1. CIS 330 -.NET Applications1 Chapter 6 – Events

2. CIS 330 -.NET Applications2 Objectives Delegate-based Events.NET event support Practical guides for managing events

3. CIS 330 -.NET Applications3 Delegate-Based Events Source or Publisher: an object publishing the event Sink or Subscriber: any party interested in the event Firing: publishing an event Delegate: type-safe method reference

4. CIS 330 -.NET Applications4 Delegate-Based Events (cont) Public delegate void NumberChangedEventHandler(int number); Public class MyPublisher { public NumberChangedEventHandler NumberChanged; /* other methods and members */ } Public class MySubscriber public void OnNumberChanged(int number) { string message = “New value is “ + number; MessageBox.Show(message,”MySubscriber”); }} MyPublisher publisher = new MyPublisher(); MySubscriber subscriber1 = new MySubscriber(); //Adding subscriptions Publisher.NumberChanged += new NumberChangedEventHandler (subscriber1.OnNumberChanged); //Firing the event Publisher.NumberChanged(3); //Removing subscriptions Publisher.NumberChanged -= subscriber1.OnNumberChanged; delegate inference Must have Same signature

5. CIS 330 -.NET Applications5 Delegate-Based Events (cont) Potential problem with exposing the delegate as a public member Fix: define delegates as an event which only allows the publisher class to fire the event Public delegate void NumberChangedEventHandler(int number); Public class MyPublisher { public event NumberChangedEventHandler NumberChanged; public void FireEvent(int number) { NumberChanged(number) } /* other methods and members */ }

6. CIS 330 -.NET Applications6.NET Loosely Coupled Events Can’t subscribe to type of event – only to publisher objects Events can’t be filtered Subscriber must be able to “see” Publisher – i.e., both must be running All “connections” must be done programmically

7. CIS 330 -.NET Applications7 Working with.NET Events Defining Delegate Signatures –Target methods should have void return types –Signature should contain publisher’s identity –Use argument container like EventArgs.NET provides the EventHandler delegate: public delegate void EventHandler(object sender, EventArgs eventArgs); Publish events defensively – use try – catch blocks EventsHelper class – can be used to catch all exceptions while publishing to all delegates

8. CIS 330 -.NET Applications8 Working with.NET Events (cont) Event Accessors: encapsulates the event member variable Managing large numbers of events: use.NET’s EventHandlerList class –Linear list that stores key/value pairs (event obj, delegate instance) Recommend writing Sink Interfaces –Define interface containing set of events –Subscriber implements interface –Publisher provides two methods, Subscribe() and Unsubscribe(); each uses two parameters, the interface and a way to endicate which events to subscribe to (author shows a way to do this using flags)

9. CIS 330 -.NET Applications9 Summary Delegate-based Events.NET event support Practical guides for managing events