DO NOT REPLY [Bug 32638] New: - Enhancement to to support new feature.

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http://issues.apache.org/bugzilla/show_bug.cgi?id=32638

           Summary: Enhancement to <macrodef> to support new feature.
           Product: Ant
           Version: 1.6.2
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Core
        AssignedTo: dev@ant.apache.org
        ReportedBy: ddevienne@lgc.com


>From the email 'About <macrodef> in the trenches' I sent out today:

This past two weeks, I've worked on coming up with a generic build file
for our multiple projects, which I'd say are 80% to 99% common from one
project to the next. Of course, I'm using heavily <import>, target
overriding, and <macrodef>, but even with these fine features, it's not
always easy to accomodate the variations from one build to the next, 
especially if I want to avoid having to rewrite whole tasks/targets.

To achieve this maximum reuse, and minimum overriding, i.e. to avoid being
forced to rewrite whole tasks, I've defined complex macros with built-in
conditionals, and lots of default attributes. Alas, there's no default
'value' for macro elements, so providing default tags for a macro element
is currently not possible. I believe this lack of element default makes
<macrodef> less powerful that it can be. 

So finally today I had a look at the macrodef code, and tried to understand
how things worked under the hood. I was surprised to find out I understood
the code ;-) (if I glossed over most of the UE/RC cuisine that is).

  BTW, I'm amazed this can all be implemented in a couple of tasks with no
  changes to the framework itself. The new fully dynamic nature of Ant 1.6
  (everything's a UE) is powerful indeed!

I then proceeded to copy MacroDef/MacroInstance to my own antlib, and
amazingly after repackaging and adding a single import, everything worked
fine. (I duplicate the code because we only use an official Ant release.)
I then tweaked <macrodef> to add the following features:

1) Everytime a macro attribute or element is explicitly defined in a macro
   instance (where the macro is used), I define an additional macro attribute
   (local property in the code) which allows to find out in the macro body
   whether the attribute or element was explicitly used/specified.

   When an 'foo' attribute is used, I define '@foo?' with value of true.
   When an 'bar' element is used, I define 'bar?' with value of true

   My macro bodies/impls then do things or not (or differently) based on
   this info. For example:

     <bm:macrodef name="my-copy">
      ...
      <attribute name="tos" default="/dev/null" />
      <sequential>
        ...
        <bm:sequential ifTrue="@{@tos?}">
          <copy file="@{tos}" tofile="..." />
        </bm:sequential>
      </sequential>
    </bm:macrodef>

    <bm:macrodef name="my-module-image">
      ...
      <element name="sources" optional="true" />
      <sequential>
        ...
        <bm:sequential ifTrue="@{@sources?}">
          <zip destfile="@{todir}/sdk/src.zip">
            <sources />
          </zip>
        </bm:sequential>
      </sequential>
    </bm:macrodef>

2) Allow defining an macro element 'default value'. Instead of the default
   being inside the <macrodef> <element>, its inside the macro <sequential>
   where the element is used. I did it this way because the implementation
   was easier, and it makes it easy to read the macro impl, although it
   could be confusing to some I guess?!

   If a macro element is not explicitly specified in the macro instance,
   and the element is optional, then the 'default value' from the macro
   definition is used. If specified, the macro instance value is used as
   usual otherwise. For example:

    <bm:macrodef name="my-register">
      ...
      <element name="patterns" optional="true" />
      <sequential>
        <ds:register ...>
          ...
          <fileset dir="@{dir}">
            <patterns>
              <include name="**/*.class" />
              <exclude name="**/test/*Test.class" />
            </patterns>
          </fileset>
        </ds:register>
      </sequential>
    </bm:macrodef>

    The macro above defaults the patterns element to 1 include + 1 exclude.

3) When coming up with elements a macro could contain, I often end up
   wanting to contain a classpath macro element that I want to pass in
   to <java> or another task taking a classpath. If I can't use the single
   implicit element for some reason, I then have to do something ugly:

   <macrodef name="ugly">
     ...
     <element name="my-classpath" optional="true" />
     <sequential>
       ...
       <java ...>
         <my-classpath />
       </java>
     </sequential>
   </macrodef>

  and I'm forced to use:

  <ugly>
    <my-classpath>
      <classpath refid="classpath" />
    </my-classpath>
  </ugly>

  If I throw in element defaults from (2), the macro becomes:

   <macrodef name="ugly">
     ...
     <element name="my-classpath" optional="true" />
     <sequential>
       ...
       <java ...>
         <my-classpath>
           <classpath refid="classpath" />
         </my-classpath>
       </java>
     </sequential>
   </macrodef>

  To work around this, I added a new useContentOnly attribute to <element>,
  which defaults to true for BC, but when false, allows to pass in the element
  itself as-is, as defined in the macro definition or instance. I can now do

   <macrodef name="nicer">
     ...
     <element name="classpath" optional="true" useContentOnly="false" />
     <sequential>
       ...
       <java ...>
         <classpath refid="classpath" />
       </java>
     </sequential>
   </macrodef>

  and I can use it as

  <nicer />

  or

  <nicer>
    <classpath refid="alt.classpath" />
  </nicer>

  or 

  <nicer>
    <classpath>
      <pathelement location="..." />
      <path refid="classpath" />
    </classpath>
  </nicer>

  With (2) + (3), you can take some ant code composed of 3 tasks for example,
  put it in a macro, and define an optional element for each task with
  useContentOnly="false", and have the macro user be able to override only
  one of the macro's task.

  To be truly complete, we'd also need a way to refer to the default content
  of the element to reuse it in the macro instance itself (i.e super).
  I haven't done that.

4) Finally, the last thing I did was to allow using the macro attributes
   in the macro instance, to benefit from the default values computed by
   the macro. Currently, if one defines:

   <macrodef name="macrosub">
     <attribute name="primary" default="foo" />
     <attribute name="secondary" default="@{primary}bar" />
     <element name="nested-elements" optional="true" implicit="true" />
     <sequential>
       <nested-elements />
     </sequential>
   </macrodef>

   and does:

   <macrosub>
     <echo>secondary = @{secondary}</echo>
   </macrosub>

   One will not get secondary = foobar, because the macro instance does
   not have access to the macro 'local properties', which includes the
   computed default attributes. I consider 'secondary' to be part of the
   macro API, and thus logic that it can be used in the macro instance.
   Implementation wise, it means using copy() more often.

My changes are not extensive, and are mostly in MacroInstance#copy and
the addition of MacroDef.TemplateElement#setUseContentOnly.

I'd appreciate some feedback on whether these new features are desirable.
I believe there would be zero BC issues. Thanks, --DD

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