導(dǎo)論
在學(xué)習(xí)c#中的委托和事件過(guò)程中，我讀了許多文章來(lái)理解他們二者究竟是怎么一回事，以及如何使用他們，現(xiàn)在我將整個(gè)的理解過(guò)程陳述以下，我學(xué)到的每一方面，恐怕也是你們需要掌握的 ：-）。
什么是委托？
委托和事件這兩個(gè)概念是完全配合的。委托僅僅是函數(shù)指針，那就是說(shuō)，它能夠引用函數(shù)，通過(guò)傳遞地址的機(jī)制完成。委托是一個(gè)類，當(dāng)你對(duì)它實(shí)例化時(shí)，要提供一個(gè)引用函數(shù)，將其作為它構(gòu)造函數(shù)的參數(shù)。
每一個(gè)委托都有自己的簽名，例如：delegate int somedelegate(string s, bool b);是一個(gè)委托申明，在這里，提及的簽名，就是說(shuō)somedelegate 這個(gè)委托 有 string 和 bool 類型的形參，返回一個(gè)int 類型。
上面提及的：當(dāng)你對(duì)委托實(shí)例化時(shí)，要提供一個(gè)引用函數(shù)，將其作為它構(gòu)造函數(shù)的參數(shù)。這里要注意了：被引用的這個(gè)函數(shù)必須和委托有相同的簽名。
看下面的函數(shù)：
private int somefunction(string str, bool bln){...}
你可以把這個(gè)函數(shù)傳給somedelegate的構(gòu)造函數(shù)，因?yàn)樗麄冇邢嗨频暮灻╥n other words，他們都有相同的形參類型和個(gè)數(shù)，并且返回相同的數(shù)據(jù)類型）。
somedelegate sd = new somedelegate(somefunction);
sd 引用了 somefunction，也就是說(shuō)，somefunction已被sd所登記注冊(cè)，如果你調(diào)用 sd，somefunction 這個(gè)函數(shù)也會(huì)被調(diào)用，記住：我所說(shuō) somefunction的含義，后面，我們會(huì)用到它。
現(xiàn)在，你應(yīng)該知道如何使用委托了，讓我們繼續(xù)理解事件之旅……
事件的理解
我們知道，在c#中：
l 按鈕（button）就是一個(gè)類，當(dāng)我們單擊它時(shí)，就觸發(fā)一次click事件。
l 時(shí)鐘（timer）也是一個(gè)類，每過(guò)一毫秒，就觸發(fā)一次tick事件。
讓我們通過(guò)一個(gè)例子來(lái)學(xué)習(xí)，假定有這樣的情節(jié)：
現(xiàn)在有一個(gè)counter的類，它有一個(gè)方法 countto(int countto, int reachablenum)，該方法表示：在指定的時(shí)間段內(nèi)（0~~countto），當(dāng)?shù)竭_(dá)指定的時(shí)間點(diǎn)reachablenum時(shí)，就觸發(fā)一次numberreached事件。
它還有一個(gè)事件：numberreached，事件是委托類型的變量。意思是：如果給事件命名，用event關(guān)鍵字和要使用的委托類型申明它即可，如下所示：
public event numberreachedeventhandler numberreached;
在上面的申明中，numberreachedeventhandle 僅是一個(gè)委托，更確切的表示應(yīng)該是：numberreacheddelegate。但是微軟從不這樣認(rèn)為mousedelegate或者paintdelegate，，而是稱謂：mouseeventhandler 或者 painteventhandler。所以
numberreachedeventhandler 比numberreacheddelegate聽(tīng)起來(lái)更方便一些，ok?好了，讓我們繼續(xù)，現(xiàn)在你知道了，在我們聲明事件之前，需要象下面這樣的形式來(lái)定義委托：
public delegate void numberreachedeventhandler(object sender, numberreachedeventargs e);
現(xiàn)在聲明的委托 numberreachedeventhandle，它有一個(gè)void 返回值，和object，numberreachedeventargs兩個(gè)形參。就像我們?cè)诘谝还?jié)中強(qiáng)調(diào)的那樣，當(dāng)實(shí)例化委托時(shí)，作為實(shí)參傳入的函數(shù)也必須擁有和委托同樣的簽名。
在你的代碼中， 你是否用過(guò)painteventargs 或者 mouseeventargs來(lái)確定鼠標(biāo)的移動(dòng)位置？是否在觸發(fā)paint事件的對(duì)象中用過(guò)graphics 屬性？實(shí)際上，為用戶提供數(shù)據(jù)的類都是繼承于system.eventargs類，就是我們常說(shuō)的事件參數(shù)類，如果事件不提供參數(shù)，就不定義該類。在我們的例子中，我們通過(guò)下面的類提供預(yù)期的時(shí)間點(diǎn)。
public class numberreachedeventargs : eventargs
{
private int _reached;
public numberreachedeventargs(int num)
{
this._reached = num;
}
public int reachednumber
{
get
{
return _reached;
}
}
}
好，有了前面的介紹，讓我們到counter類里面看看：
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，如果到達(dá)指定的時(shí)間點(diǎn)，就觸發(fā)一次事件，有以下幾個(gè)方面需要注意：
l 通過(guò)調(diào)用numberreached（它是numberreachedeventhandler委托的實(shí)例）來(lái)完成一次觸發(fā)事件。
numberreached(this, e); 通過(guò)這種方式，可以調(diào)用所有的注冊(cè)函數(shù)。
l 通過(guò) numberreachedeventargs e = new numberreachedeventargs(reachablenum); 為所有的注冊(cè)函數(shù)提供事件數(shù)據(jù)。
l 看了上面的代碼，你可能要問(wèn)了：為什么我們直接用 onnumberreached(numberreachedeventargs e)方法來(lái)調(diào)用numberreached（this，e），而不用下面的代碼呢？
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
}
這個(gè)問(wèn)題問(wèn)得很好，那就讓我們?cè)倏匆幌耾nnumberreached 簽名：
protected virtual void onnumberreached(numberreachedeventargs e)
①你也明白 關(guān)鍵字protected限定了 只有從該類繼承的類才能調(diào)用該類中的所有方法。
②關(guān)鍵字 virtual 表明了 在繼承類中可以重寫該方法。
這兩點(diǎn)非常有用，假設(shè)你在寫一個(gè)從counter繼承而來(lái)的類，通過(guò)重寫onnumberreached 方法，你可以在事件觸發(fā)之前，進(jìn)行一次其他的工作。
protected override void onnumberreached(numberreachedeventargs e)
{
//do additional work
base.onnumberreached(e);
}
注意：如果你沒(méi)有調(diào)用base.onnumberreached(e), 那么從不會(huì)觸發(fā)這個(gè)事件！在你繼承該類而想剔出它的一些其他事件時(shí)，使用該方式是非常有用的。
l 還要注意到：委托 numberreachedeventhandler 是在類定義的外部，命名空間內(nèi)定義的，對(duì)所有類來(lái)說(shuō)是可見(jiàn)的。
好，該我們來(lái)實(shí)際操作使用counter類了。
在我們簡(jiǎn)單的應(yīng)用程序中，我們有兩個(gè)文本框，分別是：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());
}
初始化事件處理的語(yǔ)法如下：
ocounter = new counter();
ocounter.numberreached += new numberreachedeventhandler(ocounter_numberreached);
現(xiàn)在你明白了你剛才所做的一切，僅僅初始化 numberreachedeventhandler 委托類型的對(duì)象（就像你實(shí)例化其他對(duì)象一樣），注意到 ocounter_numberreached 方法的簽名與我前面提到的相似。
還要注意我們用的是+= 而不是=；這是因?yàn)槲惺翘厥獾膶?duì)象，它可以引用多個(gè)對(duì)象（在這里是指它可以引用多個(gè)函數(shù)）。for example 如果有另外一個(gè)
和ocounter_numberreached一樣具有相同簽名的函數(shù)ocounter_numberreached2，這兩個(gè)函數(shù)都可以被引用：
ocounter = new counter();
ocounter.numberreached += new numberreachedeventhandler(ocounter_numberreached);
ocounter.numberreached += new numberreachedeventhandler(ocounter_numberreached2);
現(xiàn)在，觸發(fā)一個(gè)事件后，上面兩個(gè)函數(shù)被依次調(diào)用。
視情況而定，如果你想讓ocounter_numberreached2在numberreached事件發(fā)生后不再被調(diào)用，可以簡(jiǎn)單地這樣寫：ocounter.numberreached -= new numberreachedeventhandler(ocounter_numberreached2);
最后
讓我們看一下完整的源代碼,以供參考：
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;
}
}
}
}
