Adapting a Custom Html Helper for Razor (it uses HtmlTextWriter so returns void)

问题

The Problem

I have a very nifty menu Html helper written for WebFormViewEngine views. This engine allows your helpers to return void, and still be able to use:

@Html.Theseus

This is great for my helper, because it can then render the menu using HtmlTextWriter, that renders directly to the output stream.

In Razor views, however, the Html helpers are expected to return a value (usually MvcHtmlString) which is what gets added to the output. Small difference, big consequence.

There is a way around this, as pointed out to me by GvS (see ASP.NET MVC 2 to MVC 3: Custom Html Helpers in Razor) as follows:

If the helper returns void, then do the following:

@{Html.Theseus;}

(Essentially, you are just calling the method, rather than rendering into the view).

Whilst still neat, this is not quite the same as @Html.Theseus. So...

My code is complex but works very well, so am loath to go through major edits, ie, replacing the HtmlTextWriter with another writer. A snippet of the code goes like:

writer.AddAttribute(HtmlTextWriterAttribute.Href, n.Url);
writer.AddAttribute(HtmlTextWriterAttribute.Title, n.Description);
writer.RenderBeginTag(HtmlTextWriterTag.A);
writer.WriteEncodedText(n.Title);
writer.RenderEndTag();

// Recursion, if any
// Snip off the recursion at this level if specified by depth
// Use a negative value for depth if you want to render the entire sitemap from the starting node

    if ((currentDepth < depth) || (depth < 0))
    {
         if (hasChildNodes)
         {
              // Recursive building starts here

              // Open new ul tag for the child nodes 
              // "<ul class='ChildNodesContainer {0} Level{1}'>"; 
              writer.AddAttribute(HtmlTextWriterAttribute.Class, "Level" + currentDepth.ToString());
              writer.RenderBeginTag(HtmlTextWriterTag.Ul);

              // BuildMenuLevel calls itself here to 
              // recursively traverse the sitemap hierarchy, 
              // building the menu as I go.
              // Note: this is where I increase the currentDepth variable!
               BuildChildMenu(currentDepth + 1, depth, n, writer);

              // Close ul tag for the child nodes
              writer.RenderEndTag();
          }
    }

It wouldn't be fun to re write with TagBuilders. As it stands, it renders any type of menu, including the "Incremental Navigation" as described in my 4guysfromrolla article: Implementing Incremental Navigation with ASP.NET

The Options:

I guess I could return an empty MvcHtmlString, but that is pretty much the definition of a hack...

The only alternative is to head off into the sunset and rewrite the helper using the TagBuilder to build each tag, add that to a StringBuilder, then build the next tag, etc, and then use the StringBuilder instance to create the MvcHtmlString. Seriously ugly, unless I could do something like...

The Question:

Is there a way to:

Stop the HtmlTextWriter rendering to the stream and instead use it like a StringBuilder that at the end of the process I use to create an MvcHtmlString (or HtmlString)?

Sounds unlikely, even as I write...

PS:

The great thing about the HtmlTextWriter is that you can build large quantities of tags, instead of building them one by one as with a TagBuilder.

回答1:

Contrary to the responses you received for your other question Razor does not require that you return an HtmlString. The problem with your code right now is that you are writing directly to the response stream. Razor executes things inside-out which means that you can mess up the response order (see a similar question).

So in your case you could probably do this (though i haven't tested it):

public static void Theseus(this HtmlHelper html)
{
    var writer = new HtmlTextWriter(html.ViewContext.Writer);
    ...
}

Edit (follow up to address your comments):

Html Helpers are perfectly capable of either returning a HtmlString directly or void and writing to the context writer. For example, both Html.Partial and Html.RenderPartial work fine in Razor. I think what you are confusing is the syntax required to call one version and not the other.

For example, consider an Aspx view:

<%: Html.Partial("Name") %>
<% Html.RenderPartial("Name") %>

You call each method differently. If you flip things around, things will just not work. Similarly in Razor:

@Html.Partial("Name")
@{ Html.RenderPartial("Name"); }

Now it just so happens that the syntax to use a void helper is a lot more verbose in Razor compared to Aspx. However, both work just fine. Unless you meant something else by "the issue is with a html helper not being able to return void".

By the way, if you really want to call your helper using this syntax: @Html.Theseus() you could do this:

public static IHtmlString Theseus(this HtmlHelper html)
{
    var writer = new HtmlTextWriter(html.ViewContext.Writer);
    ...
    return new HtmlString("");
}

But that's a bit of a hack.

来源：https://stackoverflow.com/questions/4217611/adapting-a-custom-html-helper-for-razor-it-uses-htmltextwriter-so-returns-void

标签

asp.net-mvc

html-helper

razor