C# Cookbook

3.19. Adding a Notification Callback Using an Interface

Problem

You need a flexible, well-performing callback mechanism that does not make use of a delegate because you need more than one callback method. So the relationship between the caller and the callee is more complex than can easily be represented through the one method signature that you get with a delegate.

Solution

Use an interface to provide callback methods. The INotificationCallbacks interface contains two methods that will be used by a client as callback methods. The first method, FinishedProcessingSubGroup, is called when an amount specified in the amount parameter is reached. The second method, FinishedProcessingGroup, is called when all processing is complete:

public interface INotificationCallbacks
{
    void FinishedProcessingSubGroup(int amount);
    void FinishedProcessingGroup( );
}

The NotifyClient class implements the INotificationCallbacks interface. This class contains the implementation details of each of the callback methods:

public class NotifyClient : INotificationCallbacks
{
    public void FinishedProcessingSubGroup(int amount)
    {
        Console.WriteLine("Finished processing " + amount + " items");
    }

    public void FinishedProcessingGroup( )
    {
        Console.WriteLine("Processing complete");
    }
}

The Task class is the main class that implements its callbacks through the NotifyClient object. The Task class contains a field called notificationObj, which stores a reference to the NotifyClient object that is passed to it either through ...