Created modular validation plugin system instead of being hard-coded to just INotifyDataErrorInfo. Current plugins are for Exception validation and INotifyDataErrorInfo. Currently not in use anywhere.

src/Markup/Perspex.Markup/Data/Plugins/ExceptionValidationCheckerPlugin.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Perspex.Data;
+
+namespace Perspex.Markup.Data.Plugins
+{
+    public class ExceptionValidationCheckerPlugin : IValidationCheckerPlugin
+    {
+        public ValidationCheckerBase Start(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback)
+        {
+            return new ExceptionValidationChecker(reference, name, accessor, callback);
+        }
+
+        private class ExceptionValidationChecker : ValidationCheckerBase
+        {
+            public ExceptionValidationChecker(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback)
+                : base(reference, name, accessor, callback)
+            {
+            }
+
+            public override bool SetValue(object value, BindingPriority priority)
+            {
+                try
+                {
+                    var success = base.SetValue(value, priority);
+                    SendValidationCallback(new ExceptionValidationStatus(null));
+                    return success;
+                }
+                catch (Exception ex)
+                {
+                    SendValidationCallback(new ExceptionValidationStatus(ex));
+                }
+                return false;
+            }
+        }
+
+        private class ExceptionValidationStatus : ValidationStatus
+        {
+            public ExceptionValidationStatus(Exception exception)
+            {
+                Exception = exception;
+            }
+
+            public Exception Exception { get; }
+
+            public override bool IsValid => Exception != null;
+        }
+    }
+}

src/Markup/Perspex.Markup/Data/Plugins/IValidationCheckerPlugin.cs

@@ -0,0 +1,14 @@
+using Perspex.Data;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Perspex.Markup.Data.Plugins
+{
+    public interface IValidationCheckerPlugin
+    {
+        ValidationCheckerBase Start(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback);
+    }
+}

src/Markup/Perspex.Markup/Data/Plugins/IndeiValidationCheckerPlugin.cs

@@ -0,0 +1,73 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Perspex.Data;
+using System.ComponentModel;
+using System.Collections;
+using Perspex.Utilities;
+
+namespace Perspex.Markup.Data.Plugins
+{
+    class IndeiValidationCheckerPlugin : IValidationCheckerPlugin
+    {
+        public ValidationCheckerBase Start(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback)
+        {
+            return new IndeiValidationChecker(reference, name, accessor, callback);
+        }
+
+        private class IndeiValidationChecker : ValidationCheckerBase, IWeakSubscriber<DataErrorsChangedEventArgs>
+        {
+            public IndeiValidationChecker(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback)
+                : base(reference, name, accessor, callback)
+            {
+                var target = reference.Target as INotifyDataErrorInfo;
+                if (target != null)
+                {
+                    if (target.HasErrors)
+                    {
+                        SendValidationCallback(new IndeiValidationStatus(target.GetErrors(name)));
+                    }
+                    WeakSubscriptionManager.Subscribe(
+                        target,
+                        nameof(target.ErrorsChanged),
+                        this);
+                }
+            }
+
+            public override void Dispose()
+            {
+                base.Dispose();
+                var target = _reference.Target as INotifyDataErrorInfo;
+                if (target != null)
+                {
+                    WeakSubscriptionManager.Unsubscribe(
+                        target,
+                        nameof(target.ErrorsChanged),
+                        this);
+                }
+            }
+
+            public void OnEvent(object sender, DataErrorsChangedEventArgs e)
+            {
+                if (e.PropertyName == _name || string.IsNullOrEmpty(e.PropertyName))
+                {
+                    var indei = _reference.Target as INotifyDataErrorInfo;
+                    SendValidationCallback(new IndeiValidationStatus(indei.GetErrors(e.PropertyName)));
+                }
+            }
+        }
+
+        private class IndeiValidationStatus : ValidationStatus
+        {
+            public IndeiValidationStatus(IEnumerable errors)
+            {
+                Errors = errors;
+            }
+            public override bool IsValid => !Errors.OfType<object>().Any();
+
+            public IEnumerable Errors { get; }
+        }
+    }
+}

src/Markup/Perspex.Markup/Data/Plugins/ValidationCheckerBase.cs

@@ -0,0 +1,34 @@
+using System;
+using Perspex.Data;
+
+namespace Perspex.Markup.Data.Plugins
+{
+    public abstract class ValidationCheckerBase : IPropertyAccessor
+    {
+        protected readonly WeakReference _reference;
+        protected readonly string _name;
+        private readonly IPropertyAccessor _accessor;
+        private readonly Action<ValidationStatus> _callback;
+
+        protected ValidationCheckerBase(WeakReference reference, string name, IPropertyAccessor accessor, Action<ValidationStatus> callback)
+        {
+            _reference = reference;
+            _name = name;
+            _accessor = accessor;
+            _callback = callback;
+        }
+
+        public Type PropertyType => _accessor.PropertyType;
+
+        public object Value => _accessor.Value;
+
+        public virtual void Dispose() => _accessor.Dispose();
+
+        public virtual bool SetValue(object value, BindingPriority priority) => _accessor.SetValue(value, priority);
+
+        protected void SendValidationCallback(ValidationStatus status)
+        {
+            _callback?.Invoke(status);
+        }
+    }
+}

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

@@ -46,6 +46,10 @@
     <Compile Include="Data\ExpressionParseException.cs" />
     <Compile Include="Data\ExpressionSubject.cs" />
     <Compile Include="ControlLocator.cs" />
+    <Compile Include="Data\Plugins\ExceptionValidationCheckerPlugin.cs" />
+    <Compile Include="Data\Plugins\IndeiValidationCheckerPlugin.cs" />
+    <Compile Include="Data\Plugins\ValidationCheckerBase.cs" />
+    <Compile Include="Data\Plugins\IValidationCheckerPlugin.cs" />
     <Compile Include="Data\Plugins\PerspexPropertyAccessorPlugin.cs" />
     <Compile Include="Data\Plugins\InpcPropertyAccessorPlugin.cs" />
     <Compile Include="Data\Plugins\IPropertyAccessor.cs" />

src/Perspex.Base/Data/ValidationStatus.cs

@@ -0,0 +1,20 @@
+// 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.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Perspex.Data
+{
+    /// <summary>
+    /// Contains information on if the current object passed validation.
+    /// Subclasses of this class contain additional information depending on the method of validation checking. 
+    /// </summary>
+    public abstract class ValidationStatus
+    {
+        public abstract bool IsValid { get; }
+    }
+}

src/Perspex.Base/Perspex.Base.csproj

@@ -44,6 +44,7 @@
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
     <Compile Include="Data\BindingError.cs" />
+    <Compile Include="Data\ValidationStatus.cs" />
     <Compile Include="Diagnostics\INotifyCollectionChangedDebug.cs" />
     <Compile Include="Data\AssignBindingAttribute.cs" />
     <Compile Include="Data\BindingOperations.cs" />

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

@@ -19,7 +19,11 @@ namespace Perspex.Markup.UnitTests.Data
             public int NoValidationTest
             {
                 get { return noValidationTest; }
-                set { noValidationTest = value; NotifyPropertyChanged(); }
+                set
+                {
+                    noValidationTest = value;
+                    NotifyPropertyChanged();
+                }
             }
 
             public bool HasErrors
@@ -79,7 +83,7 @@ namespace Perspex.Markup.UnitTests.Data
             var plugin = new InpcPropertyAccessorPlugin();
             var source = new InpcTest { NoValidationTest = 0 };
             var changeFired = false;
-            plugin.Start(new WeakReference(source), nameof(InpcTest.NoValidationTest), _ => changeFired = true, _ => { });
+            var accessor = plugin.Start(new WeakReference(source), nameof(InpcTest.NoValidationTest), _ => changeFired = true, _ => { });
             source.NoValidationTest = 1;
 
             Assert.True(changeFired);