第二篇：呈现内容_第二节：WebControl呈现

一、WebControl的呈现过程

WebControl派生自Control类，所以WebControl的呈现功能基于Control的呈现逻辑之上，但有了比较大的扩展。

首先，WebControl重写了Render(HtmlTextWriter writer)方法，将呈现的逻辑一分为三：RenderBeginTag()、RenderContents()、RenderEndTag()。WebControl的这种设计基于一种假设：每个WebControl最终生成一个HTML控件（当然这个HTML控件中可能还包含其他HTML子控件），所以WebControl的呈现过程就可分为：呈现开始标签、呈现标签中的内容和呈现结尾标签。

protected internal override void Render(HtmlTextWriter writer)
{
       this.RenderBeginTag(writer);
       this.RenderContents(writer);
       this.RenderEndTag(writer);
}

①呈现开始标签：

public virtual void RenderBeginTag(HtmlTextWriter writer)
{
    this.AddAttributesToRender(writer);
    HtmlTextWriterTag htmlTextWriterTag = this.TagKey;
    if (htmlTextWriterTag != HtmlTextWriterTag.Unknown)
    {
        writer.RenderBeginTag(htmlTextWriterTag);
        return;
    }
    writer.RenderBeginTag(this.TagName);
}

由上面的实现代码可知：RenderBeginTag()方法所生成的HTML标签是由WebControl.TagKey或WebControl.TagName属性决定的。

[Browsable(false), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
protected virtual HtmlTextWriterTag TagKey
{
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    get
    {
        return this.tagKey;
    }
}

TagKey的类型为HtmlTextWriterTag 枚举，如果你的标签不在这个枚举中，则你需要把TagKey属性设为Unkown，并设置TagName属性，TagName属性为String类型，这意味着你可以将该属性设为任意值。

[Browsable(false), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
protected virtual string TagName
{
    get
    {
         if (this.tagName == null && this.TagKey != HtmlTextWriterTag.Unknown)
         {
              this.tagName = Enum.Format(typeof(HtmlTextWriterTag), this.TagKey, "G").ToLower(CultureInfo.InvariantCulture);
         }
         return this.tagName;
     }
}

• 注意：在通常情况下，我们不重写RenderBeginTag()方法，而通过重写TagKey或TagName属性来实现对生成的外围HTML标签的控制。

②呈现标签中的内容：

protected internal virtual void RenderContents(HtmlTextWriter writer)
{
    base.Render(writer);
}

③呈现结尾标签：

public virtual void RenderEndTag(HtmlTextWriter writer)
{
    writer.RenderEndTag();
}

到这里WebControl的呈现过程，都说完了，但有一个疑问是：最外围的HTML标签是怎么添加属性和样式属性的呢？

相信细心的同学一定发现在WebControl类RenderBeginTag()方法中第一行代码：

this.AddAttributesToRender(writer);

故名思议就是添加属性到呈现内容，我们看下该方法的实现：

protected virtual void AddAttributesToRender(HtmlTextWriter writer)
{
    if (this.ID != null)
    {
        writer.AddAttribute(HtmlTextWriterAttribute.Id, this.ClientID);
    }
    if (this._webControlFlags[4])
    {
        string accessKey = this.AccessKey;
        if (accessKey.Length > 0)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Accesskey, accessKey);
        }
    }
    if (!this.Enabled)
    {
        if (this.SupportsDisabledAttribute)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Disabled, "disabled");
        }
        if (this.RenderingCompatibility >= VersionUtil.Framework40 && !string.IsNullOrEmpty(WebControl.DisabledCssClass))
        {
            if (string.IsNullOrEmpty(this.CssClass))
            {
                this.ControlStyle.CssClass = WebControl.DisabledCssClass;
            }
            else
            {
                this.ControlStyle.CssClass = WebControl.DisabledCssClass + " " + this.CssClass;
            }
        }
    }
    if (this._webControlFlags[16])
    {
        int tabIndex = (int)this.TabIndex;
        if (tabIndex != 0)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Tabindex, tabIndex.ToString(NumberFormatInfo.InvariantInfo));
        }
    }
    if (this._webControlFlags[8])
    {
        string toolTip = this.ToolTip;
        if (toolTip.Length > 0)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Title, toolTip);
        }
    }
    if (this.TagKey == HtmlTextWriterTag.Span || this.TagKey == HtmlTextWriterTag.A)
    {
        this.AddDisplayInlineBlockIfNeeded(writer);
    }
    if (this.ControlStyleCreated && !this.ControlStyle.IsEmpty)
    {
        this.ControlStyle.AddAttributesToRender(writer, this);
    }
    if (this.attrState != null)
    {
        AttributeCollection attributes = this.Attributes;
        IEnumerator enumerator = attributes.Keys.GetEnumerator();
        while (enumerator.MoveNext())
        {
            string text = (string)enumerator.Current;
            writer.AddAttribute(text, attributes[text]);
        }
    }
}

首先，AddAttributesToRender()是一个虚方法，我们可以重写，又因为它在RenderBeginTag()开始时被调用，很显然我们可以将最外围的HTML标签添加属性和样式属性的逻辑在这里实现。其次，我们注意到WebControl类实现RenderBeginTag()方法时完成了（例如：Enabled与否，不同呈现）等基本的功能。所以在我们重写该方法为外围标签添加自定义属性和样式属性时，为了获得WebControl提供的基本功能。需在派生的控件类中调用父类的该方法：

protected override void AddAttributesToRender(HtmlTextWriter writer)
{
     base.AddAttributesToRender(writer);
     //......添加所需属性和样式的代码
}

二、从WebControl类派生"相册"控件

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace CustomServerControls
{
    public class Albumn : WebControl
    {
        //（一）：WebControl默认TagKey为span，这里重写为Div
        protected override HtmlTextWriterTag TagKey
        {
            get
            {
                return HtmlTextWriterTag.Div;
            }
        }
        //（二：）为主标签（Div）添加一些样式属性
        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            base.AddAttributesToRender(writer);
            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");
        }
        //（三）：主标签的内容项是一个Img标签
        protected override void RenderContents(HtmlTextWriter writer)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Src, "Images/nature.jpg");
            writer.AddAttribute(HtmlTextWriterAttribute.Width, "160px");
            writer.AddAttribute(HtmlTextWriterAttribute.Height,"160px");
            writer.AddStyleAttribute(HtmlTextWriterStyle.BorderStyle, "none");
            writer.AddStyleAttribute(HtmlTextWriterStyle.Padding, "0px");
            writer.AddStyleAttribute(HtmlTextWriterStyle.MarginTop, "16px");
            writer.RenderBeginTag(HtmlTextWriterTag.Img);
            writer.RenderEndTag();
        }
    }
}