Allow flushing collectors to only emit when complete.

[blackwinter]

Primarily needed for EqualsFilter but may also be useful as a generic option for flushing collectors:

• EqualsFilter: Use case described below.
• Combine, Entity: No actual use case yet.
• All: Already built-in (as an internal condition in emit()).
• Choose, Concat, Group, Range, Square, Tuples: Inherently "incomplete" (isComplete() always false).

When transforming Alma publishing data, we need to select fields based on some other field's subfield value:

<combine name="date" value="${value}" sameEntity="true">
  <if>
    <equalsFilter name="" value="" flushWith="POC  .a" reset="true">
      <data name="" source="@portfolio" />
      <data name="" source="POC  .a" />
    </equalsFilter>
  </if>
  <data name="value" source="POC  .b" />
</combine>

But this fires (emit()s) on every flush, even if the target value (@portfolio) wasn't received (it's only populated at a later point in the stream). Having the option to restrict firing to only when the collector isComplete(), fixes the issue:

<combine name="date" value="${value}" sameEntity="true">
  <if>
    <equalsFilter name="" value="" flushWith="POC  .a" flushIncomplete="false" reset="true">
      <data name="" source="@portfolio" />
      <data name="" source="POC  .a" />
    </equalsFilter>
  </if>
  <data name="value" source="POC  .b" />
</combine>

NOTE: This option is only applicable in combination with flushWith. Maybe the name should reflect that? (flushIncomplete with inverted default?) [EDIT: Changed attribute from complete="true" to flushIncomplete="false".]

[blackwinter]

In hindsight, I think I'd prefer flushIncomplete=false (default: true). Let me know if you disagree.

[dr0i]

@blackwinter your proposal makes it clearer - so go on!

[blackwinter]

Done.

[blackwinter]

Thank you!

[dr0i]

Thx a lot. I think this is a very good enhancement which would probably saved me some headaches (and stupid workarounds, working with "@ variables" to postpone the execution of rules ).
I added your description to https://github.com/metafacture/metafacture-core/wiki/Metamorph-Cookbook.