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Conceptual Children: A powerful new concept in WPF

, 6 Apr 2008
This article describes a new approach by which an element can remove its visual and logical relationships to its children while maintaining a conceptual parental relationship with those children.
Prize winner in Competition "Best C# article of April 2008"
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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Controls;
using System.Windows;
using System.Collections.Specialized;
using System.Windows.Media;

namespace DrWPF.Windows.Controls
{
    /// <summary>
    /// This panel maintains a collection of conceptual children that are neither logical
    /// children nor visual children of the panel.  This allows those visuals to be connected 
    /// to other parts of the UI, if necessary, or even to remain disconnected. 
    /// </summary>
    public abstract class ConceptualPanel : Panel
    {
        public ConceptualPanel()
        {
            Loaded += OnLoaded;
        }

        void OnLoaded(object sender, RoutedEventArgs e)
        {
            Loaded -= OnLoaded;
            (Children as DisconnectedUIElementCollection).Initialize();
        }

        protected override sealed UIElementCollection CreateUIElementCollection(FrameworkElement logicalParent)
        {
            DisconnectedUIElementCollection children = new DisconnectedUIElementCollection(this);
            children.CollectionChanged += new NotifyCollectionChangedEventHandler(OnChildrenCollectionChanged);
            return children;
        }

        protected virtual void OnChildAdded(UIElement child)
        {
        }

        protected virtual void OnChildRemoved(UIElement child)
        {
        }

        /// <summary>
        /// For simplicity, this class will listen to change notifications on the DisconnectedUIElementCollection
        /// and provide them to descendants through the OnChildAdded and OnChildRemoved members.  
        /// </summary>
        private void OnChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    OnChildAdded(e.NewItems[0] as UIElement);
                    break;

                case NotifyCollectionChangedAction.Remove:
                    OnChildRemoved(e.OldItems[0] as UIElement);
                    break;
            }
        }
        
        protected override int VisualChildrenCount
        {
            get { return _visualChildren.Count; }
        }

        protected override Visual GetVisualChild(int index)
        {
            if (index < 0 || index >= _visualChildren.Count)
                throw new ArgumentOutOfRangeException();
            return _visualChildren[index];
        }

        protected override void OnVisualChildrenChanged(DependencyObject visualAdded, DependencyObject visualRemoved)
        {
            if (visualAdded is Visual)
            {
                _visualChildren.Add(visualAdded as Visual);
            }

            if (visualRemoved is Visual)
            {
                _visualChildren.Remove(visualRemoved as Visual);
            }

            base.OnVisualChildrenChanged(visualAdded, visualRemoved);
        }

        private readonly List<Visual> _visualChildren = new List<Visual>();
    }
}

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License

This article, along with any associated source code and files, is licensed under The BSD License

About the Author

Dr. WPF

United States United States
Dr. WPF is a WPF Disciple! Check out the doctor's blog and bio for more information.

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