在上个章节““生死有序”的控件生命周期”中，我们提到Render是控件开发的主角，但在控件树的“合成模式（Composite）”部分这位主角却缺席了（戏份太多的缘由）。哦，好吧。主角现在登场。
控件树的呈现过程是一个华丽的大圈，它从RenderControl(HtmlTextWriter writer)开始、从RenderChildrenInternal(HtmlTextWriter writer, ICollection children)结束。其过程涉及Control类的6个方法。期间种种，我们慢慢道来。
monospace; width: 100%; border-bottom-style: none; color: black; padding-bottom: 0px; direction: ltr; text-align: left; padding-top: 0px; border-right-style: none; padding-left: 0px; margin: 0em; border-left-style: none; line-height: 12pt; padding-right: 0px; background-color: white">public virtual void RenderControl(HtmlTextWriter writer);
protected void RenderControl(HtmlTextWriter writer, ControlAdapter adapter);
private void RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter);
protected internal virtual void Render(HtmlTextWriter writer);
protected internal virtual void RenderChildren(HtmlTextWriter writer);
internal void RenderChildrenInternal(HtmlTextWriter writer, ICollection children);
①Page中RenderControl()的调用：故事开始的地方
我们知道，Page类实现了IHttpHandler接口，所以ASP.NET框架得以最终把对请求进行响应的任务通过调用Page的ProcessRequest()方法交给aspx页面。在Page执行ProcessRequest()方法处理请求时，它完成了大量的工作：维持状态、处理回传数据、处理事件等等，而最后一个环节是基于HttpContext中Response.Output流创建HtmlTextWrite，并调用Page从Control类那里继承来的RenderControl()方法把页面内容发送给请求者。
//......呈现的入口this.RenderControl(this.CreateHtmlTextWriter(this.Response.Output));②RenderControl(HtmlTextWriter writer)：
[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]public virtual void RenderControl(HtmlTextWriter writer)
{this.RenderControl(writer, this.Adapter);
}
③RenderControl(HtmlTextWriter writer, ControlAdapter adapter)：
protected void RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
{if (this.flags[16] || this.flags[512]) //if(this.Visible == false):详见下面附的代码
{ this.TraceNonRenderingControlInternal(writer); return;}
HttpContext httpContext = (this.Page == null) ? null : this.Page._context;
if (httpContext != null && httpContext.TraceIsEnabled)
{ int bufferedLength = httpContext.Response.GetBufferedLength(); this.RenderControlInternal(writer, adapter); int bufferedLength2 = httpContext.Response.GetBufferedLength(); httpContext.Trace.AddControlSize(this.UniqueID, bufferedLength2 - bufferedLength); return;}
this.RenderControlInternal(writer, adapter);}
[Bindable(true), DefaultValue(true), WebCategory("Behavior"), WebSysDescription("Control_Visible")]
public virtual bool Visible
{get
{return !this.flags[16] && (this._parent == null || this.DesignMode || this._parent.Visible);
}
set
{if (this.flags[2])
{bool flag = !this.flags[16];
if (flag != value)
{ this.flags.Set(32);}
}
if (!value)
{ this.flags.Set(16); return;}
this.flags.Clear(16);}
}
private const int invisible = 16;
private const int notVisibleOnPage = 512;
④RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)：
private void RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
{ try {this.BeginRenderTracing(writer, this);
if (adapter != null) //控件是否有相关的呈现适配器
{ //如果有，呈现适配器调用相关的呈现方法呈现控件adapter.BeginRender(writer);
adapter.Render(writer);
adapter.EndRender(writer);
}
else { //如果没有，使用控件类本身的呈现方法呈现控件 this.Render(writer);}
}
finally {this.EndRenderTracing(writer, this);
}
}
⑤Render(HtmlTextWriter writer)：
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]protected internal virtual void Render(HtmlTextWriter writer)
{ this.RenderChildren(writer);}
⑥RenderChildren(HtmlTextWriter writer)：
protected internal virtual void RenderChildren(HtmlTextWriter writer)
{ ICollection controls = this._controls; this.RenderChildrenInternal(writer, controls);}
⑦RenderChildrenInternal(HtmlTextWriter writer, ICollection children)：
internal void RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
{if (this.RareFields != null && this.RareFields.RenderMethod != null)
{writer.BeginRender();
this.RareFields.RenderMethod(writer, this);
writer.EndRender();
return;}
if (children != null) //如果子控件的集合不为空，继续递归；若为空，结束递归。
{foreach (Control control in children)
{control.RenderControl(writer);
}
}
}
上面我们分析了Control利用“合成模式”递归生成控件树的全过程，也注意到Control类用于呈现的6个方法中有三个是虚方法，它们是开发控件我们可重写改变呈现逻辑的部分。首先，我们用伪代码概述下Control类中这三个方法呈现控件的模型。
public virtual void RenderControl(HtmlTextWriter writer)
{if (this.Visible)
{ this.Render(writer);}
}
protected internal virtual void Render(HtmlTextWriter writer)
{--><div...
this.RenderChildren(writer);--></div>
}
protected internal virtual void RenderChildren(HtmlTextWriter writer)
{if (this._controls != null)
{foreach (Control control in this._controls)
{control.RenderControl(writer);
}
}
}
通过上面的分析，很显然我们需要重写Render()方法的呈现逻辑，以输出我们期待呈现的内容。对，你一定还记得初始ASP.NET控件开发中的“HelloWorld”吧？
那你应该能写出这样的代码：
using System;using System.Collections.Generic;using System.ComponentModel;using System.Text;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;namespace CustomServerControls{public class Albumn : Control
{protected override void Render(HtmlTextWriter writer)
{writer.WriteLine(@"<div style=""text-align:center;width:194px;height:194px;background:url(Images/background.gif) no-repeat left"">");
writer.WriteLine(@" <img src=""Images/Nature.jpg"" width=""160"" height=""160"" style=""border:none;padding:0px;margin-top:16px;"">");
writer.WriteLine(@"</div>");}
}
}
上面，我们已经实现了一个很不完善的“相册”控，勉强实现了HTML代码的输出。但这样用字符串来组织输出内容我们不能利用IDE的智能感知功能，也不能在编码时捕获错误，更谈不上不同浏览器生成不同的HTML代码。哪应该怎样呢？其实，HtmlTextWriter类已经封装了很多生成HTML代码的方法。下面介绍四个最常用的：
①RenderBeginTag():生成HTML起始标签
public virtual void RenderBeginTag(HtmlTextWriterTag tagKey);
public virtual void RenderBeginTag(string tagName);
②RenderEndTag():生成HTML结束标签
public virtual void RenderEndTag();
③AddAttribute():为HTML标签添加属性
public virtual void AddAttribute(string name, string value);
public virtual void AddAttribute(string name, string value, bool fEndode);
public virtual void AddAttribute(HtmlTextWriterAttribute key, string value);
public virtual void AddAttribute(HtmlTextWriterAttribute key, string value, bool fEncode);
④AddStyleAttribute():为HTML标签添加样式属性
public virtual void AddStyleAttribute(string name, string value);
public virtual void AddStyleAttribute(HtmlTextWriterStyle key, string value)
using System;using System.Collections.Generic;using System.ComponentModel;using System.Text;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;namespace CustomServerControls{public class Albumn : Control
{protected override void Render(HtmlTextWriter writer)
{ //最外层DIV的样式属性 writer.AddStyleAttribute(HtmlTextWriterStyle.TextAlign, "center"); writer.AddStyleAttribute(HtmlTextWriterStyle.Width, "194px"); writer.AddStyleAttribute(HtmlTextWriterStyle.Height, "194px");writer.AddStyleAttribute("background", "url(Images/background.gif) no-repeat left");
//最外层的Div开始writer.RenderBeginTag(HtmlTextWriterTag.Div);
//IMG标签的属性和样式属性 writer.AddAttribute(HtmlTextWriterAttribute.Src, "images/nature.jpg"); writer.AddAttribute(HtmlTextWriterAttribute.Width, "160"); writer.AddAttribute(HtmlTextWriterAttribute.Height, "160"); writer.AddStyleAttribute(HtmlTextWriterStyle.BorderStyle, "none"); writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, "0px"); writer.AddStyleAttribute(HtmlTextWriterStyle.MarginTop, "16px"); //生成Img标签writer.RenderBeginTag(HtmlTextWriterTag.Img);
writer.RenderEndTag();
//最外层DIV结束writer.RenderEndTag();
}
}
}
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