委托、事件
什么是委托？
委托和事件这两个概念是完全配合的。委托可以看成是函数指针，那就是说，它能够引用对象方法，通过传递地址的机制完成。委托是一个类，当你对它实例化时，要提供一个对象方法，将其作为它构造函数的参数。
每一个委托都有自己的签名，例如：Delegate int SomeDelegate(string s, bool b);是一个委托声明，在这里，提及的签名，就是说SomeDelegate 这个委托有 string 和 bool 类型的形参，返回一个int 类型。
上面提及的：当你对委托实例化时，要提供一个对象方法(引用函数)，将其作为它构造函数的参数。这里要注意了：被引用的这个函数必须和委托有相同的签名。
看下面的函数：
private int SomeFunction(string str, bool bln){...}
你可以把这个函数传给SomeDelegate的构造函数，因为他们有相似的签名（in other words，他们都有相同的形参类型和个数，并且返回相同的数据类型）。
SomeDelegate sd = new SomeDelegate(SomeFunction);
sd 引用了 SomeFunction，也就是说，SomeFunction已被sd所登记注册，如果你调用 sd，SomeFunction 这个函数也会被调用，记住：我所说 SomeFunction的含义，后面，我们会用到它。
现在，你应该知道如何使用委托了，让我们继续理解事件之旅……
事件的理解
我们知道，在C#中：
- 按钮（Button）就是一个类，当我们单击它时，就触发一次click事件。
- 2.时钟（Timer）也是一个类，每过一毫秒，就触发一次tick事件。
让我们通过一个例子来学习，假定有这样的情节：
现在有一个Counter的类，它有一个方法 CountTo(int countTo, int reachableNum)，该方法表示：在指定的时间段内（0~~countTo），当到达指定的时间点reachableNum时，就触发一次NumberReached事件。
它还有一个事件：NumberReached，事件是委托类型的变量。意思是：如果给事件命名，用event关键字和要使用的委托类型申明它即可，如下所示：
public event NumberReachedEventHandler NumberReached;
在上面的申明中，NumberReachedEventHandle 仅是一个委托，更确切的表示应该是：NumberReachedDelegate。但是微软从不这样认为MouseDelegate或者PaintDelegate,而是称谓：MouseEventHandler 或者 PaintEventHandler。所以NumberReachedEventHandler 比NumberReachedDelegate听起来更方便一些.现在你知道了，在我们声明事件之前，需要象下面这样的形式来定义委托：
public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);
现在声明的委托 NumberReachedEventHandle，它有一个void 返回值，和object，NumberReachedEventArgs两个形参。就像我们在第一节中强调的那样，当实例化委托时，作为实参传入的函数也必须拥有和委托同样的签名。
在你的代码中， 你是否用过PaintEventArgs 或者 MouseEventArgs来确定鼠标的移动位置？是否在触发Paint事件的对象中用过Graphics 属性？实际上，为用户提供数据的类都是继承于System.EventArgs类，就是我们常说的事件参数类，如果事件不提供参数，就不定义该类。在我们的例子中，我们通过下面的类提供预期的时间点。
public class NumberReachedEventArgs : EventArgs
{
private int _reached;
public NumberReachedEventArgs(int num)
{
this._reached = num;
}
public int ReachedNumber
{
get
{
return _reached;
}
}
}
好，有了前面的介绍，让我们到Counter类里面看看：
namespace Events
{
public delegate void NumberReachedEventHandler(object sender,
NumberReachedEventArgs e);
/// <summary>
/// Summary description for Counter.
/// </summary>
public class Counter
{
public event NumberReachedEventHandler NumberReached;
public Counter()
{
//
// TODO: Add constructor logic here
//
}
public void CountTo(int countTo, int reachableNum)
{
if(countTo < reachableNum)
throw new ArgumentException( "reachableNum should be less than countTo");
for(int ctr=0;ctr<=countTo;ctr++)
{
if(ctr == reachableNum)
{
NumberReachedEventArgs e = new NumberReachedEventArgs( reachableNum);
OnNumberReached(e);
return;//don't count any more
}
}
}
protected virtual void OnNumberReached(NumberReachedEventArgs e)
{
if(NumberReached != null)
{
NumberReached(this, e);//Raise the event
}
}
}
在Counter中，如果到达指定的时间点，就触发一次事件，有以下几个方面需要注意：
- 通过调用NumberReached（它是NumberReachedEventHandler委托的实例）来完成一次触发事件。NumberReached(this, e); 通过这种方式，可以调用所有的注册函数。
- 通过 NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum); 为所有的注册函数提供事件数据。
- 看了上面的代码，你可能要问了：为什么我们直接用 OnNumberReached(NumberReachedEventArgs e)方法来调用NumberReached（this，e），而不用下面的代码呢？
if(ctr == reachableNum)
{
NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum);
//OnNumberReached(e);
if(NumberReached != null)
{
NumberReached(this, e);//Raise the event
}
return;//don't count any more
}
这个问题问得很好，那就让我们再看一下OnNumberReached 签名：
protected virtual void OnNumberReached(NumberReachedEventArgs e)
①你也明白 关键字protected限定了 只有从该类继承的类才能调用该类中的所有方法。
②关键字 virtual 表明了 在继承类中可以重写该方法。
这两点非常有用，假设你在写一个从Counter继承而来的类，通过重写OnNumberReached 方法，你可以在事件触发之前，进行一次其他的工作。
protected override void OnNumberReached(NumberReachedEventArgs e)
{
//Do additional work
base.OnNumberReached(e);
}
注意：如果你没有调用base.OnNumberReached(e), 那么从不会触发这个事件！在你继承该类而想剔出它的一些其他事件时，使用该方式是非常有用的。
还要注意到：委托 NumberReachedEventHandler 是在类定义的外部，命名空间内定义的，对所有类来说是可见的。
好，该我们来实际操作使用Counter类了。
在我们简单的应用程序中，我们有两个文本框，分别是：txtCountTo和txtReachable：
下面是btnRun的click事件：
private void btnRun_Click(object sender, System.EventArgs e)
{
if(txtCountTo.Text == " " || txtReachable.Text==" ")
return;
oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
}
private void oCounter_NumberReached(object sender, NumberReachedEventArgs e)
{
MessageBox.Show("Reached: " + e.ReachedNumber.ToString());
}
初始化事件处理的语法如下：
oCounter = new Counter();
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
现在你明白了你刚才所做的一切，仅仅初始化 NumberReachedEventHandler 委托类型的对象（就像你实例化其他对象一样），注意到 oCounter_NumberReached 方法的签名与我前面提到的相似。
还要注意我们用的是+= 而不是=；这是因为委托是特殊的对象，它可以引用多个对象（在这里是指它可以引用多个函数）。For example 如果有另外一个和oCounter_NumberReached一样具有相同签名的函数oCounter_NumberReached2，这两个函数都可以被引用：
oCounter = new Counter();
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
现在，触发一个事件后，上面两个函数被依次调用。
视情况而定，如果你想让oCounter_NumberReached2在NumberReached事件发生后不再被调用，可以简单地这样写：
oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2);
最后
让我们看一下完整的源代码,以供参考：
[Form1.cs ]
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
namespace Events
{
/**//// <summary>
/// Summary description for Form1.
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
Counter oCounter = null;
private System.Windows.Forms.Button cmdRun;
private System.Windows.Forms.TextBox txtReachable;
private System.Windows.Forms.TextBox txtCountTo;
private System.Windows.Forms.Label label1;
private System.Windows.Forms.Label label2;
private System.Windows.Forms.Button btnRemoveDelegate;
/**//// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.Container components = null;
public Form1()
{
//
// Required for Windows Form Designer support
//
InitializeComponent();
//
// TODO: Add any constructor code after InitializeComponent call
//
oCounter = new Counter();
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
}
/**//// <summary>
/// Clean up any resources being used.
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}
Windows Form Designer generated code#region Windows Form Designer generated code
/**//// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.cmdRun = new System.Windows.Forms.Button();
this.txtReachable = new System.Windows.Forms.TextBox();
this.txtCountTo = new System.Windows.Forms.TextBox();
this.label1 = new System.Windows.Forms.Label();
this.label2 = new System.Windows.Forms.Label();
this.btnRemoveDelegate = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// cmdRun
//
this.cmdRun.Location = new System.Drawing.Point(16, 72);
this.cmdRun.Name = "cmdRun";
this.cmdRun.Size = new System.Drawing.Size(48, 23);
this.cmdRun.TabIndex = 2;
this.cmdRun.Text = "Run";
this.cmdRun.Click += new System.EventHandler(this.cmdRun_Click);
//
// txtReachable
//
this.txtReachable.Location = new System.Drawing.Point(144, 40);
this.txtReachable.Name = "txtReachable";
this.txtReachable.Size = new System.Drawing.Size(56, 20);
this.txtReachable.TabIndex = 1;
this.txtReachable.Text = " ";
//
// txtCountTo
//
this.txtCountTo.Location = new System.Drawing.Point(144, 16);
this.txtCountTo.Name = "txtCountTo";
this.txtCountTo.Size = new System.Drawing.Size(56, 20);
this.txtCountTo.TabIndex = 0;
this.txtCountTo.Text = " ";
//
// label1
//
this.label1.AutoSize = true;
this.label1.Location = new System.Drawing.Point(16, 16);
this.label1.Name = "label1";
this.label1.Size = new System.Drawing.Size(51, 13);
this.label1.TabIndex = 3;
this.label1.Text = "Count To";
//
// label2
//
this.label2.AutoSize = true;
this.label2.Location = new System.Drawing.Point(16, 40);
this.label2.Name = "label2";
this.label2.Size = new System.Drawing.Size(99, 13);
this.label2.TabIndex = 4;
this.label2.Text = "Reach this number";
//
// btnRemoveDelegate
//
this.btnRemoveDelegate.Location = new System.Drawing.Point(16, 104);
this.btnRemoveDelegate.Name = "btnRemoveDelegate";
this.btnRemoveDelegate.Size = new System.Drawing.Size(168, 23);
this.btnRemoveDelegate.TabIndex = 5;
this.btnRemoveDelegate.Text = "Remove second handler";
this.btnRemoveDelegate.Click += new System.EventHandler(this.btnRemoveDelegate_Click);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.ClientSize = new System.Drawing.Size(224, 134);
this.Controls.AddRange(new System.Windows.Forms.Control[] {
this.btnRemoveDelegate,
this.label2,
this.label1,
this.txtCountTo,
this.txtReachable,
this.cmdRun});
this.Name = "Form1";
this.Text = "Events";
this.ResumeLayout(false);
}
#endregion
/**//// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}
private void btnRun_Click(object sender, System.EventArgs e)
{
if(txtCountTo.Text == " " || txtReachable.Text==" ")
return;
oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
}
private void oCounter_NumberReached(object sender, NumberReachedEventArgs e)
{
MessageBox.Show("Reached: " + e.ReachedNumber.ToString());
}
private void oCounter_NumberReached2(object sender, NumberReachedEventArgs e)
{
MessageBox.Show("Reached2: " + e.ReachedNumber.ToString());
}
private void btnRemoveDelegate_Click(object sender, System.EventArgs e)
{
oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2);
oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
}
}
}
[Counter.cs ]
using System;
namespace Events
{
public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);
/**//// <summary>
/// Summary description for Counter.
/// </summary>
public class Counter
{
public event NumberReachedEventHandler NumberReached;
public Counter()
{
//
// TODO: Add constructor logic here
//
}
public void CountTo(int countTo, int reachableNum)
{
if(countTo < reachableNum)
throw new ArgumentException("reachableNum should be less than countTo");
for(int ctr=0;ctr<=countTo;ctr++)
{
if(ctr == reachableNum)
{
NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum);
OnNumberReached(e);
return;//don't count any more
}
}
}
protected virtual void OnNumberReached(NumberReachedEventArgs e)
{
if(NumberReached!=null)
{
NumberReached(this, e);
}
}
}
public class NumberReachedEventArgs : EventArgs
{
private int _reached;
public NumberReachedEventArgs(int num)
{
this._reached = num;
}
public int ReachedNumber
{
get
{
return _reached;
}
}
}
}
posted on 2011-05-02 10:51 beyond1008 阅读(196) 评论(0) 编辑 收藏 举报
