Events
Event handling is essentially a process in which one object can notify other objects that an event has occurred. This process is largely encapsulated by multicast delegates, which have this ability built-in.
Defining a Delegate for an Event
The .NET Framework provides many event-handling delegates, but you can write your own. For example:
delegate void MoveEventHandler(object source, MoveEventArgs e);
By convention, the delegate’s first parameter denotes the source
of the event, and the delegate’s second parameter derives from System.EventArgs
and contains data about the event.
Storing Data for an Event with EventArgs
The EventArgs class may be derived from to include
information relevant to a particular event:
public class MoveEventArgs : EventArgs {
public int newPosition;
public bool cancel;
public MoveEventArgs(int newPosition) {
this.newPosition = newPosition;
}
}Declaring and Firing and Event
A class or struct can declare an event by applying the event modifier
to a delegate field. In this example, the slider class has a Position property
that fires a Move event whenever its Position changes:
class Slider {
int position;
public event MoveEventHandler Move;
public int Position {
get { return position; }
set {
if (position != value) { // if position changed
if (Move != null) { // if invocation list not empty
MoveEventArgs args = new MoveEventArgs(value);
Move(this, args); // fire event
if (args.cancel)
return;
}
position = value;
}
}
}
}The event keyword promotes encapsulation ...