Events: Custom Add and Remove
/*
A Programmer's Introduction to C# (Second Edition)
by Eric Gunnerson
Publisher: Apress L.P.
ISBN: 1-893115-62-3
*/
// 23 - Events\Custom Add and Remove
// copyright 2000 Eric Gunnerson
using System;
using System.Collections;
using System.Runtime.CompilerServices;
public class EventsCustomAddandRemove
{
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();
}
}
//
// 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();
}
}
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