Application Development Using Visual Basic® and .NET

Events

Delegates are the foundation for a design pattern known as events. Conceptually, servers implement incoming interfaces, which are called by clients. In a diagram, such an interface may be shown with a small bubble (a notation used in COM). Sometimes a client may wish to receive notifications from a server when certain “events” occur. In such a case the server will specify an outgoing interface. The server defines the interface and the client implements it. In a diagram, such an interface may be shown with an arrow (again, a notation used in COM). Figure 6-4 illustrates a server with one incoming and one outgoing interface. In the case of the outgoing interface, the client will implement an incoming interface, which the server will call. ...