第二篇：呈现内容_第一节：Control呈现

一、Control的呈现过程

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。

1）控件树呈现的“合成模式（Composite）”

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。

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);
        }
    }
}

2）控件树呈现的简单模型

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);
        }
    }
}

二、从Control类派生"相册"控件

1）“相册”控件初实现

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;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>");
        }
    }
}

2）HtmlTextWriter的四个方法

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)

• 值得注意的是：当需要向一个标签添加属性和样式属性时，在调用生成HTML其实标签的RenderBeginTag()方法之前先调用AddAttribute()方法和AddStyleAttribute()方法添加所有必要的属性和样式属性。

3）“相册”控件再实现

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();
        }
    }
}