I've been asked lately about the use of the ViewFilter mechanism in AspView.

I've once written about it briefly here on my blog, and you can see it at http://www.kenegozi.com/Blog/2007/01/08/introducing-viewfilters.aspx

However, I'll post another (a bit more realistic) example here.

Scenario: some kind of a CMS thing. You want to present the user with some markup, in both "preview" mode and "Source" modes.

If the server had direct access to the markup in a string literal, things were easy. That usually happens when the markup is to be supplied by an end user, either directly or through a WYSIWYG Html editor. You'd end up with something like:

public interface IContentItem
{
    public string Markup { get; }
}

your view would look like:

...
<h3>Preview:</h3>
<div><%=view.ContentItem.Markup %> </div>




<h3>Source:</h3>
<div><%=Helpers.Html.HtmlEncode(view.ContentItem.Markup) %> </div>
...

easy enough.

However, what if the piece of markup that you want to show, has some view-logic, so you have a template generating the markup from an entity? For example, this blog has a view "Posts/One" that gets a Post entity, and fits it into a single post markup, putting the title in a <h4>, tags in <span> with theit title and href, etc.

How can you show the markup source for that?

ViewFilter to the rescue.

In short - A ViewFilter is a way to transform a chunk of a view, using simple manipulations. Do not look for that on other View Engines, as it's currently an AspView-only feature.

Let's code our needed filter:

public class HtmlEncodeViewFilter : IViewFilter
{
    public string ApplyOn(string input)
    {
        return HttpUtility.HtmlEncode(input);
    }
}

and in the view:

...
<% foreach (Post post in view.Posts)  { %>
<h3>Preview:</h3>
<subview:.Posts.One post="<%=post %>" > </subview:.Posts.One>

<h3>Source:</h3>
<filter:HtmlEncode>
    <subview:.Posts.One post="<%=post %>" > </subview:.Posts.One>
</filter:HtmlEncode>
<% } %>

Hey - you won't even need to create that filter. AspView is supplied with four basic (however useful) filters:

• HtmlDecodeViewFilter
• HtmlEncodeViewFilter
• LowerCaseViewFilter
• LowerCaseViewFilter

Can you think of more reusable view filters? why not post them here, or better yet, supply a patch to AspView with you filters?