Question

I have worker class with

public event EventHandler<EventArgs> DataModified;

which can be raised from other than UI thread (it is network client which fetches updates from server).
I have ModelView with

ObservableCollection<DataModel> DataItems;

to which View binds. My modelview must subscribe to ModifiedEvent, so it can reflect changes in DataItems. How do I update DataItems from callback event? I do not have access to UI Dispatcher from my model view (because view should not be aware of modelview). What is proper .NET 4.5 way for handling this?

Answer 1

You can create a service wich holds a CoreDispatcher and inject into your view model (or make it static)

public static class SmartDispatcher
{
    private static CoreDispatcher _instance;
    private static void RequireInstance()
    {
        try
        {
            _instance = Window.Current.CoreWindow.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.");
        }
    }

    public static void Initialize(CoreDispatcher dispatcher)  
    {
        if (dispatcher == null)
        {
            throw new ArgumentNullException("dispatcher");
        }

        _instance = dispatcher;
    }

    public static bool CheckAccess()
    {
        if (_instance == null)
        {
            RequireInstance();
        }
        return _instance.HasThreadAccess;
    }

    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 (CheckAccess())
        {
            a();
        }
        else
        {
            _instance.RunAsync(CoreDispatcherPriority.Normal, () => { a(); });
        }
    }
}

You should initialize SmartDispatcher in App.xaml.cs:

 var rootFrame = new Frame();
 SmartDispatcher.Initialize(rootFrame.Dispatcher);

 Window.Current.Content = rootFrame;
 Window.Current.Activate();

use it in this way:

SmartDispatcher.BeginInvoke(() => _collection.Add(item));

This class is based on Jeff Wilcox’s analog for windows phone 7.