    001// Copyright 2007-2013 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 * A ValueEncoder is used to convert server side objects to unique client-side
    019 * strings (typically IDs) and back. This mechanism is widely used in Tapestry
    020 * to allow you to work more seamlessly with objects rather than manually
    021 * managing the encoding and decoding process throughout your application.
    022 * <p/>
    023 * Tapestry uses a ValueEncoder when generating an
    024 * {@link org.apache.tapestry5.EventContext} as part of a URL, and when
    025 * components (such as {@link org.apache.tapestry5.corelib.components.Select})
    026 * need to generate unique client-side strings to be rendered within form
    027 * elements.
    028 * <p/>
    029 * Tapestry can automatically generate ValueEncoders for enums as well as
    030 * Collections of any object types for which a coercion can be found from a
    031 * formatted String, such as primitives, primitive wrappers, Dates, Calendars,
    032 * "name=value" strings, and any types for which a {@linkplain org.apache.tapestry5.ioc.services.TypeCoercer
    033 * custom type coercion} has been contributed.
    034 * <p/>
    035 * Custom ValueEncoder implementations will need to be supplied for entity type
    036 * objects. In such cases the {@link #toClient(Object)} method typically returns
    037 * an object's database primary key, and the {@link #toValue(String)}
    038 * re-acquires the corresponding entity object, perhaps by doing a database
    039 * lookup by that ID.
    040 * <p/>
    041 * Some optional modules, such as Tapestry's own Hibernate and JPA modules, can
    042 * automatically create a ValueEncoder for each of your entity types and then
    043 * configure Tapestry to use them whenever a ValueEncoder is needed for those
    044 * types. If you don't use one of those modules, you can still configure
    045 * Tapestry to automatically use your custom ValueEncoder implementations by
    046 * having your ValueEncoder implement the
    047 * {@link org.apache.tapestry5.services.ValueEncoderFactory} interface and then
    048 * contributing a ValueEncoderSource that adds your encoder, like this, in your
    049 * application's module class:
    050 *
    051* <pre>
    052 * public static void contributeValueEncoderSource(
    053 *         MappedConfiguration&lt;Class&lt;Color&gt;, ValueEncoderFactory&lt;Color&gt;&gt; configuration)
    054 * {
    055 *     configuration.addInstance(Color.class, ColorEncoder.class);
    056 * }
    057 * </pre>
    058 *
    059 * @see SelectModel
    060 * @see org.apache.tapestry5.services.ValueEncoderSource
    061 * @see org.apache.tapestry5.services.ValueEncoderFactory
    062 * @see org.apache.tapestry5.annotations.PageActivationContext
    063 * @see org.apache.tapestry5.annotations.RequestParameter
    064 * @see org.apache.tapestry5.annotations.ActivationRequestParameter
    065 */
    066public interface ValueEncoder<V>
    067{
    068    /**
    069     * Converts a value into a client-side representation. The value should be parseable by {@link #toValue(String)}. In
    070     * some cases, what is returned is an identifier used to locate the true object, rather than a string representation
    071     * of the value itself.
    072     *
    073     * @param value to be encoded
    074     * @return a string representation of the value, or the value's identity
    075     */
    076    String toClient(V value);
    077
    078    /**
    079     * Converts a client-side representation, provided by {@link #toClient(Object)}, back into a server-side value.
    080     *
    081     * @param clientValue string representation of the value's identity
    082     * @return the corresponding entity, or null if not found
    083     */
    084    V toValue(String clientValue);
    085}