The Event model in C# finds its roots in the event programming model that is popular in asynchronous programming. The basic foundation behind this programming model is the idea of "publisher and subscribers." In this model, you have publishers who will do some logic and publish an "event." Publishers will then send out their event only to subscribers who have subscribed to receive the specific event.
In C#, any object can publish a set of events to which other applications can subscribe. When the publishing class raises an event, all the subscribed applications are notified. The following figure shows this mechanism.
Conventions:
The following important conventions are used with events:
- Event Handlers in the .NET Framework return void and take two parameters.
- The first paramter is the source of the event; that is the publishing object.
- The second parameter is an object derived from EventArgs.
- Events are properties of the class publishing the event.
- The keyword event controls how the event property is accessed by the subscribing classes.
Simple Event:
Let's modify our logging example from above to use an event rather than a delegate:
using System;
using System.IO;
namespace Akadia.SimpleEvent
{
/* ========= Publisher of the Event ============== */
public class MyClass
{
// Define a delegate named LogHandler, which will encapsulate
// any method that takes a string as the parameter and returns no value
public delegate void LogHandler(string message);
// Define an Event based on the above Delegate
public event LogHandler Log;
// Instead of having the Process() function take a delegate
// as a parameter, we've declared a Log event. Call the Event,
// using the OnXXXX Method, where XXXX is the name of the Event.
public void Process()
{
OnLog("Process() begin");
OnLog("Process() end");
}
// By Default, create an OnXXXX Method, to call the Event
protected void OnLog(string message)
{
if (Log != null)
{
Log(message);
}
}
}
// The FileLogger class merely encapsulates the file I/O
public class FileLogger
{
FileStream fileStream;
StreamWriter streamWriter;
// Constructor
public FileLogger(string filename)
{
fileStream = new FileStream(filename, FileMode.Create);
streamWriter = new StreamWriter(fileStream);
}
// Member Function which is used in the Delegate
public void Logger(string s)
{
streamWriter.WriteLine(s);
}
public void Close()
{
streamWriter.Close();
fileStream.Close();
}
}
/* ========= Subscriber of the Event ============== */
// It's now easier and cleaner to merely add instances
// of the delegate to the event, instead of having to
// manage things ourselves
public class TestApplication
{
static void Logger(string s)
{
Console.WriteLine(s);
}
static void
{
FileLogger fl = new FileLogger("process.log");
MyClass myClass = new MyClass();
// Subscribe the Functions Logger and fl.Logger
myClass.Log += new MyClass.LogHandler(Logger);
myClass.Log += new MyClass.LogHandler(fl.Logger);
// The Event will now be triggered in the Process() Method
myClass.Process();
fl.Close();
}
}
}
Compile a test:
# csc SimpleEvent.cs
# SimpleEvent.exe
Process() begin
Process() end
# cat process.log
Process() begin
Process() end
The Second Change Event Example:
Suppose you want to create a Clock class that uses events to notify potential subscribers whenever the local time changes value by one second. Here is the complete, documented example:
using System;
using System.Threading;
namespace SecondChangeEvent
{
/* ======================= Event Publisher =============================== */
// Our subject -- it is this class that other classes
// will observe. This class publishes one event:
// SecondChange. The observers subscribe to that event.
public class Clock
{
// Private Fields holding the hour, minute and second
private int _hour;
private int _minute;
private int _second;
// The delegate named SecondChangeHandler, which will encapsulate
// any method that takes a clock object and a TimeInfoEventArgs
// object as the parameter and returns no value. It's the
// delegate the subscribers must implement.
public delegate void SecondChangeHandler (
object clock,
TimeInfoEventArgs timeInformation
);
// The event we publish
public event SecondChangeHandler SecondChange;
// The method which fires the Event
protected void OnSecondChange(
object clock,
TimeInfoEventArgs timeInformation
)
{
// Check if there are any Subscribers
if (SecondChange != null)
{
// Call the Event
SecondChange(clock,timeInformation);
}
}
// Set the clock running, it will raise an
// event for each new second
public void Run()
{
for(;;)
{
// Sleep 1 Second
Thread.Sleep(1000);
// Get the current time
System.DateTime dt = System.DateTime.Now;
// If the second has changed
// notify the subscribers
if (dt.Second != _second)
{
// Create the TimeInfoEventArgs object
// to pass to the subscribers
TimeInfoEventArgs timeInformation =
new TimeInfoEventArgs(
dt.Hour,dt.Minute,dt.Second);
// If anyone has subscribed, notify them
OnSecondChange (this,timeInformation);
}
// update the state
_second = dt.Second;
_minute = dt.Minute;
_hour = dt.Hour;
}
}
}
// The class to hold the information about the event
// in this case it will hold only information
// available in the clock class, but could hold
// additional state information
public class TimeInfoEventArgs : EventArgs
{
public TimeInfoEventArgs(int hour, int minute, int second)
{
this.hour = hour;
this.minute = minute;
this.second = second;
}
public readonly int hour;
public readonly int minute;
public readonly int second;
}
/* ======================= Event Subscribers =============================== */
// An observer. DisplayClock subscribes to the
// clock's events. The job of DisplayClock is
// to display the current time
public class DisplayClock
{
// Given a clock, subscribe to
// its SecondChangeHandler event
public void Subscribe(Clock theClock)
{
theClock.SecondChange +=
new Clock.SecondChangeHandler(TimeHasChanged);
}
// The method that implements the
// delegated functionality
public void TimeHasChanged(
object theClock, TimeInfoEventArgs ti)
{
Console.WriteLine("Current Time: {0}:{1}:{2}",
ti.hour.ToString(),
ti.minute.ToString(),
ti.second.ToString());
}
}
// A second subscriber whose job is to write to a file
public class LogClock
{
public void Subscribe(Clock theClock)
{
theClock.SecondChange +=
new Clock.SecondChangeHandler(WriteLogEntry);
}
// This method should write to a file
// we write to the console to see the effect
// this object keeps no state
public void WriteLogEntry(
object theClock, TimeInfoEventArgs ti)
{
Console.WriteLine("Logging to file: {0}:{1}:{2}",
ti.hour.ToString(),
ti.minute.ToString(),
ti.second.ToString());
}
}
/* ======================= Test Application =============================== */
// Test Application which implements the
// Clock Notifier - Subscriber Sample
public class Test
{
public static void
{
// Create a new clock
Clock theClock = new Clock();
// Create the display and tell it to
// subscribe to the clock just created
DisplayClock dc = new DisplayClock();
dc.Subscribe(theClock);
// Create a Log object and tell it
// to subscribe to the clock
LogClock lc = new LogClock();
lc.Subscribe(theClock);
// Get the clock started
theClock.Run();
}
}
}
What you study:
The Clock class from the last sample could simply print the time rather tahn raising an event, so why bother with the introduction of using delegates? The advantage of the publisg / subscribe idiom is that any number of classes can be notified when an event is raised. The subscribing classes do not need to know how the Clock works, and the Clock does not need to know what they are going to do in response to the event. Similarly a button can publish an Onclick event, and any number of unrelated objects can subscribe to that event, receiving notification when the button is clicked.
The publisher and the subscribers are decoupled by the delegate. This is highly desirable as it makes for more flexible and robust code. The clock can chnage how it detects time without breaking any of the subscribing classes. The subscribing classes can change how they respond to time changes without breaking the Clock. The two classes spin indepentdently of one another, which makes for code that is easier to maintain.
Regards,
Paresh Bhole
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