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  package org.apache.cayenne.access;
  
  import org.apache.cayenne.DataRow;
  import org.apache.cayenne.Persistent;
  import org.apache.cayenne.query.Query;
  
  /**
   * Defines API for a DataContext "delegate" - an object that is temporarily passed control
   * by DataContext at some critical points in the normal flow of execution. A delegate thus
   * can modify the flow, abort an operation, modify the objects participating in an
   * operation, or perform any other tasks it deems necessary. DataContextDelegate is shared
   * by DataContext and its ObjectStore.
   * 
   * @see org.apache.cayenne.access.DataContext
   * @since 1.1
   */
  public interface DataContextDelegate {
  
      /**
       * Invoked before a Query is executed via <em>DataContext.performQuery</em>. The
       * delegate may substitute the Query with a different one or may return null to discard
       * the query.
       * 
       * @since 1.2
       */
      Query willPerformQuery(DataContext context, Query query);
  
      /**
       * Invoked before a Query is executed via <em>DataContext.performGenericQuery</em>.
       * The delegate may substitute the Query with a different one or may return null to
       * discard the query.
       * 
       * @since 1.2
       */
      Query willPerformGenericQuery(DataContext context, Query query);
  
      /**
       * Invoked by parent DataContext whenever an object change is detected. This can be a
       * change to the object snapshot, or a modification of an "independent" relationship
       * not resulting in a snapshot change. In the later case snapshot argument may be
       * null. If a delegate returns <code>true</code>, ObjectStore will attempt to merge
       * the changes into an object.
       */
      boolean shouldMergeChanges(Persistent object, DataRow snapshotInStore);
  
      /**
       * Called after a successful merging of external changes to an object. If previosly a
       * delegate returned <code>false</code> from
       * {@link #shouldMergeChanges(Persistent, DataRow)}, this method is not invoked,
       * since changes were not merged.
       */
      void finishedMergeChanges(Persistent object);
  
      /**
       * Invoked by ObjectStore whenever it is detected that a database row was deleted for
       * object. If a delegate returns <code>true</code>, ObjectStore will change
       * MODIFIED objects to NEW (resulting in recreating the deleted record on next commit)
       * and all other objects - to TRANSIENT. To block this behavior, delegate should
       * return <code>false</code>, and possibly do its own processing.
       * 
       * @param object Persistent that was deleted externally and is still present in the
       *            ObjectStore associated with the delegate.
       */
      boolean shouldProcessDelete(Persistent object);
  
      /**
       * Called after a successful processing of externally deleted object. If previosly a
       * delegate returned <code>false</code> from
       * {@link #shouldProcessDelete(Persistent)}, this method is not invoked, since no
       * processing was done.
       */
      void finishedProcessDelete(Persistent object);
  }
  