Buttons are disabled when there is a null in the binding chain for Command.

Fixes #846.

samples/BindingTest/BindingTest.csproj

@@ -85,6 +85,7 @@
     <Compile Include="ViewModels\IndeiErrorViewModel.cs" />
     <Compile Include="ViewModels\ExceptionErrorViewModel.cs" />
     <Compile Include="ViewModels\MainWindowViewModel.cs" />
+    <Compile Include="ViewModels\NestedCommandViewModel.cs" />
     <Compile Include="ViewModels\TestItem.cs" />
   </ItemGroup>
   <ItemGroup>

samples/BindingTest/MainWindow.xaml

@@ -95,8 +95,9 @@
         <Button Content="Button" Command="{Binding StringValueCommand}" CommandParameter="Button"/>
         <ToggleButton Content="ToggleButton" IsChecked="{Binding BooleanFlag, Mode=OneWay}" Command="{Binding StringValueCommand}" CommandParameter="ToggleButton"/>
         <CheckBox Content="CheckBox" IsChecked="{Binding !BooleanFlag, Mode=OneWay}" Command="{Binding StringValueCommand}" CommandParameter="CheckBox"/>
-        <RadioButton Content="RadionButton" IsChecked="{Binding !!BooleanFlag, Mode=OneWay}" Command="{Binding StringValueCommand}" CommandParameter="RadioButton"/>
+        <RadioButton Content="Radio Button" IsChecked="{Binding !!BooleanFlag, Mode=OneWay}" Command="{Binding StringValueCommand}" CommandParameter="RadioButton"/>
         <TextBox Text="{Binding Path=StringValue}"/>
+        <Button Content="Nested View Model Button" Name="NestedTest" Command="{Binding NestedModel.Command}" />
       </StackPanel>
     </TabItem>
   </TabControl>

samples/BindingTest/ViewModels/MainWindowViewModel.cs

@@ -15,6 +15,7 @@ namespace BindingTest.ViewModels
         private string _stringValue = "Simple Binding";
         private bool _booleanFlag = false;
         private string _currentTime;
+        private NestedCommandViewModel _nested;
 
         public MainWindowViewModel()
         {
@@ -39,6 +40,7 @@ namespace BindingTest.ViewModels
             {
                 BooleanFlag = !BooleanFlag;
                 StringValue = param.ToString();
+                NestedModel = _nested ?? new NestedCommandViewModel();
             });
 
             Task.Run(() =>
@@ -90,5 +92,11 @@ namespace BindingTest.ViewModels
         public DataAnnotationsErrorViewModel DataAnnotationsValidation { get; } = new DataAnnotationsErrorViewModel();
         public ExceptionErrorViewModel ExceptionDataValidation { get; } = new ExceptionErrorViewModel();
         public IndeiErrorViewModel IndeiDataValidation { get; } = new IndeiErrorViewModel();
+
+        public NestedCommandViewModel NestedModel
+        {
+            get { return _nested; }
+            private set { this.RaiseAndSetIfChanged(ref _nested, value); }
+        }
     }
 }

samples/BindingTest/ViewModels/NestedCommandViewModel.cs

@@ -0,0 +1,20 @@
+using ReactiveUI;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+namespace BindingTest.ViewModels
+{
+    public class NestedCommandViewModel : ReactiveObject
+    {
+        public NestedCommandViewModel()
+        {
+            Command = ReactiveCommand.Create();
+        }
+
+        public ICommand Command { get; }
+    }
+}

src/Avalonia.Controls/Button.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using System.Windows.Input;
+using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Rendering;
@@ -41,8 +42,9 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Command"/> property.
         /// </summary>
-        public static readonly StyledProperty<ICommand> CommandProperty =
-            AvaloniaProperty.Register<Button, ICommand>(nameof(Command));
+        public static readonly DirectProperty<Button, ICommand> CommandProperty =
+            AvaloniaProperty.RegisterDirect<Button, ICommand>(nameof(Command),
+                button => button.Command, (button, command) => button.Command = command, enableDataValidation: true);
 
         /// <summary>
         /// Defines the <see cref="HotKey"/> property.
@@ -68,6 +70,8 @@ namespace Avalonia.Controls
         public static readonly RoutedEvent<RoutedEventArgs> ClickEvent =
             RoutedEvent.Register<Button, RoutedEventArgs>("Click", RoutingStrategies.Bubble);
 
+        private ICommand _command;
+
         /// <summary>
         /// Initializes static members of the <see cref="Button"/> class.
         /// </summary>
@@ -102,8 +106,8 @@ namespace Avalonia.Controls
         /// </summary>
         public ICommand Command
         {
-            get { return GetValue(CommandProperty); }
-            set { SetValue(CommandProperty, value); }
+            get { return _command; }
+            set { SetAndRaise(CommandProperty, ref _command, value); }
         }
 
         /// <summary>
@@ -240,6 +244,15 @@ namespace Avalonia.Controls
             }
         }
 
+        protected override void UpdateDataValidation(AvaloniaProperty property, BindingNotification status)
+        {
+            base.UpdateDataValidation(property, status);
+            if(property == CommandProperty)
+            {
+                IsEnabled = status?.ErrorType != BindingErrorType.Error;
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="Command"/> property changes.
         /// </summary>

src/Avalonia.Controls/MenuItem.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Command"/> property.
         /// </summary>
         public static readonly StyledProperty<ICommand> CommandProperty =
-            Button.CommandProperty.AddOwner<MenuItem>();
+            AvaloniaProperty.Register<MenuItem, ICommand>(nameof(Command));
 
         /// <summary>
         /// Defines the <see cref="HotKey"/> property.