FIL-4 Completed refactoring of Sections to ItemFilterCommentBlocks

Filtration/Utility/RoutedCommandHandler.cs

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+using System.Windows;
+using System.Windows.Input;
+
+namespace Filtration.Utility
+{
+    /// <summary>
+    ///  Allows associated a routed command with a non-routed command.  Used by
+    ///  <see cref="RoutedCommandHandlers"/>.
+    /// </summary>
+    public class RoutedCommandHandler : Freezable
+    {
+        public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
+            "Command",
+            typeof(ICommand),
+            typeof(RoutedCommandHandler),
+            new PropertyMetadata(default(ICommand)));
+
+        /// <summary> The command that should be executed when the RoutedCommand fires. </summary>
+        public ICommand Command
+        {
+            get => (ICommand)GetValue(CommandProperty);
+            set => SetValue(CommandProperty, value);
+        }
+
+        public static readonly DependencyProperty IsActiveProperty = DependencyProperty.Register(
+            "IsActive", typeof(bool), typeof(RoutedCommandHandler), new PropertyMetadata(default(bool)));
+
+        public bool IsActive
+        {
+            get { return (bool) GetValue(IsActiveProperty); }
+            set { SetValue(IsActiveProperty, value); }
+        }
+
+        /// <summary> The command that triggers <see cref="ICommand"/>. </summary>
+        public ICommand RoutedCommand { get; set; }
+
+        /// <inheritdoc />
+        protected override Freezable CreateInstanceCore()
+        {
+            return new RoutedCommandHandler();
+        }
+
+        /// <summary>
+        ///  Register this handler to respond to the registered RoutedCommand for the
+        ///  given element.
+        /// </summary>
+        /// <param name="owner"> The element for which we should register the command
+        ///  binding for the current routed command. </param>
+        internal void Register(FrameworkElement owner)
+        {
+            var binding = new CommandBinding(RoutedCommand, HandleExecuted, HandleCanExecute);
+            owner.CommandBindings.Add(binding);
+        }
+
+        /// <summary> Proxy to the current Command.CanExecute(object). </summary>
+        private void HandleCanExecute(object sender, CanExecuteRoutedEventArgs e)
+        {
+            if (!IsActive) return;
+
+            e.CanExecute = Command?.CanExecute(e.Parameter) != null;
+            e.Handled = true;
+        }
+
+        /// <summary> Proxy to the current Command.Execute(object). </summary>
+        private void HandleExecuted(object sender, ExecutedRoutedEventArgs e)
+        {
+            if (!IsActive) return;
+
+            Command?.Execute(e.Parameter);
+            e.Handled = true;
+        }
+    }
+}

Filtration/Utility/RoutedCommandHandlers.cs

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+using System.Collections;
+using System.Collections.Specialized;
+using System.Windows;
+
+namespace Filtration.Utility
+{
+    /// <summary>
+    ///  Holds a collection of <see cref="RoutedCommandHandler"/> that should be
+    ///  turned into CommandBindings.
+    /// </summary>
+    public class RoutedCommandHandlers : FreezableCollection<RoutedCommandHandler>
+    {
+        /// <summary>
+        ///  Hide this from WPF so that it's forced to go through
+        ///  <see cref="GetCommands"/> and we can auto-create the collection
+        ///  if it doesn't already exist.  This isn't strictly necessary but it makes
+        ///  the XAML much nicer.
+        /// </summary>
+        private static readonly DependencyProperty CommandsProperty = DependencyProperty.RegisterAttached(
+            "CommandsPrivate",
+            typeof(RoutedCommandHandlers),
+            typeof(RoutedCommandHandlers),
+            new PropertyMetadata(default(RoutedCommandHandlers)));
+
+        /// <summary>
+        ///  Gets the collection of RoutedCommandHandler for a given element, creating
+        ///  it if it doesn't already exist.
+        /// </summary>
+        public static RoutedCommandHandlers GetCommands(FrameworkElement element)
+        {
+            RoutedCommandHandlers handlers = (RoutedCommandHandlers)element.GetValue(CommandsProperty);
+            if (handlers == null)
+            {
+                handlers = new RoutedCommandHandlers(element);
+                element.SetValue(CommandsProperty, handlers);
+            }
+
+            return handlers;
+        }
+
+        private readonly FrameworkElement _owner;
+
+        /// <summary> Each collection is tied to a specific element. </summary>
+        /// <param name="owner"> The element for which this collection is created. </param>
+        public RoutedCommandHandlers(FrameworkElement owner)
+        {
+            _owner = owner;
+
+            // because we auto-create the collection, we don't know when items will be
+            // added.  So, we observe ourself for changes manually. 
+            var self = (INotifyCollectionChanged)this;
+            self.CollectionChanged += (sender, args) =>
+            {
+                // note this does not handle deletions, that's left as an exercise for the
+                // reader, but most of the time, that's not needed! 
+                ((RoutedCommandHandlers)sender).HandleAdditions(args.NewItems);
+            };
+        }
+
+        /// <summary> Invoked when new items are added to the collection. </summary>
+        /// <param name="newItems"> The new items that were added. </param>
+        private void HandleAdditions(IList newItems)
+        {
+            if (newItems == null)
+                return;
+
+            foreach (RoutedCommandHandler routedHandler in newItems)
+            {
+                routedHandler.Register(_owner);
+            }
+        }
+
+        /// <inheritdoc />
+        protected override Freezable CreateInstanceCore()
+        {
+            return new RoutedCommandHandlers(_owner);
+        }
+    }
+}