001// Copyright 2008 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007//     http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5;
016
017/**
018 * Defines a strategy, used by {@link Field} components such as {@link org.apache.tapestry5.corelib.components.TextField},
019 * to handle the case where either the server-side value to be sent (as a string) to the client, or the client-side
020 * string passed back up to the server, is null or blank.
021 *
022 * @see org.apache.tapestry5.services.NullFieldStrategySource
023 */
024public interface NullFieldStrategy
025{
026    /**
027     * Provides a replacement value for null, when converting the server-side object to a client-side string. The
028     * replacement value, if non-null, will be passed to {@link org.apache.tapestry5.Translator#toClient(Object)}.
029     */
030    Object replaceToClient();
031
032    /**
033     * Provides a replacement value for a null or blank string passed from the client to the server as part of a form
034     * submission. This replacement value will be passed to {@link Translator#parseClient(Field, String, String)}  as if
035     * it were the value supplied by the user.
036     *
037     * @return replacement value (this must not be null)
038     */
039    String replaceFromClient();
040}