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Introducing the Model Thread View Thread Pattern

, 1 May 2010 BSD
Reduce threading code, and increase UI responsiveness with a new pattern extending MVVM.
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//===================================================================================
// 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.Linq;
using System.Windows.Threading;
using Microsoft.Practices.Composite.Events;

namespace Microsoft.Practices.Composite.Presentation.Events
{
    /// <summary>
    /// Defines a class that manages publication and subscription to events.
    /// </summary>
    /// <typeparam name="TPayload">The type of message that will be passed to the subscribers.</typeparam>
    public class CompositePresentationEvent<TPayload> : EventBase
    {
        private IDispatcherFacade uiDispatcher;

        /// <summary>
        /// Gets the <see cref="Dispatcher"/> that is bound to the UI elements.
        /// </summary>
        /// <value>The <see cref="Dispatcher"/> to use when subscribing using <see cref="ThreadOption.UIThread"/>.</value>
        private IDispatcherFacade UIDispatcher
        {
            get
            {
                if (uiDispatcher == null)
                {
                    this.uiDispatcher = new DefaultDispatcher();
                }

                return uiDispatcher;
            }
        }

        /// <summary>
        /// Subscribes a delegate to an event that will be published on the <see cref="ThreadOption.PublisherThread"/>.
        /// <see cref="CompositePresentationEvent{TPayload}"/> will maintain a <seealso cref="WeakReference"/> to the target of the supplied <paramref name="action"/> delegate.
        /// </summary>
        /// <param name="action">The delegate that gets executed when the event is published.</param>
        /// <returns>A <see cref="SubscriptionToken"/> that uniquely identifies the added subscription.</returns>
        /// <remarks>
        /// The CompositePresentationEvent collection is thread-safe.
        /// </remarks>
        public SubscriptionToken Subscribe(Action<TPayload> action)
        {
            return Subscribe(action, ThreadOption.PublisherThread);
        }

        /// <summary>
        /// Subscribes a delegate to an event.
        /// CompositePresentationEvent will maintain a <seealso cref="WeakReference"/> to the Target of the supplied <paramref name="action"/> delegate.
        /// </summary>
        /// <param name="action">The delegate that gets executed when the event is raised.</param>
        /// <param name="threadOption">Specifies on which thread to receive the delegate callback.</param>
        /// <returns>A <see cref="SubscriptionToken"/> that uniquely identifies the added subscription.</returns>
        /// <remarks>
        /// The CompositePresentationEvent collection is thread-safe.
        /// </remarks>
        public SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption)
        {
            return Subscribe(action, threadOption, false);
        }

        /// <summary>
        /// Subscribes a delegate to an event that will be published on the <see cref="ThreadOption.PublisherThread"/>.
        /// </summary>
        /// <param name="action">The delegate that gets executed when the event is published.</param>
        /// <param name="keepSubscriberReferenceAlive">When <see langword="true"/>, the <seealso cref="CompositePresentationEvent{TPayload}"/> keeps a reference to the subscriber so it does not get garbage collected.</param>
        /// <returns>A <see cref="SubscriptionToken"/> that uniquely identifies the added subscription.</returns>
        /// <remarks>
        /// If <paramref name="keepSubscriberReferenceAlive"/> is set to <see langword="false" />, <see cref="CompositePresentationEvent{TPayload}"/> will maintain a <seealso cref="WeakReference"/> to the Target of the supplied <paramref name="action"/> delegate.
        /// If not using a WeakReference (<paramref name="keepSubscriberReferenceAlive"/> is <see langword="true" />), the user must explicitly call Unsubscribe for the event when disposing the subscriber in order to avoid memory leaks or unexepcted behavior.
        /// 
        /// The CompositePresentationEvent collection is thread-safe.
        /// </remarks>
        public SubscriptionToken Subscribe(Action<TPayload> action, bool keepSubscriberReferenceAlive)
        {
            return Subscribe(action, ThreadOption.PublisherThread, keepSubscriberReferenceAlive);
        }

        /// <summary>
        /// Subscribes a delegate to an event.
        /// </summary>
        /// <param name="action">The delegate that gets executed when the event is published.</param>
        /// <param name="threadOption">Specifies on which thread to receive the delegate callback.</param>
        /// <param name="keepSubscriberReferenceAlive">When <see langword="true"/>, the <seealso cref="CompositePresentationEvent{TPayload}"/> keeps a reference to the subscriber so it does not get garbage collected.</param>
        /// <returns>A <see cref="SubscriptionToken"/> that uniquely identifies the added subscription.</returns>
        /// <remarks>
        /// If <paramref name="keepSubscriberReferenceAlive"/> is set to <see langword="false" />, <see cref="CompositePresentationEvent{TPayload}"/> will maintain a <seealso cref="WeakReference"/> to the Target of the supplied <paramref name="action"/> delegate.
        /// If not using a WeakReference (<paramref name="keepSubscriberReferenceAlive"/> is <see langword="true" />), the user must explicitly call Unsubscribe for the event when disposing the subscriber in order to avoid memory leaks or unexepcted behavior.
        /// 
        /// The CompositePresentationEvent collection is thread-safe.
        /// </remarks>
        public SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive)
        {
            return Subscribe(action, threadOption, keepSubscriberReferenceAlive, null);
        }

        /// <summary>
        /// Subscribes a delegate to an event.
        /// </summary>
        /// <param name="action">The delegate that gets executed when the event is published.</param>
        /// <param name="threadOption">Specifies on which thread to receive the delegate callback.</param>
        /// <param name="keepSubscriberReferenceAlive">When <see langword="true"/>, the <seealso cref="CompositePresentationEvent{TPayload}"/> keeps a reference to the subscriber so it does not get garbage collected.</param>
        /// <param name="filter">Filter to evaluate if the subscriber should receive the event.</param>
        /// <returns>A <see cref="SubscriptionToken"/> that uniquely identifies the added subscription.</returns>
        /// <remarks>
        /// If <paramref name="keepSubscriberReferenceAlive"/> is set to <see langword="false" />, <see cref="CompositePresentationEvent{TPayload}"/> will maintain a <seealso cref="WeakReference"/> to the Target of the supplied <paramref name="action"/> delegate.
        /// If not using a WeakReference (<paramref name="keepSubscriberReferenceAlive"/> is <see langword="true" />), the user must explicitly call Unsubscribe for the event when disposing the subscriber in order to avoid memory leaks or unexepcted behavior.
        /// 
        /// The CompositePresentationEvent collection is thread-safe.
        /// </remarks>
        public virtual SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive, Predicate<TPayload> filter)
        {
            IDelegateReference actionReference = new DelegateReference(action, keepSubscriberReferenceAlive);
            IDelegateReference filterReference;
            if (filter != null)
            {
                filterReference = new DelegateReference(filter, keepSubscriberReferenceAlive);
            }
            else
            {
                filterReference = new DelegateReference(new Predicate<TPayload>(delegate { return true; }), true);
            }
            EventSubscription<TPayload> subscription;
            switch (threadOption)
            {
                case ThreadOption.PublisherThread:
                    subscription = new EventSubscription<TPayload>(actionReference, filterReference);
                    break;
                case ThreadOption.BackgroundThread:
                    subscription = new BackgroundEventSubscription<TPayload>(actionReference, filterReference);
                    break;
                case ThreadOption.UIThread:
                    subscription = new DispatcherEventSubscription<TPayload>(actionReference, filterReference, UIDispatcher);
                    break;
                default:
                    subscription = new EventSubscription<TPayload>(actionReference, filterReference);
                    break;
            }


            return base.InternalSubscribe(subscription);
        }



        /// <summary>
        /// Publishes the <see cref="CompositePresentationEvent{TPayload}"/>.
        /// </summary>
        /// <param name="payload">Message to pass to the subscribers.</param>
        public virtual void Publish(TPayload payload)
        {
            base.InternalPublish(payload);
        }

        /// <summary>
        /// Removes the first subscriber matching <seealso cref="Action{TPayload}"/> from the subscribers' list.
        /// </summary>
        /// <param name="subscriber">The <see cref="Action{TPayload}"/> used when subscribing to the event.</param>
        public virtual void Unsubscribe(Action<TPayload> subscriber)
        {
            lock (Subscriptions)
            {
                IEventSubscription eventSubscription = Subscriptions.Cast<EventSubscription<TPayload>>().FirstOrDefault(evt => evt.Action == subscriber);
                if (eventSubscription != null)
                {
                    Subscriptions.Remove(eventSubscription);
                }
            }
        }

        /// <summary>
        /// Returns <see langword="true"/> if there is a subscriber matching <seealso cref="Action{TPayload}"/>.
        /// </summary>
        /// <param name="subscriber">The <see cref="Action{TPayload}"/> used when subscribing to the event.</param>
        /// <returns><see langword="true"/> if there is an <seealso cref="Action{TPayload}"/> that matches; otherwise <see langword="false"/>.</returns>
        public virtual bool Contains(Action<TPayload> subscriber)
        {
            IEventSubscription eventSubscription;
            lock (Subscriptions)
            {
                eventSubscription = Subscriptions.Cast<EventSubscription<TPayload>>().FirstOrDefault(evt => evt.Action == subscriber);
            }
            return eventSubscription != null;
        }

    }
}

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About the Author

Daniel Vaughan
President Outcoder
Switzerland Switzerland
Daniel Vaughan is a Microsoft MVP and co-founder of Outcoder, a Swiss software and consulting company dedicated to creating best-of-breed user experiences and leading-edge back-end solutions, using the Microsoft stack of technologies--in particular WPF, WinRT, Windows Phone, and also Xamarin.Forms.
 
Daniel is the author of Windows Phone 8 Unleashed and Windows Phone 7.5 Unleashed, both published by SAMS.
 
Daniel is the developer behind several acclaimed Windows Phone apps including Surfy, Intellicam, and Splashbox; and is the creator of a number of popular open-source projects including Calcium SDK, and Clog.
 
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