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    
015    package 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     */
024    public 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    }