事件

一.事件概述

事件具有以下特点：

• 发行者确定何时引发事件，订户确定执行何种操作来响应该事件。

• 一个事件可以有多个订户。一个订户可处理来自多个发行者的多个事件。

• 没有订户的事件永远不会被调用。

• 事件通常用于通知用户操作（如：图形用户界面中的按钮单击或菜单选择操作）。

• 如果一个事件有多个订户，当引发该事件时，会同步调用多个事件处理程序。要异步调用事件，请参见使用异步方式调用同步方法。

• 可以利用事件同步线程。

• 在 .NET Framework 类库中，事件是基于 EventHandler 委托和 EventArgs 基类的。

二. 使用.

     1.定义委托. (也可以基于系统的委托,这样就可以不用定义啦.)

     2.定义一个事件变量(基于定义的委托.)    //event 委托类型  变量名称.

     3.发布

     4.订阅

namespace ConsoleApplication1
{
    using System;
    using System.Collections.Generic;

    // Define a class to hold custom event info
    public class CustomEventArgs : EventArgs
    {
        public CustomEventArgs(string s)
        {
            message = s;
        }
        private string message;

        public string Message
        {
            get { return message; }
            set { message = value; }
        }
    }

    // Class that publishes an event
    class Publisher
    {

        // Declare the event using EventHandler<T>
        public event EventHandler<CustomEventArgs> RaiseCustomEvent;   //2.定义一个事件变量.

        public void DoSomething()
        {
            // Write some code that does something useful here
            // then raise the event. You can also raise an event
            // before you execute a block of code.
            OnRaiseCustomEvent(new CustomEventArgs("Did something"));

        }

        // Wrap event invocations inside a protected virtual method
        // to allow derived classes to override the event invocation behavior
        protected virtual void OnRaiseCustomEvent(CustomEventArgs e)
        {
            // Make a temporary copy of the event to avoid possibility of
            // a race condition if the last subscriber unsubscribes
            // immediately after the null check and before the event is raised.
            EventHandler<CustomEventArgs> handler = RaiseCustomEvent;

            // Event will be null if there are no subscribers
            if (handler != null)
            {
                // Format the string to send inside the CustomEventArgs parameter
                e.Message += String.Format(" at {0}", DateTime.Now.ToString());

                // Use the () operator to raise the event.
                handler(this, e);                                          //3.发布
            }
        }
    }

    //Class that subscribes to an event
    class Subscriber
    {
        private string id;
        public Subscriber(string ID)
        {
            id = ID;
            // Subscribe to the event using C# 2.0 syntax
            //pub.RaiseCustomEvent += HandleCustomEvent;
        }

        // Define what actions to take when the event is raised.
        public void HandleCustomEvent(object sender, CustomEventArgs e)
        {
            Console.WriteLine(id + " received this message: {0}", e.Message);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Publisher pub = new Publisher();
            Subscriber sub1 = new Subscriber("sub1");
            Subscriber sub2 = new Subscriber("sub2");

            pub.RaiseCustomEvent += sub1.HandleCustomEvent;   //4.订阅
            pub.RaiseCustomEvent += sub2.HandleCustomEvent;

            // Call the method that raises the event.
            pub.DoSomething();

            // Keep the console window open
            Console.WriteLine("Press Enter to close this window.");
            Console.ReadLine();

        }
    }

}