将HTML转成XHTML并清除一些无用的标签和属性
2018-07-20 来源：编程学习网
介绍
这是一个能帮你从HTML生成有效XHTML的经典库。它还提供对标签以及属性过滤的支持。你可以指定允许哪些标签和属性可在出现在输出中，而其他的标签过滤掉。你也可以使用这个库清理Microsoft Word文档转化成HTML时生成的臃肿的HTML。你也在将HTML发布到博客网站前清理一下，否则像WordPress、b2evolution等博客引擎会拒绝的。
它是如何工作的
里面有两个类：HtmlReader和HtmlWriter
HtmlReader拓展了著名的由Chris Clovett开发的SgmlReader。当它读取HTML时，它跳过所有有前缀的节点。其中，所有像<o:p>、<o:Document>、<st1:personname>等上百的无用标签被滤除了。这样你读取的HTML就剩下核心的HTML标签了。
HtmlWriter拓展了常规的XmlWriter，XmlWriter生成XML。XHTML本质上是XML格式的HTML。所有你熟悉使用的标签——比如<img>、<br>和<hr>，都不是闭合的标签——在XHTML中必需是空元素形式，像<img .. />、<br/>和<hr/>。由于XHTML是常见的XML格式，你可以方便的使用XML解析器读取XHTML文档。这使得有了应用XPath搜索的机会。
HtmlReader
HtmlReader很简单，下面是完整的类：
////// This class skips all nodes which has some
/// kind of prefix. This trick does the job
/// to clean up MS Word/Outlook HTML markups.
///public class HtmlReader : Sgml.SgmlReader
{
public HtmlReader( TextReader reader ) : base( )
{
base.InputStream = reader;
base.DocType = "HTML";
}
public HtmlReader( string content ) : base( )
{
base.InputStream = new StringReader( content );
base.DocType = "HTML";
}
public override bool Read()
{
bool status = base.Read();
if( status )
{
if( base.NodeType == XmlNodeType.Element )
{
// Got a node with prefix. This must be one
// of those "" or something else.
// Skip this node entirely. We want prefix
// less nodes so that the resultant XML
// requires not namespace.
if( base.Name.IndexOf(':') > 0 )
base.Skip();
}
}
return status;
}
}
HtmlWriter
这个类是有点麻烦。下面是使用技巧：
-
重写WriteString方法并避免使用常规的XML编码。对HTML文件手动更改编码。
-
重写WriteStartElementis以避免不被允许的标签写到输出中。
-
重写WriteAttributesis以避免不需求的属性。
让我们分部分来看下整个类：
可配置性
你可以通过修改下面的部分配置HtmlWriter：
public class HtmlWriter : XmlTextWriter
{
////// If set to true, it will filter the output
/// by using tag and attribute filtering,
/// space reduce etc
///public bool FilterOutput = false;
////// If true, it will reduce consecutive with one instance
///public bool ReduceConsecutiveSpace = true;
////// Set the tag names in lower case which are allowed to go to output
///public string [] AllowedTags =
new string[] { "p", "b", "i", "u", "em", "big", "small",
"div", "img", "span", "blockquote", "code", "pre", "br", "hr",
"ul", "ol", "li", "del", "ins", "strong", "a", "font", "dd", "dt"};
////// If any tag found which is not allowed, it is replaced by this tag.
/// Specify a tag which has least impact on output
///public string ReplacementTag = "dd";
////// New lines \r\n are replaced with space
/// which saves space and makes the
/// output compact
///public bool RemoveNewlines = true;
////// Specify which attributes are allowed.
/// Any other attribute will be discarded
///public string [] AllowedAttributes = new string[]
{
"class", "href", "target", "border", "src",
"align", "width", "height", "color", "size"
};
}
WriteString方法
////// The reason why we are overriding
/// this method is, we do not want the output to be
/// encoded for texts inside attribute
/// and inside node elements. For example, all the
/// gets converted to   in output. But this does not
/// apply to HTML. In HTML, we need to have as it is.
//////public override void WriteString(string text)
{
// Change all non-breaking space to normal space
text = text.Replace( " ", " " );
/// When you are reading RSS feed and writing Html,
/// this line helps remove those CDATA tags
text = text.Replace("", "");
// Do some encoding of our own because
// we are going to use WriteRaw which won't
// do any of the necessary encoding
text = text.Replace( "<", "<" );
text = text.Replace( ">", ">" );
text = text.Replace( "'", "'" );
text = text.Replace( "\"", ""e;" );
if( this.FilterOutput )
{
text = text.Trim();
// We want to replace consecutive spaces
// to one space in order to save horizontal width
if( this.ReduceConsecutiveSpace )
text = text.Replace(" ", " ");
if( this.RemoveNewlines )
text = text.Replace(Environment.NewLine, " ");
base.WriteRaw( text );
}
else
{
base.WriteRaw( text );
}
}
WriteStartElement: 应用标签过滤
public override void WriteStartElement(string prefix,
string localName, string ns)
{
if( this.FilterOutput )
{
bool canWrite = false;
string tagLocalName = localName.ToLower();
foreach( string name in this.AllowedTags )
{
if( name == tagLocalName )
{
canWrite = true;
break;
}
}
if( !canWrite )
localName = "dd";
}
base.WriteStartElement(prefix, localName, ns);
}
WriteAttributes方法: 应用属性过滤
bool canWrite = false;
string attributeLocalName = reader.LocalName.ToLower();
foreach( string name in this.AllowedAttributes )
{
if( name == attributeLocalName )
{
canWrite = true;
break;
}
}
// If allowed, write the attribute
if( canWrite )
this.WriteStartAttribute(reader.Prefix,
attributeLocalName, reader.NamespaceURI);
while (reader.ReadAttributeValue())
{
if (reader.NodeType == XmlNodeType.EntityReference)
{
if( canWrite ) this.WriteEntityRef(reader.Name);
continue;
}
if( canWrite )this.WriteString(reader.Value);
}
if( canWrite ) this.WriteEndAttribute();
结论
示例应用是一个你可以立即用来清理HTML文件的实用工具。你可以将这个类应用在像博客等需要发布一些HTML到Web服务的工具中。
原文地址：http://www.codeproject.com/Articles/10792/Convert-HTML-to-XHTML-and-Clean-Unnecessary-Tags-a
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