两篇关于委托和事件的文章之一

什么是委托？

委托和事件这两个概念是完全配合的。委托仅仅是函数指针，那就是说，它能够引用函数，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，要提供一个引用函数，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);是一个委托申明，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：

l 按钮（Button）就是一个类，当我们单击它时，就触发一次click事件。

l 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，OK?好了，让我们继续，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：

l 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。

l 通过 NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum); 为所有的注册函数提供事件数据。

l 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。

l 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

  1using System;
  2using System.Drawing;
  3using System.Collections;
  4using System.ComponentModel;
  5using System.Windows.Forms;
  6using System.Data;
  7
  8namespace Events
  9{
10    /**//**//**//// <summary>
11    /// Summary description for Form1.
12    /// </summary>
13    public class Form1 : System.Windows.Forms.Form
14    {
15        Counter oCounter = null;
16
17        private System.Windows.Forms.Button cmdRun;
18        private System.Windows.Forms.TextBox txtReachable;
19        private System.Windows.Forms.TextBox txtCountTo;
20        private System.Windows.Forms.Label label1;
21        private System.Windows.Forms.Label label2;
22        private System.Windows.Forms.Button btnRemoveDelegate;
23        /**//**//**//// <summary>
24        /// Required designer variable.
25        /// </summary>
26        private System.ComponentModel.Container components = null;
27
28        public Form1()
29        {
30            //
31            // Required for Windows Form Designer support
32            //
33            InitializeComponent();
34
35            //
36            // TODO: Add any constructor code after InitializeComponent call
37            //
38            oCounter = new Counter();
39            oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
40            oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
41        }
42
43        /**//**//**//// <summary>
44        /// Clean up any resources being used.
45        /// </summary>
46        protected override void Dispose( bool disposing )
47        {
48            if( disposing )
49            {
50                if (components != null)
51                {
52                    components.Dispose();
53                }
54            }
55            base.Dispose( disposing );
56        }
57
58        Windows Form Designer generated codeWindows Form Designer generated code#region Windows Form Designer generated code
59        /**//**//**//// <summary>
60        /// Required method for Designer support - do not modify
61        /// the contents of this method with the code editor.
62        /// </summary>
63        private void InitializeComponent()
64        {
65            this.cmdRun = new System.Windows.Forms.Button();
66            this.txtReachable = new System.Windows.Forms.TextBox();
67            this.txtCountTo = new System.Windows.Forms.TextBox();
68            this.label1 = new System.Windows.Forms.Label();
69            this.label2 = new System.Windows.Forms.Label();
70            this.btnRemoveDelegate = new System.Windows.Forms.Button();
71            this.SuspendLayout();
72            // 
73            // cmdRun
74            // 
75            this.cmdRun.Location = new System.Drawing.Point(16, 72);
76            this.cmdRun.Name = "cmdRun";
77            this.cmdRun.Size = new System.Drawing.Size(48, 23);
78            this.cmdRun.TabIndex = 2;
79            this.cmdRun.Text = "Run";
80            this.cmdRun.Click += new System.EventHandler(this.cmdRun_Click);
81            // 
82            // txtReachable
83            // 
84            this.txtReachable.Location = new System.Drawing.Point(144, 40);
85            this.txtReachable.Name = "txtReachable";
86            this.txtReachable.Size = new System.Drawing.Size(56, 20);
87            this.txtReachable.TabIndex = 1;
88            this.txtReachable.Text = "";
89            // 
90            // txtCountTo
91            // 
92            this.txtCountTo.Location = new System.Drawing.Point(144, 16);
93            this.txtCountTo.Name = "txtCountTo";
94            this.txtCountTo.Size = new System.Drawing.Size(56, 20);
95            this.txtCountTo.TabIndex = 0;
96            this.txtCountTo.Text = "";
97            // 
98            // label1
99            // 
100            this.label1.AutoSize = true;
101            this.label1.Location = new System.Drawing.Point(16, 16);
102            this.label1.Name = "label1";
103            this.label1.Size = new System.Drawing.Size(51, 13);
104            this.label1.TabIndex = 3;
105            this.label1.Text = "Count To";
106            // 
107            // label2
108            // 
109            this.label2.AutoSize = true;
110            this.label2.Location = new System.Drawing.Point(16, 40);
111            this.label2.Name = "label2";
112            this.label2.Size = new System.Drawing.Size(99, 13);
113            this.label2.TabIndex = 4;
114            this.label2.Text = "Reach this number";
115            // 
116            // btnRemoveDelegate
117            // 
118            this.btnRemoveDelegate.Location = new System.Drawing.Point(16, 104);
119            this.btnRemoveDelegate.Name = "btnRemoveDelegate";
120            this.btnRemoveDelegate.Size = new System.Drawing.Size(168, 23);
121            this.btnRemoveDelegate.TabIndex = 5;
122            this.btnRemoveDelegate.Text = "Remove second handler";
123            this.btnRemoveDelegate.Click += new System.EventHandler(this.btnRemoveDelegate_Click);
124            // 
125            // Form1
126            // 
127            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
128            this.ClientSize = new System.Drawing.Size(224, 134);
129            this.Controls.AddRange(new System.Windows.Forms.Control[] {
130                                                                          this.btnRemoveDelegate,
131                                                                          this.label2,
132                                                                          this.label1,
133                                                                          this.txtCountTo,
134                                                                          this.txtReachable,
135                                                                          this.cmdRun});
136            this.Name = "Form1";
137            this.Text = "Events";
138            this.ResumeLayout(false);
139
140        }
141        #endregion
142
143        /**//**//**//// <summary>
144        /// The main entry point for the application.
145        /// </summary>
146        [STAThread]
147        static void Main()
148        {
149            Application.Run(new Form1());
150        }
151
152        private void btnRun_Click(object sender, System.EventArgs e)
153        {
154            if(txtCountTo.Text == "" || txtReachable.Text=="")
155                return;
156            oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
157        }
158
159        private void oCounter_NumberReached(object sender, NumberReachedEventArgs e)
160        {
161            MessageBox.Show("Reached: " + e.ReachedNumber.ToString());
162        }
163        private void oCounter_NumberReached2(object sender, NumberReachedEventArgs e)
164        {
165            MessageBox.Show("Reached2: " + e.ReachedNumber.ToString());
166        }
167
168        private void btnRemoveDelegate_Click(object sender, System.EventArgs e)
169        {
170            oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2);
171            oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
172        }
173    }
174} 

 

 

Counter.cs

 

using System;
2
3namespace Events
4{
5    public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);
6
7    /**//**//**//// <summary>
8    /// Summary description for Counter.
9    /// </summary>
10    public class Counter
11    {
12        public event NumberReachedEventHandler NumberReached;
13       
14        public Counter()
15        {
16            //
17            // TODO: Add constructor logic here
18            //
19        }
20        public void CountTo(int countTo, int reachableNum)
21        {
22            if(countTo < reachableNum)
23                throw new ArgumentException("reachableNum should be less than countTo");
24            for(int ctr=0;ctr<=countTo;ctr++)
25            {
26                if(ctr == reachableNum)
27                {
28                    NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum);
29                    OnNumberReached(e);
30                    return;//don't count any more
31                }
32            }
33        }
34
35        protected virtual void OnNumberReached(NumberReachedEventArgs e)
36        {
37            if(NumberReached!=null)
38            {
39                NumberReached(this, e);
40            }
41        }
42    }
43
44    public class NumberReachedEventArgs : EventArgs
45    {
46        private int _reached;
47        public NumberReachedEventArgs(int num)
48        {
49            this._reached = num;
50        }
51        public int ReachedNumber
52        {
53            get
54            {
55                return _reached;
56            }
57        }
58    }
59}
60