Event (computing) - Delegate Event Model

A very common and very "programmer-friendly" variant is the delegate event model, which is provided by the most popular graphic frameworks.

This model is based on three entities:

• a control, which is the event source,
• consumers, also called listeners, that receive the events from the source,
• interfaces (in the broader meaning of the term) that describe the protocol by which every event is to be communicated.

Furthermore, the model requires that

• every listener must implement the interface for the event it wants to listen to
• every listener must register with the source to declare its desire to listen to some particular event
• every time the source generates an event, it communicates it to the registered listeners, following the protocol of the interface.

C# uses events as special delegates that can only be fired by the class that declares it. This allows for better abstraction. Here's an example:

delegate void Notifier (string sender); class Model { public event Notifier notifyViews; public void Change { ... notifyViews("Model"); } } class View1 { public View1(Model m) { m.notifyViews += new Notifier(this.Update1); } void Update1(string sender) { Console.WriteLine(sender + " was changed during update"); } } class View2 { public View2(Model m) { m.notifyViews += new Notifier(this.Update2); } void Update2(string sender) { Console.WriteLine(sender + " was changed"); } } class Test { static void Main { Model m = new Model; new View1(m); new View2(m); m.Change; } }