Simplifying Asynchronous Calls in Silverlight using Action

This post explores a way to encapsulate web service calls from Silverlight in a way that is easy to use and understand.

As anyone who works with Silverlight knows, Silverlight forces web service calls to be asynchronous. There is a good reason for this, which I won't get into with this post. What I would like to do is demonstrate an approach that advocates a clean separation of the internals of the service from the application that is consuming it.

First, I am assuming you understand web services and WCF. I assume you've called services from Silverlight, and are aware of the nuances of configuring the endpoint, etc. I am also going to assume you are familiar with some frameworks such as Prism, MEF, Caliburn, etc that advocate clean separation of concerns and modular design.

Let's assume I have a service on my web page that is a calculator (oh, where did I get that idea?) It looks something like this:

[ServiceContract(Namespace = "http://www.wintellect.com/sample")]    
public interface ICalculatorService
{
    [OperationContract]
    long Multiply(int x, int y);
}

Simple, correct? The first thing I'm going to do is to create a similar interface on my Silverlight client. This interface will hide the details of how the service is connected and wired. I also want to make it easier to deal with the asynchronous nature of the calls. I've seen some pretty exotic implementations to try to make an asynchronous call look synchronous, and don't quite understand why we'd want to do it. Instead, let's just define a contract like this somewhere in the Silverlight application that is "seen" globally:

public interface ICalculator 
{
   void Multiply(int x, int y, Action<long> result); 
}

As you can see, it's a simple void method ... in fact, all methods we use to abstract the asynchronous calls will most likely be void. But the key is last parameter. This delegate allows me to specify how I want to deal with the result.

Now I create a project that is solely used to implement ICalculator and connect with the web service. First, if you haven't read my post about abstracting WCF service calls, be sure to read it: Abstracting WCF Service Calls in Silverlight. It explains how I use a base class to avoid having to change the ServiceReferences.ClientConfig file everytime I deploy an application, and also will give you the concept for the BaseService class you see below.

public class CalculatorService : BaseService<CalculatorServiceClient,ICalculatorService>, ICalculator
{
    public CalculatorService()
    {
        _GetClientChannel().MultiplyCompleted += new EventHandler<MultiplyCompletedEventArgs>(CalculatorService_MultiplyCompleted);
    }        

    #region Event handlers

    void CalculatorService_MultiplyCompleted(object sender, MultiplyCompletedEventArgs e)
    {
        Action<long> result = e.UserState as Action<long>;
        if (result != null)
        {
            result(e.Result); 
        }
    }

    #endregion 

    #region ICalculator Members

    public void Multiply(int x, int y, Action<long> result)
    {
        _GetClientChannel().MultiplyAsync(x, y, result);
    }

    #endregion
}

The base class preps the channel and credentials, etc for me. In the constructor of the implementation, I simply register for the completion event for the call. When a consumer of the service makes a call, the consumer provides a delegate to call back to when the service is done. This is passed in the user state, then cast back to an action and called once the service returns. If you wanted to have error handling, you'd just extend the action delegate to have an error-related parameter and check the e.Error when the call completed.

Now we can put it into use. In my bootstrapper, I'll bind the service to the contract like this:

Container.RegisterType<ICalculator,CalculatorService>(new ContainerControlledLifetimeManager());
            

Now I can throw a view model together that looks something like this:

public class CalculatorViewModel : INotifyPropertyChanged
{

    private bool _busy = false;

    public bool BusyState
    {
        get { return _busy; }
        set
        {
            if (!value.Equals(_busy))
            {
                _busy = value;
                OnPropertyChanged("BusyState");
                CalculateCommand.RaiseCanExecuteChanged();
            }
        }
    }

   public DelegateCommand<object> CalculateCommand { get; set; } 

    public CalculatorViewModel(ICalculator calculator)
    {
        Calculator = calculator;
        CalculateCommand = new DelegateCommand<object>(o => CommandCalculate(),
            o => !BusyState);
    }

    public ICalulator Calculator { get; set; }

    private int _x;

    public int X
    {
        get { return _x; }
        set
        {
            if (!value.Equals(_x))
            {
                _x = value;
                OnPropertyChanged("X"); 
            }
        }
    }

     private int _y;

    public int Y
    {
        get { return _y; }
        set
        {
            if (!value.Equals(_y))
            {
                _y = value;
                OnPropertyChanged("Y"); 
            }
        }
    }

    private long _answer;

    public long Answer
    {
        get { return _answer; }
        set
        {
            if (!value.Equals(_answer))
            {
                _answer = value;
                OnPropertyChanged("Answer"); 
            }
        }
    }

    public void CommandCalculate()
    {
        BusyState = true;
        Calculator.Multiply(_x, _y, answer => {
              BusyState = false;
              Answer = answer;
         }); 
    }

    private void OnPropertyChanged(string property)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(property));
        }
    }

    #endregion 

    #region INotifyPropertyChanged Members

   public event PropertyChangedEventHandler PropertyChanged;

    #endregion
}

Now I can simply data bind my XAML to the X, Y, and Answer properties, and bind a button to the Click command. The button will even disable itself while the client is waiting on the server to return the answer. As you can see, the implementation is very straightforward and I don't have to worry about how the call happens. In fact, using this type of abstraction, I could easily wire in a mock class to satisfy the service for testing like this... or maybe decide that a web service for multiplication is overkill and just satisy the contract locally:

public class MockCalculator : ICalculator 
{
   public void Multiply(int x, int y, Action<long> result) 
   {
      result(x*y);
   }
}

While the Action command is really a different way of expressing a delegate, it also makes the intent of the code clear and allows for a clean separation of mechanism from result when dealing with asynchronous calls.