Events using delegates (c#) – An additional example
A former colleague wrote an excellent blog post about using Events with Delegates, in C#. He is one of the smartest people I know and very active in the community, with a deep understanding of a dozen technologies with never ending passion for technology. I haven't found a simpler example on the web yet. I wanted to add an additional real world example written in ASP.net – The example is of a News paper (Publisher) and Customers (Subscribers). Here it is:
1: /* Event delegation works on the Publisher <-> Subscriber Model. The Publisher exposes events that clients who are interested can subscribe
2: * to and when an event occurs, the subscribers are notified.
3: * If we were to consider a common real world example, we could think of implementing a Newspapaer and its Subscibers
4: * The code below implements the Newspaper as the Publisher and its Subscribers as customers who subscribe to be notified of breaking news.
5: */
6:
7: using System;
8: using System.Web;
9:
10: namespace Example4
11: {
12: public partial class EventsDemo : System.Web.UI.Page
13: {
14: protected void Page_Load(object sender, EventArgs e)
15: {
16: //We will first instantiate our Publisher - Which is our news paper - TimesOfIndia
17: TimesOfIndia TOI = new TimesOfIndia();
18:
19: //Add subscribers
20: TOICustomer Sudeepg = new TOICustomer(TOI, "Sudeepg");
21: TOICustomer Jack = new TOICustomer(TOI, "Jack");
22: TOICustomer George = new TOICustomer(TOI, "George");
23:
24: //Times now will get the breaking news for us
25: TOI.getBreakingNews();
26: }
27: }
28:
29: //This is our public delegate that will be hooked up by our subscribers.
30: public delegate void BreakingNewsEventHandler(object sender, BreakingNewsEventArgs e);
31:
32: public class TimesOfIndia // Times of india news paper is our publisher.
33: {
34: public event BreakingNewsEventHandler breakingNewsNotifyList;
35:
36: public void getBreakingNews()
37: {
38: string BreakingNews = "Annual Sale - Best deals on electronic devices!"; //Sample breaking news
39:
40: //Display the breaking news - NOTE: The Marquee tag only works in IE. Remove it for other browsers.
41: HttpContext.Current.Response.Write("<h1><marquee> Current Breaking News : " + BreakingNews.ToString() + "</marquee></h1><hr />");
42:
43: //Create and add event args
44: BreakingNewsEventArgs e = new BreakingNewsEventArgs(BreakingNews);
45:
46: //We need to invoke the notification, ONLY if there are subscribers to breaking news notification. That's why we check for NULL.
47: if (breakingNewsNotifyList != null)
48: breakingNewsNotifyList(this, e);
49:
50: }
51: }
52:
53: public class BreakingNewsEventArgs : EventArgs //Class used to pass data to event consumers when sending notification.
54: {
55: public BreakingNewsEventArgs(string BreakingNews)
56: {
57: this.m_BreakingNews = BreakingNews; //Save the breaking news into a property.
58: }
59:
60: string m_BreakingNews = "";
61: public string BreakingNews //Data we are passing to consumers is the breaking news itself (Stored as property).
62: {
63: get { return m_BreakingNews; }
64: }
65: }
66:
67:
68: public class TOICustomer //Instances of this class will be the subscribers of Times of India (Publisher).
69: {
70: private TimesOfIndia TOI; //Copy of the newsPaper
71: private string m_CustomerName = ""; //Store the customer name
72:
73: public TOICustomer(TimesOfIndia TOICopy, string customerName)
74: {
75: this.TOI = TOICopy;
76: m_CustomerName = customerName;
77: TOI.breakingNewsNotifyList += new BreakingNewsEventHandler(OnRecvBreakingNews); //Subscribe to breaking news notification
78: }
79:
80: //This method will be invoked upon a breaking news Notification.
81: private void OnRecvBreakingNews(object sender, BreakingNewsEventArgs e)
82: {
83: HttpContext.Current.Response.Write(m_CustomerName + " Acknowledging breaking news : " + e.BreakingNews.ToString() + "<br />");
84: }
85: }
86: }