Asp.net MVC FluentHTML and Fluent Interface

赏花归去马如飞, 去马如飞酒力微.
酒力微醒时已暮, 醒时已暮赏花归.---苏轼

我们力求页面层代码简洁并具有较好的可读性,在Asp.net MVC的平台上,我们以新的起点来实现这一目标.MvcContrib.FluentHtml和Spark ViewEngine给我们做出了榜样.本文将以MvcContrib.FluentHtml为例探究它的实现机制:Fluent Interface.

读过开篇的诗句,不知是否感受到文字之美.不仅仅是在文学作品中,在代码中,这种美一样存在.

在MvcContrib.FluentHtml的应用中,我们随处可以见到下面的代码:

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> <% = this.TextBox(x => x.Person.Name).Title( " Entertheperson'sname " ).Label( " Name: " ) %> < br />

……
<% = this.Select(x => x.Person.Gender).Options(Model.Genders).Size( 5 ).Label( " Gender: " )
.Title( " Selecttheperson'sgender " ) %> < br />
浏览器中生成的代码为:
< label id ="Person_Name_Label" for ="Person_Name" > Name: </ label >
< input id ="Person_Name" type ="text" value ="Jeremy" title ="Entertheperson'sname" name ="Person.Name" maxlength ="50" />
.
< select id ="Person_Gender" title ="Selecttheperson'sgender" size ="5" name ="Person.Gender" >
< option value ="M" selected ="selected" > Male </ option >
< option value ="F" > Female </ option >
</ select >

上面对动态生成TextBox和Select的代码很有意思,我们使用普通的方式在页面上生成同样的客户端代码,CS代码大致是这样的:

Label label = new Label();

label.Text = "Name";

TextBox textbox= new TextBox();

textbox.ToolTip ="Enter the person's name";

textbox.ID = "No.10001";

textbox.ID = "Person.Name";

而FluentHtml创建页面元素的方式让我们很容易联想到StringBuilder的使用:

StringBuilder stringbuilder = new StringBuilder();

stringbuilder.Append("Hello").Append(" ").Append("World!");

Fulent Interface

这种实现编程方式就是"Fluent Interface",这并不是什么新概念,2005年Eric Evans 和Martin Fowler就为这种实现方式命名.源文档 <http://www.martinfowler.com/bliki/FluentInterface.html> 可以通过维基百科中对Fluent Interface的描述获得一个基本的了解:In software engineering, a fluent interface (as first coined by Eric Evans and Martin Fowler) is a way of implementing an object oriented API in a way that aims to provide for more readable code.

我们分解上面的话:

• 它是面向对象API的一种实现方式
• 目的是增加代码的可读性

既然我们最熟悉的是StringBuilder,我们就从这个线索追下去:打开Re

flector,很容易找到StringBuilder的Append方法:

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> public StringBuilderAppend( string value)
{
if(value!=null)
{
stringstringValue=this.m_StringValue;
IntPtrcurrentThread=Thread.InternalGetCurrentThread();
if(this.m_currentThread!=currentThread)
{
stringValue=string.GetStringForStringBuilder(stringValue,stringValue.Capacity);
}
intlength=stringValue.Length;
intrequiredLength=length+value.Length;
if(this.NeedsAllocation(stringValue,requiredLength))
{
stringnewString=this.GetNewString(stringValue,requiredLength);
newString.AppendInPlace(value,length);
this.ReplaceString(currentThread,newString);
}
else
{
stringValue.AppendInPlace(value,length);
this.ReplaceString(currentThread,stringValue);
}
}
returnthis;
}

阅读这段有两个特别要注意的点:1.方法的返回值是StringBuilder类型 2.最后一句:return this;为了深刻理解,我们写一个简单的StringBuilder:

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> public interface IContentBuilder
{
void WriteContent();
IContentBuilderAppend( string partialContent);
}
public class TestContentBuilder:IContentBuilder
{
string temp;
#region IContentBuilderMembers

void IContentBuilder.WriteContent()
{
Console.Write(temp);
}

IContentBuilderIContentBuilder.Append( string partialContent)
{
temp += partialContent;
return this ;
}

#endregion
}
……
// 调用代码
IContentBuildert = new TestContentBuilder();
t.Append( " test " ).Append( " Hello " ).WriteContent();

跑一下代码,和StringBuilder效果是一样的.从上面的应用也可以看出:Fluent Interface经常用来完成对象的构造和属性赋值.

言归正传:FluentHTML

了解了Fluent Interface,我们来看一下MVCContrib.FluentHTML的实现,这里以TextBox为例进行考察,首先看一下它的继承关系:

public class TextBox : TextInput<TextBox>

public abstract class TextInput<T> : Input<T>, ISupportsMaxLength where T : TextInput<T>

public abstract class Input<T> : FormElement<T> where T : Input<T>, Ielement

泛型是一种高层次的算法抽象,我们就通过Input<T>一窥端倪:

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> public abstract class Input < T > :FormElement < T > where T:Input < T > ,IElement
{
protected object elementValue;

protected Input( string type, string name): base (HtmlTag.Input,name)
{
builder.MergeAttribute(HtmlAttribute.Type,type, true );
}

protected Input( string type, string name,MemberExpressionforMember,IEnumerable < IBehaviorMarker > behaviors)
: base (HtmlTag.Input,name,forMember,behaviors)
{
builder.MergeAttribute(HtmlAttribute.Type,type, true );
}

/// <summary>
/// Setthe'value'attribute.
/// </summary>
/// <paramname="value"> Thevaluefortheattribute. </param>
public virtual TValue( object value)
{
elementValue = value;
return (T) this ;
}

/// <summary>
/// Setthe'size'attribute.
/// </summary>
/// <paramname="value"> Thevaluefortheattribute. </param>
public virtual TSize( int value)
{
Attr(HtmlAttribute.Size,value);
return (T) this ;
}

protected override void PreRender()
{
Attr(HtmlAttribute.Value,elementValue);
base .PreRender();
}
}

以Size方法为例,可以看出这是一种典型的Fluent Interface实现:

public virtual T Size(int value)

{

Attr(HtmlAttribute.Size, value);

return (T)this;

}

分析到这里,上面的语句中还有一点比较奇怪,就是Lambda表达式的部分:

this.TextBox(x => x.Person.Name).Title("Enter the person's name").Label("Name:")

TextBox的实现代码里面我们没有看到对Lambda表达式的支持.那是在什么地方完成的呢?通过跟进,我们来到了ViewDataContainerExtensions,它是IViewDataContainer 的Extension Method:

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> namespace MvcContrib.FluentHtml
{
/// <summary>
/// ExtensionstoIViewDataContainer
/// </summary>
public static class ViewDataContainerExtensions
{
/// <summary>
/// GenerateanHTMLinputelementoftype'text'andsetitsvaluefromViewDatabasedonthenameprovided.
/// </summary>
/// <paramname="view"> Theview. </param>
/// <paramname="name"> Valueofthe'name'attributeoftheelement.Alsousedtoderivethe'id'attribute. </param>
public static TextBoxTextBox( this IViewDataContainerview, string name)
{
return new TextBox(name).Value(view.ViewData.Eval(name));
}
……

看一下return new TextBox(name).Value(view.ViewData.Eval(name));所以这里就成了TextBox定义方法链的第一步.

总结

为了能够在View中能够简洁清晰的构造HTML元素,Asp.net MVC中通过htmlHelper.InputHelper来实现页面元素的构造. 页面层所使用的<%= Html.TextBox("username") %>,HTML也是htmlHelper的Extension Method.相比较起来,htmlHelper提供了基础的页面控件定义和构造,而FluentHTML表现的更为灵活.除了FluentHTML,著名的Spark View Engine也有类似的实现,大家可以关注一下.

嗯哼,全文完.

P.S:最近要找工作了,有好机会联系我:mailto:ligaoren@gmail.com