area,
+        object source,
+        string messageTemplate,
+        params object[] propertyValues);
+
+    public class TestLogSink : ILogSink
+    {
+        private LogCallback _callback;
+
+        public TestLogSink(LogCallback callback)
+        {
+            _callback = callback;
+        }
+
+        public static IDisposable Start(LogCallback callback)
+        {
+            var sink = new TestLogSink(callback);
+            Logger.Sink = sink;
+            return Disposable.Create(() => Logger.Sink = null);
+        }
+
+        public void Log(LogEventLevel level, string area, object source, string messageTemplate, params object[] propertyValues)
+        {
+            _callback(level, area, source, messageTemplate, propertyValues);
+        }
+    }
+}