Update UI thread from portable class library

Question

I have an MVVM Cross application running on Windows Phone 8 which I recently ported across to using Portable Class Libraries.

The view models are within the portable class library and one of them exposes a property which enables and disables a PerformanceProgressBar from the Silverlight for WP toolkit through data binding.

When the user presses a button a RelayCommand kicks off a background process which sets the property to true which should enable the progress bar and does the background processing.

Before I ported it to a PCL I was able to invoke the change from the UI thread to ensure the progress bar got enabled, but the Dispatcher object isn't available in a PCL. How can I work around this?

Thanks

Dan

Answer 1

If you don't have access to the Dispatcher, you can just pass a delegate of the BeginInvoke method to your class:

public class YourViewModel
{
    public YourViewModel(Action<Action> beginInvoke)
    {
        this.BeginInvoke = beginInvoke;
    }

    protected Action<Action> BeginInvoke { get; private set; }

    private void SomeMethod()
    {
        this.BeginInvoke(() => DoSomething());
    }
}

Then to instanciate it (from a class that has access to the dispatcher):

var dispatcherDelegate = action => Dispatcher.BeginInvoke(action);

var viewModel = new YourViewModel(dispatcherDelegate);

---

Or you can also create a wrapper around your dispatcher.

First, define a IDispatcher interface in your portable class library:

public interface IDispatcher
{
    void BeginInvoke(Action action);
}

Then, in the project who has access to the dispatcher, implement the interface:

public class DispatcherWrapper : IDispatcher
{
    public DispatcherWrapper(Dispatcher dispatcher)
    {
        this.Dispatcher = dispatcher;
    }

    protected Dispatcher Dispatcher { get; private set; }

    public void BeginInvoke(Action action)
    {
        this.Dispatcher.BeginInvoke(action);
    }
}

Then you can just pass this object as a IDispatcher instance to your portable class library.