Attaching Behaviors from the Expression Blend SDK using Styles (Part 2)

In the previous post in this series, I introduce the reader to a new attached property that gives the ability to attach a collection of behaviors to an item through a style. Since that post, I have discovered a bug in the original implementation that prevented databindings inside the behavior to work correctly.

Rather than do a walk through, here is the full, revised version of the StylizedBehaviors class:

#region
using System;
using System.Windows;
using System.Windows.Interactivity;
#endregion
 
namespace LivingAgile.Common.Presentation.WPF.Behaviors
{
    public class StylizedBehaviors
    {
        #region Fields (private)
        private static readonly DependencyProperty BehaviorIdProperty =
            DependencyProperty.RegisterAttached(
                @"BehaviorId", typeof ( Guid ), typeof ( StylizedBehaviors ), new UIPropertyMetadata( Guid.Empty ) );
        #endregion
 
        #region Fields (public)
        public static readonly DependencyProperty BehaviorsProperty = DependencyProperty.RegisterAttached(
            @"Behaviors",
            typeof ( StylizedBehaviorCollection ),
            typeof ( StylizedBehaviors ),
            new FrameworkPropertyMetadata( null, OnPropertyChanged ) );
        #endregion
 
        #region Static Methods (public)
        public static StylizedBehaviorCollection GetBehaviors ( DependencyObject uie )
        {
            return ( StylizedBehaviorCollection )uie.GetValue( BehaviorsProperty );
        }
 
        public static void SetBehaviors ( DependencyObject uie, StylizedBehaviorCollection value )
        {
            uie.SetValue( BehaviorsProperty, value );
        }
        #endregion
 
        #region Static Methods (private)
        private static Guid GetBehaviorId ( DependencyObject obj )
        {
            return ( Guid )obj.GetValue( BehaviorIdProperty );
        }
 
        private static int GetIndexOf ( BehaviorCollection itemBehaviors, Behavior behavior )
        {
            int index = -1;
 
            Guid behaviorId = GetBehaviorId( behavior );
 
            for ( int i = 0; i < itemBehaviors.Count; i++ )
            {
                Behavior currentBehavior = itemBehaviors[ i ];
 
                if ( currentBehavior == behavior )
                {
                    index = i;
                    break;
                }
 
                Guid cloneId = GetBehaviorId( currentBehavior );
 
                if ( cloneId == behaviorId )
                {
                    index = i;
                    break;
                }
            }
 
            return index;
        }
 
        private static void OnPropertyChanged ( DependencyObject dpo, DependencyPropertyChangedEventArgs e )
        {
            var uie = dpo as UIElement;
 
            if ( uie == null )
            {
                return;
            }
 
            BehaviorCollection itemBehaviors = Interaction.GetBehaviors( uie );
 
            var newBehaviors = e.NewValue as StylizedBehaviorCollection;
            var oldBehaviors = e.OldValue as StylizedBehaviorCollection;
 
            if ( newBehaviors == oldBehaviors )
            {
                return;
            }
 
            if ( oldBehaviors != null )
            {
                foreach ( var behavior in oldBehaviors )
                {
                    int index = GetIndexOf( itemBehaviors, behavior );
 
                    if ( index >= 0 )
                    {
                        itemBehaviors.RemoveAt( index );
                    }
                }
            }
 
            if ( newBehaviors != null )
            {
                foreach ( var behavior in newBehaviors )
                {
                    Guid behaviorId = GetBehaviorId( behavior );
 
                    if ( behaviorId == Guid.Empty )
                    {
                        behaviorId = Guid.NewGuid( );
                        SetBehaviorId( behavior, behaviorId );
                    }
 
                    int index = GetIndexOf( itemBehaviors, behavior );
 
                    if ( index < 0 )
                    {
                        var clone = ( Behavior )behavior.Clone( );
                        Guid cloneId = GetBehaviorId( clone );
 
                        if ( behaviorId != cloneId )
                        {
                        }
 
                        itemBehaviors.Add( clone );
                    }
                }
            }
        }
 
        private static void SetBehaviorId ( DependencyObject obj, Guid value )
        {
            obj.SetValue( BehaviorIdProperty, value );
        }
        #endregion
    }
}

The major change is that we now copy a cloned version of the behavior to the item that inherits the style. This seems to be necessary because the behavior gets the incorrect inheritance context since it really lives inside a freezable collection. Cloning the behavior seems to clear out the inheritance context and make things work correctly. To support that change, I had to add tracking ID's to the behaviors that are attached so they can be related during the remove operation.

Happy databinding on behaviors inside styles!