Understanding the Silverlight Dispatcher

Question

I had a Invalid Cross Thread access issue, but a little research and I managed to fix it by using the Dispatcher.

Now in my app I have objects with lazy loading. I'd make an Async call using WCF and as usual I use the Dispatcher to update my objects DataContext, however it didn't work for this scenario. I did however find a solution here. Here's what I don't understand.

In my UserControl I have code to call an Toggle method on my object. The call to this method is within a Dispatcher like so.

Dispatcher.BeginInvoke( () => _CurrentPin.ToggleInfoPanel() );

As I mentioned before this was not enough to satisfy Silverlight. I had to make another Dispatcher call within my object. My object is NOT a UIElement, but a simple class that handles all its own loading/saving.

So the problem was fixed by calling

Deployment.Current.Dispatcher.BeginInvoke( () => dataContext.Detail = detail );

within my class.

Why did I have to call the Dispatcher twice to achieve this? Shouldn't a high-level call be enough? Is there a difference between the Deployment.Current.Dispatcher and the Dispatcher in a UIElement?

Answer 1

Ideally, store a single instance of Dispatcher that you can use elsewhere without having the thread check on it.

Calling any singleton .Current instance may in fact cause a cross-thread access check to be invoked. By storing it first, you can avoid this to actually get the shared instance.

I use a "SmartDispatcher" that uses a dispatcher when called off-thread, and just invokes otherwise. It solves this sort of issue.

Post: http://www.jeff.wilcox.name/2010/04/propertychangedbase-crossthread/

Code:

// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License (Ms-PL).
// Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
// All other rights reserved.

using System.ComponentModel;

namespace System.Windows.Threading
{
    /// <summary>
    /// A smart dispatcher system for routing actions to the user interface
    /// thread.
    /// </summary>
    public static class SmartDispatcher
    {
        /// <summary>
        /// A single Dispatcher instance to marshall actions to the user
        /// interface thread.
        /// </summary>
        private static Dispatcher _instance;

        /// <summary>
        /// Backing field for a value indicating whether this is a design-time
        /// environment.
        /// </summary>
        private static bool? _designer;

        /// <summary>
        /// Requires an instance and attempts to find a Dispatcher if one has
        /// not yet been set.
        /// </summary>
        private static void RequireInstance()
        {
            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }

            // Design-time is more of a no-op, won't be able to resolve the
            // dispatcher if it isn't already set in these situations.
            if (_designer == true)
            {
                return;
            }

            // Attempt to use the RootVisual of the plugin to retrieve a
            // dispatcher instance. This call will only succeed if the current
            // thread is the UI thread.
            try
            {
                _instance = Application.Current.RootVisual.Dispatcher;
            }
            catch (Exception e)
            {
                throw new InvalidOperationException("The first time SmartDispatcher is used must be from a user interface thread. Consider having the application call Initialize, with or without an instance.", e);
            }

            if (_instance == null)
            {
                throw new InvalidOperationException("Unable to find a suitable Dispatcher instance.");
            }
        }

        /// <summary>
        /// Initializes the SmartDispatcher system, attempting to use the
        /// RootVisual of the plugin to retrieve a Dispatcher instance.
        /// </summary>
        public static void Initialize()
        {
            if (_instance == null)
            {
                RequireInstance();
            }
        }

        /// <summary>
        /// Initializes the SmartDispatcher system with the dispatcher
        /// instance.
        /// </summary>
        /// <param name="dispatcher">The dispatcher instance.</param>
        public static void Initialize(Dispatcher dispatcher)
        {
            if (dispatcher == null)
            {
                throw new ArgumentNullException("dispatcher");
            }

            _instance = dispatcher;

            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }
        }

        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public static bool CheckAccess()
        {
            if (_instance == null)
            {
                RequireInstance();
            }

            return _instance.CheckAccess();
        }

        /// <summary>
        /// Executes the specified delegate asynchronously on the user interface
        /// thread. If the current thread is the user interface thread, the
        /// dispatcher if not used and the operation happens immediately.
        /// </summary>
        /// <param name="a">A delegate to a method that takes no arguments and 
        /// does not return a value, which is either pushed onto the Dispatcher 
        /// event queue or immediately run, depending on the current thread.</param>
        public static void BeginInvoke(Action a)
        {
            if (_instance == null)
            {
                RequireInstance();
            }

            // If the current thread is the user interface thread, skip the
            // dispatcher and directly invoke the Action.
            if (_instance.CheckAccess() || _designer == true)
            {
                a();
            }
            else
            {
                _instance.BeginInvoke(a);
            }
        }
    }
}

Answer 2

If you use the MVVM light toolkit you could use the DispatcherHelper class in the Galasoft.MvvmLight.Threading namespace in the Extras dll. It checks the access and uses a static property for the dispatcher, similar to the SmartDispatcher.

In your App.xaml.cs startup event call:

DispatcherHelper.Initialize();

Then anywhere you need to use a dispatcher use:

   DispatcherHelper.CheckBeginInvokeOnUI(() => // do stuff; );