发布基于 EventHandler 模式的事件

一、随事件一起发送自定义数据且不使用泛型

1、如果随事件一起发送自定义数据，将自定义数据的类声明为对发布服务器和订阅者类均可见的范围。 然后添加所需成员以保留自定义事件数据。 在此示例中，将返回一个简单的字符串。

public class CustomEventArgs : EventArgs  
{  
    public CustomEventArgs(string s)  
    {  
        msg = s;  
    }  
    private string msg;  
    public string Message  
    {  
        get { return msg; }  
    }   
}

 2、声明发布类中的委托。 为以 EventHandler 结尾的委托命名。 第二个参数指定自定义 EventArgs 类型。

public delegate void CustomEventHandler(object sender, CustomEventArgs a);

 3、定义事件，将委托声明为类型。

public event CustomEventHandler RaiseCustomEvent;

 

二、如果不随事件发送自定义数据。

如果没有任何自定义 EventArgs 类，事件类型将为非泛型 EventHandler 委托。 你无需声明该委托，因为它已在创建 C# 项目时包括的 System 命名空间中声明。 将以下代码添加到发布服务器类。

public event EventHandler RaiseCustomEvent;

 三、随事件一起发送自定义数据且使用泛型。

则无需自定义委托。 而是在发布类中，将事件类型指定为 EventHandler<CustomEventArgs>，替换尖括号中自定义类的名称。

public event EventHandler<CustomEventArgs> RaiseCustomEvent;

 

示例

下例通过使用自定义 EventArgs 类和 EventHandler<TEventArgs> 作为事件类型来演示之前的步骤。

namespace DotNetEvents
{
    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;

        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 += $" at {DateTime.Now}";

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

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

        // Define what actions to take when the event is raised.
        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", pub);
            Subscriber sub2 = new Subscriber("sub2", pub);

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

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

        }
    }
}