001// Licensed under the Apache License, Version 2.0 (the "License");
002// you may not use this file except in compliance with the License.
003// You may obtain a copy of the License at
004//
005//     http://www.apache.org/licenses/LICENSE-2.0
006//
007// Unless required by applicable law or agreed to in writing, software
008// distributed under the License is distributed on an "AS IS" BASIS,
009// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
010// See the License for the specific language governing permissions and
011// limitations under the License.
012
013package org.apache.tapestry5;
014
015/**
016 * A ValueEncoder is used to convert server side objects to unique client-side
017 * strings (typically IDs) and back. This mechanism is widely used in Tapestry
018 * to allow you to work more seamlessly with objects rather than manually
019 * managing the encoding and decoding process throughout your application.
020 *
021 * Tapestry uses a ValueEncoder when generating an
022 * {@link org.apache.tapestry5.EventContext} as part of a URL, and when
023 * components (such as {@link org.apache.tapestry5.corelib.components.Select})
024 * need to generate unique client-side strings to be rendered within form
025 * elements.
026 *
027 * Tapestry can automatically generate ValueEncoders for enums as well as
028 * Collections of any object types for which a coercion can be found from a
029 * formatted String, such as primitives, primitive wrappers, Dates, Calendars,
030 * "name=value" strings, and any types for which a {@linkplain org.apache.tapestry5.commons.services.TypeCoercer
031 * custom type coercion} has been contributed.
032 *
033 * Custom ValueEncoder implementations will need to be supplied for entity type
034 * objects. In such cases the {@link #toClient(Object)} method typically returns
035 * an object's database primary key, and the {@link #toValue(String)}
036 * re-acquires the corresponding entity object, perhaps by doing a database
037 * lookup by that ID.
038 *
039 * Some optional modules, such as Tapestry's own Hibernate and JPA modules, can
040 * automatically create a ValueEncoder for each of your entity types and then
041 * configure Tapestry to use them whenever a ValueEncoder is needed for those
042 * types. If you don't use one of those modules, you can still configure
043 * Tapestry to automatically use your custom ValueEncoder implementations by
044 * having your ValueEncoder implement the
045 * {@link org.apache.tapestry5.services.ValueEncoderFactory} interface and then
046 * contributing a ValueEncoderSource that adds your encoder, like this, in your
047 * application's module class:
048 * 
049* <pre>
050 * public static void contributeValueEncoderSource(
051 *         MappedConfiguration&lt;Class&lt;Color&gt;, ValueEncoderFactory&lt;Color&gt;&gt; configuration)
052 * {
053 *     configuration.addInstance(Color.class, ColorEncoder.class);
054 * }
055 * </pre>
056 * 
057 * @see SelectModel
058 * @see org.apache.tapestry5.services.ValueEncoderSource
059 * @see org.apache.tapestry5.services.ValueEncoderFactory
060 * @see org.apache.tapestry5.annotations.PageActivationContext
061 * @see org.apache.tapestry5.annotations.RequestParameter
062 * @see org.apache.tapestry5.annotations.ActivationRequestParameter
063 */
064public interface ValueEncoder<V>
065{
066    /**
067     * Converts a value into a client-side representation. The value should be parseable by {@link #toValue(String)}. In
068     * some cases, what is returned is an identifier used to locate the true object, rather than a string representation
069     * of the value itself.
070     *
071     * @param value to be encoded
072     * @return a string representation of the value, or the value's identity
073     */
074    String toClient(V value);
075
076    /**
077     * Converts a client-side representation, provided by {@link #toClient(Object)}, back into a server-side value.
078     *
079     * @param clientValue string representation of the value's identity
080     * @return the corresponding entity, or null if not found
081     */
082    V toValue(String clientValue);
083}