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:
public 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:
using System; public class MoveEventArgs : EventArgs { public int newPosition; public bool cancel; public MoveEventArgs(int newPosition) { this.newPosition = newPosition; } }
Declaring and Firing an 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:
public 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 ...
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