Mikel
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:
<%= this.TextBox(x => x.Person.Name).Title("Enter the person's name").Label("Name:") %><br /> … … <%= this.Select(x => x.Person.Gender).Options(Model.Genders).Size(5).Label("Gender:") .Title("Select the person's gender") %><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="Enter the person's name" name="Person.Name" maxlength="50"/>
.<select id="Person_Gender" title="Select the person's gender" 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:
public StringBuilder Append(string value)
{
if (value != null)
{
string stringValue = this.m_StringValue; IntPtr currentThread = Thread.InternalGetCurrentThread(); if (this.m_currentThread != currentThread) { stringValue = 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:
{
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<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:
{
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<IBehaviorMarker> behaviors)
: base(HtmlTag.Input, name, forMember, behaviors)
{
builder.MergeAttribute(HtmlAttribute.Type, type, true);
}
/// <summary>
/// Set the 'value' attribute.
/// </summary>
/// <param name="value">The value for the attribute.</param>
public virtual T Value(object value)
{
elementValue = value;
return (T)this;
}
/// <summary>
/// Set the 'size' attribute.
/// </summary>
/// <param name="value">The value for the attribute.</param>
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IViewDataContainer 的Extension Method:
{
/// <summary>
/// Extensions to IViewDataContainer
/// </summary>
public static class ViewDataContainerExtensions
{
/// <summary>
/// Generate an HTML input element of type 'text' and set its value from ViewData based on the name provided.
/// </summary>
/// <param name="view">The view.</param>
/// <param name="name">Value of the 'name' attribute of the element. Also used to derive the 'id' attribute.</param>
public static TextBox TextBox(this IViewDataContainer view, 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.
