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TransformationFilter
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This is the primary interface to implement if you are building a :ref:`Filter <extensions-filters>`. It will allow your Apex class to inspect records that are being processed, and modify them if need be.

Filters are given an opportunity to inspect and modify :doc:`/classes/record-in-flight` instances during Link processing, after they are retrieved/handled by a Source Adapter but before they are delivered to a :doc:`/adapter-interfaces/target-adapter`.

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Definition
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.. code-block:: java

	/**
	 * TransformationFilters are applied to RecordInFlight records.
	 */
	global interface TransformationFilter {

		/**
		 * Given contextual information about a Link, let us know if this Filter is appropriate to use in this context. Perhaps your link only works
		 * in certain scenarios, like when the local Salesforce org is the source system.
		 *
		 * If you're not sure what to do with this method, just return true.
		 *
		 * @param context Information about this Link execution in case you need it
		 *
		 * @return True if this Filter is appropriate to use with this Link
		 */
		Boolean validFor(LinkContext context);

		/**
		 * This method allows the Filter to inspect and possibly manipulate RecordInFlight records. These records represent a record that is
		 * being processed inside of a Valence Link and is in the midst of syncing from a source system to a target system.
		 *
		 * Change records in the list, or the list itself, to affect what ends up in the target system. You can add/remove/change properties,
		 * add errors or warnings to the record, or even remove it from the list entirely to drop it on the floor without causing a failed record.
		 *
		 * Word of warning: Do not add new records to the list. We've carefully calculated list size in order to not hit governor limits, and also
		 * any new records are not going to be processed by filters that were applied before yours was.
		 *
		 * @param context Information about this Link execution in case you need it
		 * @param records A subset (single batch) of records being processed by the Link
		 */
		void process(LinkContext context, List<RecordInFlight> records);
	}

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Example Usage
*************

.. code-block:: java

    global with sharing class ConstantFilter implements valence.TransformationFilter {

        public Boolean validFor(valence.LinkContext context) {
            // allow this filter to be used by any Link in any circumstance
            return true;
        }

        public void process(valence.LinkContext context, List<valence.RecordInFlight> records) {
            // add a constant property to every record that goes by
            for(valence.RecordInFlight record : records)
                record.getProperties().put('SpecialField', 'HelloThere');
        }
    }