Why are event delegates required ?

[paragpdoke]

Hello VB.NET Experts.
I am new to VB.NET (using VS2008 on WinXP 32bit).

I was going through tutorials and found the topic of Events and Delegates. However, I fail to understand why should this be required ? Instead of having to raise events, can't one just call the necessary procedure ?

Sorry for posing this basic question. Any replies or even links to existing posts are welcome. [Also not sure if this question strictly falls under VB.NET category.]

Thanks in advance,
Parag Doke

[tlhintoq]

Think of an event as a man with a very loud P.A. system. When something happens he announces it to the world. He doesn't really care who's listening or what they do with that information. When he yells out "I've Started", 200 people can react at the same time.

Think of calling each method as a man with a single line telephone and a long list of people to call along with a list of instructions for each person. Now when he makes an announcement he calls each person individually and tells them what they are responsible for doing. He calls DJ and says "You play music". Then he calls Logger and says "You make a note of the time". Then he calls Progress and says "Open that graph". Then he calls Chime to say "Play the happy noise". And while he is making is 200 individual phone calls, he is too busy to do anything else. And these 200 people on his list each have to wait until the previous one gets told.

If you raise an event you don't know or care which classes are listening. You don't know or care how they are going to respond. You distribute the burden to each class. You can add new reactions by writing one small class that subscribes to the event.

Events give you a way to make things happen across classes without the calling class having to know anything about the listening class. It helps you compartmentalize.

For example:
Your very long LookForDuplicates() method can simply raise an event of ProgressUpdate... It doesn't have to know or care what other classes are listening. One of them can be a ProgressBar class. Later you can add a PlayBackgroundMusic class without ever changing your known working primary method of LookForDuplicates()

Events also give you a way to do things Asynchronously. If you
MethodAlpha();
MethodBravo();
MethodCharlie();

MethodCharlie doesn't start until MethodBravo finishes. But if you raise events the program flow continues. Your very time consuming MethodBravo doesn't hold up MethodCharlie.

How about this example:
Picture that you have written a program that does lots of things, has lots of windows and lots of tools. One of them is a single floating pallet with a Stop sign graphic. When you press it, everything needs to stop.

You *could* give your Stop button knowledge of every tool and window and operation. When you add new features, you add code to the Stop button. It has to understand ALL other features and windows. So it becomes a mile long.

Or it could simply raise an event of EmergencyStopEvent, to which all the windows subscribe. It starts and stays about 10 lines long because it does one little thing: It yells out STOP!

Later when you write a new tool and 6 new windows you don't need to go back into your EmergencyStop pallet and update it (without breaking it). Each of the new tools and windows need only to subscribe to the EmergencyStop event.

[paragpdoke]

Hello tlhintoq.
Thank you so much for taking out the time to explain this to me in simple terms ! I got the concept beyond doubt. Many many thanks.
Regards,
Parag Doke

[PRR]

:) really good ans....

[r035198x]

More formally, the world we model in OOP consists of Observers and observables. Observers can themselves be observables too. When an observable changes state, observers should respond to that change in state by performing an action (possibly a nothing action).
The event handling model is a facility for registering observers to their observables in an efficient way.
Delegates facilitate event handling by providing one name for a group of methods(events) making it possible to programmatically change the method that is run and also plug new code into existing classes.