asp.net控件开发基础四
一.从继承WebControl开始
在第二篇教程中,重点介绍了Render()方法的使用,用来呈现控件,但从Control类继承的控件尚未发挥asp.net控件的作用.大家知道web服务器控件分为HTML服务器控件(如<input id="Button2" runat="server" type="button" value="button" />这样的形式)和标准服务器控件(就是<asp:.. id="" runat="server" />这样的形式的控件)
HTML服务器控件的控件从System.Web.UI.HtmlControls.HtmlControl 类派生
标准服务器控件的控件从System.Web.UI.WebControls.WebControl 类派生
HtmlControl 类和WebControl 类则从System.Web.UI.Control 类派生,并扩展.
所以我们说,所有的服务器控件都继承自System.Web.UI.Control 类,即所有的服务器控件都具有Control 类的共同属性,如Visible,EnableViewState属性,HtmlControl 类和WebControl 类则扩充了System.Web.UI.Control 类的功能,如
HtmlControl 类定义了所有 HTML 服务器控件所通用的方法、属性 (Property) 和事件(具体参数参照MSDN)
WebControl 类定义了所有 标准服务器控件所通用的方法、属性 (Property) 和事件(具体参数参照MSDN)
如每个继承了WebControl 类的标准控件都有定义外观和行为的属性,然后不同控件再根据需要扩展功能.
![](https://images.cnblogs.com/cnblogs_com/clingingboy/ServerControl/ch4/ch1.jpg)
图一
所以我们推荐的做法是直接从WebControl 类派生,而非Control类.我们所做的非并从头开始.从WebControl 类继承可以帮我们省很多工作.
二.重写WebControl类方法,不再是Render()
WebControl类继承了Control类,当然有Render方法,在WebControl类中重写了Render方法,如下代码
示例一
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注意 RebderBeginTag方法并非是HtmlTextWriter类中的方法,而是WebControl类中的方法,表示输出HTML标签头标记,如<table .....>,RenderEndTag方法则输出HTML标签尾标记,如</table>.中间的RenderContents方法则就是Control类的Render方法. 看下面RenderContents方法的定义.
示例二
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接着再看RenderBeginTag方法的定义
示例三
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这里打个比方,假设你要输出一个表格,你就必须定义<table>标签头,然后在其内部定义<tr>,<td>,下面看Control类中Render方法的实现,表明Render方法必须完成所有的任务,包括标签头标记<table>和<table>标签的属性和样式的输出.
示例四
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在WebControl类中重写了Render方法后,直接帮你定义好了标签,默认情况下为<span>,可通过重写TagKey属性来修改标签,然后AddAttributesToRender方法为标签定义样式和属性
示例五
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示例六
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接着重写RenderContents方法,对比上面的Render方法,实现效果是一样的
示例七
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所以说重写后的Render方法在默认情况下加了一个标签,而你可以重写这个标签(默认情况下为<span>).可能大家对其感到奇怪,即然可以实现同样效果,有必要重写Render方法,再加一个RenderContents方法吗?
三.Render方法和RenderContents方法的区别
当你继承WebControl类,在RenderContents方法中实现示例四代码时,(我们再以以前的例子展示)呈现后的代码如下,看到没有,控件ID为<span>
在RenderContents方法输入的标签将成为其内部标签.再看看这个控件的属性面板,你会看到很多的继承自WebControl类的属性,设置其属性,即是设置<span>标签的属性.TagKey的作用大概就在于此吧,为了使用WebControl类的公共属性.(我是这么认为的)而非定义在<table>标签上
示例八
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......
假设我们称<span>里面的<table>等标签为子标签,在RenderContents方法应该定义控件的子标签,如果你只定义标签属性的话,只需重写AddAttributesToRender方法即可,可不须重写RenderContents方法.
下面再讲下重写标签的方法
(1) 重写TagKey属性,下面重写label控件的标签
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(2)重写基类构造函数,此方法只有在继承Control类后适用
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最后总结下:
1.控件继承自WebControl类 主要原因是WebControl类公共的东西比Control类
2.TagKey 表示控件的标签,默认情况下为<span>,可以重写此属性修改或者重写WebControl类的构造函数
3.AddAttributesToRender方法 为标签添加属性和样式
4.RenderContents方法 在标签内呈现内容
如果控件不复杂,则可直接从标准控件继承(如label),再根据需要扩展,重写AddAttributesToRender方法,还可以重写
TagKey更改默认标签,而无须重写RenderContents方法.如果控件比较复杂,不是单一的,则需要在RenderContents方法输出控件的内部的内容.
其实最大的区别就是默认情况下WebControl类为你加了一个标签,方便添加WebControl类的一些公共的东西,如果你重写Render()方法,而舍弃RenderContents方法,你就无福享受WebControl类给你提供的这么多属性和方法了.
大家多熟悉下WebControl类的一些公共属性,然后再多改改,可以明白的更加深刻.
我喜欢慢慢的把东西全讲全,不然心里不舒服,所以我就慢慢写了,当然前提是我理解的基础上.这次的例子,大家可根据第二篇的代码适当修改就可.错误之处还请指出^_^
参考文章:http://dev.yesky.com/msdn/37/2416037.shtml
此文章转自:http://www.cnblogs.com/Clingingboy/archive/2006/08/05/468694.html