.Net Framework中的標準委托和事件_1

.Net Framework中的標準委托，已經定義在命名空間System中，

namespace System
{
    public delegate void EventHandler(object sender, EventArgs e);
}

.Net Framwork類庫中的所有事件均基於EventHandler委托。

其中EventArgs參數是可以自定義，必須繼承EventArgs類：

public class CustomEventArgs:EventArgs

發佈事件有三種方式：

1. 使用.net framework標準委托 

public event EventHandler RaiseCustomEvent;

2. 自定義EventArgs參數

public event CustomEventHandler RaiseCustomEvent;

3. 自定義EventArgs參數，泛型的表達方式

public event EventHandler<CustomEventArgs> RaiseCustomEvent;

如何：發佈符合.net framework準則的事件

首先分兩個類：

1. 發行者類（Publisher），發送Sends（引發Raise）事件的類，這個類做兩件事

• 發佈事件
• 編寫事件的觸發程式

     窗體（Form）和控制項（control）的類中都發佈了事件（Load，Click等），事件的觸發程式同樣能找到，如下：

protected virtual void OnClick(EventArgs e);

2. 訂閱者類（Subscriber），接收Receive（處理Handle）事件的類，這個類做兩件事

• 註冊事件
• 編寫事件的處理程式

     當你雙擊Form1中的一個按鈕Button1，會直接進入

        private void button1_Click(object sender, EventArgs e)
        {
        }

    其實就是在寫事件的處理程式；在Form1.Designer.cs中會自動註冊按鈕雙擊事件

this.button1.Click += new System.EventHandler(this.button1_Click);

下麵來看一個例子，是一個比較標準的方式添加自定義的事件：

namespace DotNetEvents
{
    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();

            Console.WriteLine("Press Enter to close this window.");
            Console.ReadKey();
        }
    }

    //Define a class to hold custom event info
    public class CustomEventArgs:EventArgs
    {
        private string message;
        public CustomEventArgs (string s)
        {
            message = s;
        }
        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 oveeride 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 raise
            if(RaiseCustomEvent !=null )
            {
                e.Message = String.Format(" at {0}", DateTime.Now.ToString());
                RaiseCustomEvent(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 suing C# 2.0 syntax
            pub.RaiseCustomEvent += HandleCustomEvent;
        }

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