浅谈ASP.NET MVC中的FluentHtml与连续接口

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.

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

< %= this.TextBox(x => x.Person.Name).Title("Enter the person's name").Label("Name:") %> …… < %= this.Select(x => x.Person.Gender).Options(Model.Genders).Size(5).Label("Gender:") .Title("Select the person's gender") %> 浏览器中生成的代码为:

< LABEL id=Person_Name_Label for=Person_Name>Name:< /LABEL> < INPUT id=Person_Name title="Enter the person's name" value=Jeremy maxLength=50 name=Person.Name> . < SELECT id=Person_Gender title="Select the person's gender" size=5 name=Person.Gender>< OPTION selected value=M>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 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Reflector,很容易找到StringBuilder的Append方法:

public StringBuilder Append(string value)
{ if (value != null)
{ string stringValue = this.m_StringValue;
IntPtr currentThread = Thread.InternalGetCurrentThread();
if (this.m_currentThread != currentThread)
{
stringstringValue = string.GetStringForStringBuilder(stringValue, stringValue.Capacity);
} int length = stringValue.Length;
int requiredLength = length + value.Length;
if (this.NeedsAllocation(stringValue, requiredLength))
{ string newString = this.GetNewString(stringValue, requiredLength);
newString.AppendInPlace(value, length);
this.ReplaceString(currentThread, newString);
}
else
{
stringValue.AppendInPlace(value, length);
this.ReplaceString(currentThread, stringValue);
}
} return this;
}

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:

public interface IContentBuilder {
void WriteContent();
IContentBuilder Append(string partialContent);
} public class TestContentBuilder : IContentBuilder { string temp;
#region IContentBuilder Members void IContentBuilder.WriteContent() {
Console.Write(temp);
}
IContentBuilder IContentBuilder.Append(string partialContent) {
temp += partialContent;
return this;
} #endregion }
… … //调用代码 IContentBuilder t = 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 public abstract class TextInput : Input, ISupportsMaxLength where T : TextInput public abstract class Input : FormElement where T : Input, Ielement 泛型是一种高层次的算法抽象,我们就通过Input一窥端倪:

public abstract class Input : FormElement where T : Input, 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, MemberExpression forMember, IEnumerable behaviors) : base(HtmlTag.Input, name, forMember, behaviors)
{
builder.MergeAttribute(HtmlAttribute.Type, type, true);
} /// /// Set the 'value' attribute. /// /// The value for the attribute.
public virtual T Value(object value)
{ elementValue = value;
return (T)this;
}
/// /// Set the 'size' attribute. /// /// The value for the attribute.
public virtual T Size(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IViewDataCon

namespace MvcContrib.FluentHtml {
/// /// Extensions to IViewDataContainer ///
public static class ViewDataContainerExtensions
{
/// /// Generate an HTML input element of type 'text' and set its value from ViewData based on the name provided. /// /// The view.
/// Value of the 'name' attribute of the element.Also used to derive the 'id' attribute.

public static TextBox TextBox(this IViewDataContainer view, string name)
{
return new TextBox(name).Value(view.ViewData.Eval(name));
} … … tainer 的Extension Method:

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.

FluentHtml与连续接口总结

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.