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Cayenne Object Persistence Framework
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package org.apache.cayenne.access;

import org.apache.cayenne.DataObject;
import org.apache.cayenne.DataRow;
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
 * @author Mike Kienenberger
 * @author Andrus Adamchik
 * @since 1.1
 */
public interface DataContextDelegate {

    /**
     * Invoked before a GenericSelectQuery is executed. The delegate may
     * modify the GenericSelectQuery by returning a different
     * GenericSelectQuery, or may return null to discard the query.
     * 
     * @return the original or modified GenericSelectQuery or null to
     *         discard the query.
     * @deprecated since 1.2 implement either
     *             {@link #willPerformQuery(DataContext, Query)} or
     *             {@link #willPerformGenericQuery(DataContext, Query)}.
     */
    org.apache.cayenne.query.GenericSelectQuery willPerformSelect(
            DataContext context,
            org.apache.cayenne.query.GenericSelectQuery query);

    /**
     * Invoked before a Query is executed via DataContext.performQuery. The
     * delegate may subsitute 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 DataContext.performGenericQuery.
     * The delegate may subsitute 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 true, ObjectStore will attempt to merge
     * the changes into an object.
     */
    boolean shouldMergeChanges(DataObject object, DataRow snapshotInStore);

    /**
     * Called after a successful merging of external changes to an object. If previosly a
     * delegate returned false from
     * {@link #shouldMergeChanges(DataObject, DataRow)}, this method is not invoked,
     * since changes were not merged.
     */
    void finishedMergeChanges(DataObject object);

    /**
     * Invoked by ObjectStore whenever it is detected that a database row was deleted for
     * object. If a delegate returns true, 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 false, and possibly do its own processing.
     * 
     * @param object DataObject that was deleted externally and is still present in the
     *            ObjectStore associated with the delegate.
     */
    boolean shouldProcessDelete(DataObject object);

    /**
     * Called after a successful processing of externally deleted object. If previosly a
     * delegate returned false from
     * {@link #shouldProcessDelete(DataObject)}, this method is not invoked, since no
     * processing was done.
     */
    void finishedProcessDelete(DataObject object);
}