Re: Need help on write my custom velocity directive

[Nathan Bubna <nb...@gmail.com>]

On 7/26/07, 于世英 <yu...@cgsoft.com.cn> wrote:
> Hi all,
> I'm going to writing some directives to extend Velocity.  now I'm in lacking
> of documents.
> I've read the PPT 《Hacking Velocity》by Will Glass-Husain, and some souce
> code of system directive such as "literal", "execute", but I don't
> understand "foreach" till now.
> I've written the following code(Mydd.java) and it can work as coded now:
> -------------------file begin ------------------------------
> import java.io.IOException;
> import java.io.Writer;
> import java.util.Iterator;
>
> import org.apache.velocity.context.InternalContextAdapter;
> import org.apache.velocity.exception.MethodInvocationException;
> import org.apache.velocity.exception.ParseErrorException;
> import org.apache.velocity.exception.ResourceNotFoundException;
> import org.apache.velocity.runtime.directive.Directive;
> import org.apache.velocity.runtime.RuntimeServices;
> import org.apache.velocity.runtime.parser.node.Node;
> import org.apache.velocity.exception.TemplateInitException;
> import org.apache.log4j.Logger;
>
> public class Mydd extends Directive {
>
>         private static Logger logger = Logger.getLogger(Mydd.class);
>
>     public void init(RuntimeServices rs, InternalContextAdapter context,
> Node node)
>         throws TemplateInitException
>     {
>         super.init(rs, context, node);
>
>     }
>         @Override
>         public String getName() {
>                 return "mydd";
>         }
>
>         @Override
>         public int getType() {
>
>                 return LINE;
>         }
>
>     public boolean render(InternalContextAdapter context,
>                            Writer writer, Node node) throws IOException
>     {
>         for (int i = 0; i < node.jjtGetNumChildren(); i ++) {
>                 logger.debug("debug mydd parameter: " + node.jjtGetChild(i).
> toString() + " (" + node.jjtGetChild(i).literal() + ")");
>         }
>         writer.write("Hello, world!");
>         return true;
>     }
> }
> -------------------file end ----------------------------------
>
> What I want to know is:
> 1, the above directive's syntax in template is
> #mydd(arg1 arg2 arg3 ...)
> but system directives like "foreach" and "set" have COOL syntax like
> "#foreach ($a in $set)", how to let my directive support syntax like this?

IIRC, those core directives all have special privileges via parsing
code and other stuff.  You'll probably have to modify the JavaCC
grammar and rebuild the velocity.jar to achieve similar results.

> 2, does the directive mechanism provided Velocity has other convenience or
> power that I don't know ? what I know about  is all in the above code :-)

i'm not sure.  Will might know better than me on this.  You can also
explore what other custom directives do for ideas.  I believe there
are a few in the contributed code of the wiki and in the whiteboard
area of the source (via either SVN or the source distro).  Also,
Struts 2 uses custom directives pretty extensively.  With direct
access to the AST (via the Node), i'm sure there's quite a lot that
can be done.

>
> Thanks in advance, any hint will be appreciated!
>
>
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