C#委托,事件理解入门 (译稿)

501阅读　0评论2011-05-28　天海一线
分类：嵌入式

目录

l        导论

l        什么是委托

l        事件的理解

l        事件 关键字

l        最后

 

导论

    在学习C#中的委托和事件过程中，我读了许多文章来理解他们二者究竟是怎么一回事，以及如何使用他们，现在我将整个的理解过程陈述以下，我学到的每一方面，恐怕也是你们需要掌握的 ：-）。

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 description for Counter.

    ///

    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

 

  1. using System;
  2. using System.Drawing;
  3. using System.Collections;
  4. using System.ComponentModel;
  5. using System.Windows.Forms;
  6. using System.Data;

  8. namespace Events
  9. {
  10.     /**////

  12.     /// Summary description for Form1.

  14.     ///

  16.     public class Form1 : System.Windows.Forms.Form
  17.     {
  18.         Counter oCounter = null;

  20.         private System.Windows.Forms.Button cmdRun;
  21.         private System.Windows.Forms.TextBox txtReachable;
  22.         private System.Windows.Forms.TextBox txtCountTo;
  23.         private System.Windows.Forms.Label label1;
  24.         private System.Windows.Forms.Label label2;
  25.         private System.Windows.Forms.Button btnRemoveDelegate;
  26.         /**////

  28.         /// Required designer variable.

  30.         ///

  32.         private System.ComponentModel.Container components = null;

  34.         public Form1()
  35.         {
  36.             //

  38.             // Required for Windows Form Designer support

  40.             //

  42.             InitializeComponent();

  44.             //

  46.             // TODO: Add any constructor code after InitializeComponent call

  48.             //

  50.             oCounter = new Counter();
  51.             oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
  52.             oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
  53.         }

  55.         /**////

  57.         /// Clean up any resources being used.

  59.         ///

  61.         protected override void Dispose( bool disposing )
  62.         {
  63.             if( disposing )
  64.             {
  65.                 if (components != null)
  66.                 {
  67.                     components.Dispose();
  68.                 }
  69.             }
  70.             base.Dispose( disposing );
  71.         }

  73.         Windows Form Designer generated code#region Windows Form Designer generated code
  74.         /**////

  76.         /// Required method for Designer support - do not modify

  78.         /// the contents of this method with the code editor.

  80.         ///

  82.         private void InitializeComponent()
  83.         {
  84.             this.cmdRun = new System.Windows.Forms.Button();
  85.             this.txtReachable = new System.Windows.Forms.TextBox();
  86.             this.txtCountTo = new System.Windows.Forms.TextBox();
  87.             this.label1 = new System.Windows.Forms.Label();
  88.             this.label2 = new System.Windows.Forms.Label();
  89.             this.btnRemoveDelegate = new System.Windows.Forms.Button();
  90.             this.SuspendLayout();
  91.             //

  93.             // cmdRun

  95.             //

  97.             this.cmdRun.Location = new System.Drawing.Point(16, 72);
  98.             this.cmdRun.Name = "cmdRun";
  99.             this.cmdRun.Size = new System.Drawing.Size(48, 23);
  100.             this.cmdRun.TabIndex = 2;
  101.             this.cmdRun.Text = "Run";
  102.             this.cmdRun.Click += new System.EventHandler(this.cmdRun_Click);
  103.             //

  105.             // txtReachable

  107.             //

  109.             this.txtReachable.Location = new System.Drawing.Point(144, 40);
  110.             this.txtReachable.Name = "txtReachable";
  111.             this.txtReachable.Size = new System.Drawing.Size(56, 20);
  112.             this.txtReachable.TabIndex = 1;
  113.             this.txtReachable.Text = "";
  114.             //

  116.             // txtCountTo

  118.             //

  120.             this.txtCountTo.Location = new System.Drawing.Point(144, 16);
  121.             this.txtCountTo.Name = "txtCountTo";
  122.             this.txtCountTo.Size = new System.Drawing.Size(56, 20);
  123.             this.txtCountTo.TabIndex = 0;
  124.             this.txtCountTo.Text = "";
  125.             //

  127.             // label1

  129.             //

  131.             this.label1.AutoSize = true;
  132.             this.label1.Location = new System.Drawing.Point(16, 16);
  133.             this.label1.Name = "label1";
  134.             this.label1.Size = new System.Drawing.Size(51, 13);
  135.             this.label1.TabIndex = 3;
  136.             this.label1.Text = "Count To";
  137.             //

  139.             // label2

  141.             //

  143.             this.label2.AutoSize = true;
  144.             this.label2.Location = new System.Drawing.Point(16, 40);
  145.             this.label2.Name = "label2";
  146.             this.label2.Size = new System.Drawing.Size(99, 13);
  147.             this.label2.TabIndex = 4;
  148.             this.label2.Text = "Reach this number";
  149.             //

  151.             // btnRemoveDelegate

  153.             //

  155.             this.btnRemoveDelegate.Location = new System.Drawing.Point(16, 104);
  156.             this.btnRemoveDelegate.Name = "btnRemoveDelegate";
  157.             this.btnRemoveDelegate.Size = new System.Drawing.Size(168, 23);
  158.             this.btnRemoveDelegate.TabIndex = 5;
  159.             this.btnRemoveDelegate.Text = "Remove second handler";
  160.             this.btnRemoveDelegate.Click += new System.EventHandler(this.btnRemoveDelegate_Click);
  161.             //

  163.             // Form1

  165.             //

  167.             this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
  168.             this.ClientSize = new System.Drawing.Size(224, 134);
  169.             this.Controls.AddRange(new System.Windows.Forms.Control[] {
  170.                                                                           this.btnRemoveDelegate,
  171.                                                                           this.label2,
  172.                                                                           this.label1,
  173.                                                                           this.txtCountTo,
  174.                                                                           this.txtReachable,
  175.                                                                           this.cmdRun});
  176.             this.Name = "Form1";
  177.             this.Text = "Events";
  178.             this.ResumeLayout(false);

  180.         }
  181.         #endregion

  183.         /**////

  185.         /// The main entry point for the application.

  187.         ///

  189.         [STAThread]
  190.         static void Main()
  191.         {
  192.             Application.Run(new Form1());
  193.         }

  195.         private void btnRun_Click(object sender, System.EventArgs e)
  196.         {
  197.             if(txtCountTo.Text == "" || txtReachable.Text=="")
  198.                 return;
  199.             oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
  200.         }

  202.         private void oCounter_NumberReached(object sender, NumberReachedEventArgs e)
  203.         {
  204.             MessageBox.Show("Reached: " + e.ReachedNumber.ToString());
  205.         }
  206.         private void oCounter_NumberReached2(object sender, NumberReachedEventArgs e)
  207.         {
  208.             MessageBox.Show("Reached2: " + e.ReachedNumber.ToString());
  209.         }

  211.         private void btnRemoveDelegate_Click(object sender, System.EventArgs e)
  212.         {
  213.             oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2);
  214.             oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
  215.         }
  216.     }
  217. }

 

Counter.cs

  1. using System;

  3. namespace Events
  4. {
  5.     public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);

  7.     /**////

  9.     /// Summary description for Counter.

  11.     ///

  13.     public class Counter
  14.     {
  15.         public event NumberReachedEventHandler NumberReached;
  16.          
  17.         public Counter()
  18.         {
  19.             //

  21.             // TODO: Add constructor logic here

  23.             //

  25.         }
  26.         public void CountTo(int countTo, int reachableNum)
  27.         {
  28.             if(countTo < reachableNum)
  29.                 throw new ArgumentException("reachableNum should be less than countTo");
  30.             for(int ctr=0;ctr<=countTo;ctr++)
  31.             {
  32.                 if(ctr == reachableNum)
  33.                 {
  34.                     NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum);
  35.                     OnNumberReached(e);
  36.                     return;//don't count any more

  38.                 }
  39.             }
  40.         }

  42.         protected virtual void OnNumberReached(NumberReachedEventArgs e)
  43.         {
  44.             if(NumberReached!=null)
  45.             {
  46.                 NumberReached(this, e);
  47.             }
  48.         }
  49.     }

  51.     public class NumberReachedEventArgs : EventArgs
  52.     {
  53.         private int _reached;
  54.         public NumberReachedEventArgs(int num)
  55.         {
  56.             this._reached = num;
  57.         }
  58.         public int ReachedNumber
  59.         {
  60.             get
  61.             {
  62.                 return _reached;
  63.             }
  64.         }
  65.     }
  66. }

 

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