Delegate & Event

A delegate is a reference type, like the other reference types you've seen in this book, but instead of referring to an object, a delegate refers to a method. This is called encapsulating the method. When you create the delegate, you specify a method signature and return type; you can encapsulate any matching method with that delegate.

namespace DelegateTest { // 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); // 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); // 这里是相当于一个发布者，如果自身改变的话，通知注册过的 // 用户 // 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) { // 这里向发布者注册自己，然后如果发布者改变的 // 话，就可以直接调用这个函数。 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) { 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(); } } }

Events enable a subscribers.

public class Publisher { // Declare the delegate (if using non-generic pattern). public delegate void SampleEventHandler(object sender, SampleEventArgs e); // Declare the event. public event SampleEventHandler SampleEvent; // Wrap the event in a protected virtual method // to enable derived classes to raise the event. protected virtual void RaiseSampleEvent() { // Raise the event by using the () operator. SampleEvent(this, new SampleEventArgs("Hello")); } }