[讀書筆記]基於事件的進階編程

add和remove訪問器的編程用法，以及如何選擇性地呼叫事件訂閱者類的方法。

using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace AdvancedEvent{    //define a delegate    delegate void NameChangedDelegate(string name, string newValue);    class Program    {        static void Main(string[] args)        {            Customer c = new Customer();            Subscriber s1 = new Subscriber(c, "subscribe-A");            Subscriber s2 = new Subscriber(c, "subscribe-B");            Subscriber s3 = new Subscriber(c, "subscribe-C");            c.FirstName = "Kevin";            c.LastName = "Hoffman";            s2.Unsubscribe();            c.FirstName = "Joe";            c.MiddleName = "Scott";            Console.ReadLine();        }    }    class Subscriber //    {        private string subscribedId = "new subscriber";        private Customer myCustomer = null;        private NameChangedDelegate ncDel = null;        public Subscriber(Customer c, string subId)        {            subscribedId = subId;            ncDel = new NameChangedDelegate(myCustomer_OnNameChanged);            myCustomer = c;            myCustomer.onNameChanged += ncDel;        }        void myCustomer_OnNameChanged(string name, string newValue)        {            Console.WriteLine("[{0}] Customer {1} changed to {2}.", subscribedId, name, newValue);         }        public void Unsubscribe()        {            myCustomer.onNameChanged -= ncDel;        }        public string SubscriberID        {            get            {                return subscribedId;            }            set            {                subscribedId = value;            }         }    }    class Customer    {        private string firstName;        private string lastName;        private string middleName;        private event NameChangedDelegate onNameChange; // create the event by using the delegate defined        public event NameChangedDelegate onNameChanged //define the details of the event        {            add            {                onNameChange += value;                if (value.Target is Subscriber)                {                    Console.WriteLine("Subscriber '{0}' just subscribed to OnNameChanged.", (((Subscriber)value.Target).SubscriberID));                }            }            remove            {                onNameChange -= value;                if (value.Target is Subscriber)                {                    Console.WriteLine("Subscriber '{0}' just subscribed to OnNameChanged.", (((Subscriber)value.Target).SubscriberID));                }            }         }        public string FirstName        {            get            {                return firstName;            }            set            {                firstName = value;                onNameChange("firstname", value);            }        }        public string LastName        {            get            {                return lastName;            }            set            {                lastName = value;                onNameChange("lastname", value);             }        }        public string MiddleName        {            get            {                return middleName;            }            set            {                middleName = value;                NotifyAllBut("subscribe-C", "middlename", value);            }        }        private void NotifyAllBut(string subId, string name, string value)        {            foreach (NameChangedDelegate d in onNameChange.GetInvocationList())            {                if (((Subscriber)d.Target).SubscriberID.ToUpper() != subId.ToUpper())                {                    d(name, value);                }            }        }    }}

 

類或對象可以通過事件向其他類或對象通知發生的相關事情。發送（或引發）事件的類稱為“發行者”，接收（或處理）事件的類稱為“訂戶”。

 

在這個例子中，Customer就是發行者，接收和處理事件訂戶就是Subscriber。整個事件的處理流程是如下設計的：

 

1. 定義一個委託NameChangedDelegate綁定到訂戶Subscriber的事件的處理方法上myCustomer_OnNameChanged。

 

Delegate void NameChangedDelegate(string name, string newvalue); //為事件的處理方法聲明一個委託

 

Class Subscriber

{

           Private NameChangedDelegate ncDel = null; // 將委託聲明為類的成員

 

           public Subscrber(Customer c, string subId)

           {

                    ncDel = new NameChangedDelegate(myCustomer_OnNameChanged); //將委託綁定到事件處理方法上

 

                    myCustomer = c;

                    myCustomer.OnNameChanged += ncDel; //為Customer類的OnNameChanged方法訂製處理方法，如果事件發生了，則激發myCustomer_OnNameChanged事件運行

}

}

1. 在訂戶Subscriber類裡，寫出具體的事件處理方法。

Void myCustomer_OnNameChanged(string name, string newvalue)

{

           //處理方法

}

1. 以上的步驟交代好了，事件發生後的處理方法。但是事件還沒有編寫。現在開始寫關鍵的部分，編寫事件。
2. 在Customer類裡面定義了2個事件。

Private event NameChangedDelegate onNameChange;

 

         Public event NameChangedDelegate onNameChanged

         {

                   Add//add訪問器

                   {

                           

}

Remove//remove訪問器

{

}

}

1. 首先來看看onNameChange的作用。

Public string FirstName

{

Get

{

Return firstName;

}

Set

{

         firstName = value;

         onNameChange(“firstname”, value);

}

}

 

onNameChange是NameChangedDelegate的一個執行個體，所以是onNameChange(“firstname”, value)和NameChangedDelegate委託的方法myCustomer_OnNameChanged具有相同的參數結構。

通過Debug知道，上面的關於FirstName屬性裡的onNameChange(“firstname”,value)語句只有在當customer執行個體的firstname值發生改變的時候，才會運行。同時運行myCustomer_OnNameChanged方法。至於為什麼，只有當customer執行個體的firstname值發生改變的時候，onNameChange(“firstname”,value)語句才運行。我想只有和這句話有關係了 onNameChange += value

1. 分析onNameChanged事件。

public event NameChangedDelegate onNameChanged

        {

            add

            {

                onNameChange += value;

                if (value.Target is Subscriber)

                {

                    Console.WriteLine("Subscriber '{0}' just subscribed to OnNameChanged.", (((Subscriber)value.Target).SubscriberID));

                }

            }

 

            remove

            {

                onNameChange -= value;

                if (value.Target is Subscriber)

                {

                    Console.WriteLine("Subscriber '{0}' just subscribed to OnNameChanged.", (((Subscriber)value.Target).SubscriberID));

                }

 

            }

 

        }