Introducing the Model Thread View Thread Pattern

//===================================================================================
// Microsoft patterns & practices
// Composite Application Guidance for Windows Presentation Foundation and Silverlight
//===================================================================================
// Copyright (c) Microsoft Corporation.  All rights reserved.
// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY
// OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT
// LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
// FITNESS FOR A PARTICULAR PURPOSE.
//===================================================================================
// The example companies, organizations, products, domain names,
// e-mail addresses, logos, people, places, and events depicted
// herein are fictitious.  No association with any real company,
// organization, product, domain name, email address, logo, person,
// places, or events is intended or should be inferred.
//===================================================================================
using System;
using System.ComponentModel;
using System.Windows;
using Microsoft.Practices.Composite.Presentation.Properties;
using Microsoft.Practices.Composite.Regions;

namespace Microsoft.Practices.Composite.Presentation.Regions.Behaviors
{
    /// <summary>
    /// Subscribes to a static event from the <see cref="RegionManager"/> in order to register the target <see cref="IRegion"/>
    /// in a <see cref="IRegionManager"/> when one is available on the host control by walking up the tree and finding
    /// a control whose <see cref="RegionManager.RegionManagerProperty"/> property is not <see langword="null"/>.
    /// </summary>
    public class RegionManagerRegistrationBehavior : RegionBehavior, IHostAwareRegionBehavior
    {
        /// <summary>
        /// The key of this behavior.
        /// </summary>
        public static readonly string BehaviorKey = "RegionManagerRegistration";

        private WeakReference attachedRegionManagerWeakReference;
        private DependencyObject hostControl;

        /// <summary>
        /// Initializes a new instance of <see cref="RegionManagerRegistrationBehavior"/>.
        /// </summary>
        public RegionManagerRegistrationBehavior()
        {
            this.RegionManagerAccessor = new DefaultRegionManagerAccessor();
        }

        /// <summary>
        /// Provides an abstraction on top of the RegionManager static members.
        /// </summary>
        public IRegionManagerAccessor RegionManagerAccessor { get; set; }

        /// <summary>
        /// Gets or sets the <see cref="DependencyObject"/> that the <see cref="IRegion"/> is attached to.
        /// </summary>
        /// <value>A <see cref="DependencyObject"/> that the <see cref="IRegion"/> is attached to.
        /// This is usually a <see cref="FrameworkElement"/> that is part of the tree.</value>
        /// <exception cref="InvalidOperationException">When this member is set after the <see cref="IRegionBehavior.Attach"/> method has being called.</exception>
        public DependencyObject HostControl
        {
            get
            {
                return hostControl;
            }
            set
            {
                if (IsAttached)
                {
                    throw new InvalidOperationException(Resources.HostControlCannotBeSetAfterAttach);
                }
                this.hostControl = value;
            }
        }

        /// <summary>
        /// When the <see cref="IRegion"/> has a name assigned, the behavior will start monitoring the ancestor controls in the element tree
        /// to look for an <see cref="IRegionManager"/> where to register the region in.
        /// </summary>
        protected override void OnAttach()
        {
            if (string.IsNullOrEmpty(this.Region.Name))
            {
                this.Region.PropertyChanged += this.Region_PropertyChanged;
            }
            else
            {
                this.StartMonitoringRegionManager();
            }
        }

        private void Region_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == "Name" && !string.IsNullOrEmpty(this.Region.Name))
            {
                this.Region.PropertyChanged -= this.Region_PropertyChanged;
                this.StartMonitoringRegionManager();
            }
        }

        private void StartMonitoringRegionManager()
        {
            this.RegionManagerAccessor.UpdatingRegions += this.OnUpdatingRegions;
            this.TryRegisterRegion();
        }

        private void TryRegisterRegion()
        {
            DependencyObject targetElement = this.HostControl;
            if (targetElement.CheckAccess())
            {
                IRegionManager regionManager = this.FindRegionManager(targetElement);

                IRegionManager attachedRegionManager = this.GetAttachedRegionManager();

                if (regionManager != attachedRegionManager)
                {
                    if (attachedRegionManager != null)
                    {
                        this.attachedRegionManagerWeakReference = null;
                        attachedRegionManager.Regions.Remove(this.Region.Name);
                    }

                    if (regionManager != null)
                    {
                        this.attachedRegionManagerWeakReference = new WeakReference(regionManager);

                        regionManager.Regions.Add(this.Region);
                    }
                }
            }
        }

        /// <summary>
        /// This event handler gets called when a RegionManager is requering the instances of a region to be registered if they are not already.
        /// <remarks>Although this is a public method to support Weak Delegates in Silverlight, it should not be called by the user.</remarks>
        /// </summary>
        /// <param name="sender">The sender.</param>
        /// <param name="e">The arguments.</param>
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Security", "CA2109:ReviewVisibleEventHandlers", Justification = "This has to be public in order to work with weak references in partial trust or Silverlight environments.")]
        public void OnUpdatingRegions(object sender, EventArgs e)
        {
            this.TryRegisterRegion();
        }

        private IRegionManager FindRegionManager(DependencyObject dependencyObject)
        {
            var regionmanager = this.RegionManagerAccessor.GetRegionManager(dependencyObject);
            if (regionmanager != null)
            {
                return regionmanager;
            }

            DependencyObject parent = null;
#if SILVERLIGHT
            //parent = System.Windows.Media.VisualTreeHelper.GetParent(dependencyObject);
                    FrameworkElement frameworkElement = dependencyObject as FrameworkElement;
                    if (frameworkElement != null)
                    {
                        parent = frameworkElement.Parent;
                    }
#else
            parent = LogicalTreeHelper.GetParent(dependencyObject);
#endif
            if (parent != null)
            {
                return this.FindRegionManager(parent);
            }

            return null;
        }

        private IRegionManager GetAttachedRegionManager()
        {
            if (this.attachedRegionManagerWeakReference != null)
            {
                return this.attachedRegionManagerWeakReference.Target as IRegionManager;
            }

            return null;
        }
    }
}