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17: Delegates and Events
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using using using using using System; System.Collections.Generic; System.Linq; System.Text; System.Threading;
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namespace Example_17_2_ _ _ _Delegates_and_Events { // a 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 int hour; public int minute; public int second; public TimeInfoEventArgs(int hour, int minute, int second) { this.hour = hour; this.minute = minute; this.second = second; } } // The publisher: the class that other classes // will observe. This class publishes one delegate: // SecondChangeHandler. public class Clock { private int hour; private int minute; private int second; // the delegate the subscribers must implement public delegate void SecondChangeHandler(object clock, TimeInfoEventArgs timeInformation); // an instance of the delegate public SecondChangeHandler SecondChanged; // set the clock running // it will raise an event for each new second public void Run( ) { for (; ; ) { // sleep 100 milliseconds Thread.Sleep(100);
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// 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 subscriber TimeInfoEventArgs timeInformation = new TimeInfoEventArgs(dt.Hour, dt.Minute, dt.Second); // if anyone has subscribed, notify them if (SecondChanged != null) { SecondChanged(this, timeInformation); } } // update the state this.second = dt.Second; this.minute = dt.Minute; this.hour = dt.Hour; } } } // A subscriber: 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.SecondChanged += 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 LogCurrentTime { public void Subscribe(Clock theClock)
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17: Delegates and Events
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{ theClock.SecondChanged += 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( )); } } public class Tester { public void Run( ) { // 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 LogCurrentTime lct = new LogCurrentTime( ); lct.Subscribe(theClock); // Get the clock started theClock.Run( ); } } public class Program { public static void Main( ) { Tester t = new Tester( ); t.Run( ); } } }
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The output will look something like this, depending on what time it is when you run the program:
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Current Time: 14:53:56 Logging to file: 14:53:56
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Events |
Current Logging Current Logging Current Logging Current Logging
Time: 14:53:57 to file: 14:53:57 Time: 14:53:58 to file: 14:53:58 Time: 14:53:59 to file: 14:53:59 Time: 14:54:0 to file: 14:54:0
The net effect of this code is to create two classes, DisplayClock and LogCurrentTime, both of which subscribe to a third class s event (Clock.SecondChanged).
SecondChanged is a multicast delegate field, initially referring to nothing. In time, it refers to a single delegate, and then later to multiple delegates. When the observer classes wish to be notified, they create an instance of the delegate and then add these delegates to SecondChanged. For example, in DisplayClock.Subscribe( ), you see this line of code:
theClock.SecondChanged += new Clock.SecondChangeHandler(TimeHasChanged);
It turns out that the LogCurrentTime class also wants to be notified. In its Subscribe( ) method is very similar code:
public void Subscribe(Clock theClock) { theClock.SecondChanged += new Clock.SecondChangeHandler(WriteLogEntry); }
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