Events : Event : GUI Windows Forms C# Examples

    
//  copyright  2000  Eric  Gunnerson
//  From  A  Programmer's  Introduction  to  C#  2.0,  Third  Edition

using  System;
using  System.Collections;
using  System.Runtime.CompilerServices;

//
//  Global  delegate  cache.  Uses  a  two-level  hashtable.  The  delegateStore  hashtable  
//  stores  a  hashtable  keyed  on  the  object  instance,  and  the  instance  hashtable  is  
//  keyed  on  the  unique  key.  This  allows  fast  tear-down  of  the  object  when  it's  destroyed.
//
public  class  DelegateCache
{
        private  DelegateCache()  {}        //  nobody  can  create  one  of  these
        
        Hashtable  delegateStore  =  new  Hashtable();        //  top  level  hash  table
        
        static  DelegateCache  dc  =  new  DelegateCache();        //  our  single  instance
        
        Hashtable  GetInstanceHash(object  instance)
        {
                Hashtable  instanceHash  =  (Hashtable)  delegateStore[instance];
                
                if  (instanceHash  ==  null)
                {
                        instanceHash  =  new  Hashtable();
                        delegateStore[instance]  =  instanceHash;
                        
                }
                return(instanceHash);
        }
        
        public  static  void  Combine(Delegate  myDelegate,  object  instance,  object  key)
        {
                lock(instance)
                {
                        Hashtable  instanceHash  =  dc.GetInstanceHash(instance);                
                        
                        instanceHash[key]  =  Delegate.Combine((Delegate)  instanceHash[key],
                        myDelegate);
                }
        }
        
        public  static  void  Remove(Delegate  myDelegate,  object  instance,  object  key)
        {
                lock(instance)
                {
                        Hashtable  instanceHash  =  dc.GetInstanceHash(instance);                
                        
                        instanceHash[key]  =  Delegate.Remove((Delegate)  instanceHash[key],
                        myDelegate);
                }
        }
        
        public  static  Delegate  Fetch(object  instance,  object  key)
        {
                Hashtable  instanceHash  =  dc.GetInstanceHash(instance);                
                
                return((Delegate)  instanceHash[key]);
        }
        
        public  static  void  ClearDelegates(object  instance)
        {
                dc.delegateStore.Remove(instance);
        }
}

public  class  Button
{
        public  void  TearDown()
        {
                DelegateCache.ClearDelegates(this);
        }
        
        
        public  delegate  void  ClickHandler(object  sender,  EventArgs  e);
        
        static  object  clickEventKey  =  new  object();
        
        public  event  ClickHandler  Click
        {
                add
                {
                        DelegateCache.Combine(value,  this,  clickEventKey);
                }
                
                remove
                {
                        DelegateCache.Remove(value,  this,  clickEventKey);
                }
        }
        
        protected  void  OnClick()
        {
                ClickHandler  ch  =  (ClickHandler)  DelegateCache.Fetch(this,  clickEventKey);
                
                if  (ch  !=  null)
                ch(this,  null);
        }
        
        public  void  SimulateClick()
        {
                OnClick();
        }
}

class  Test
{
        static  public  void  ButtonHandler(object  sender,  EventArgs  e)
        {
                Console.WriteLine("Button  clicked");
        }
        
        public  static  void  Main()
        {
                Button  button  =  new  Button();
                
                button.Click  +=  new  Button.ClickHandler(ButtonHandler);
                
                button.SimulateClick();
                
                button.Click  -=  new  Button.ClickHandler(ButtonHandler);
                
                button.TearDown();
        }
}