    001// Copyright 2008-2014 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.annotations;
    016
    017import java.lang.annotation.*;
    018
    019import static org.apache.tapestry5.ioc.annotations.AnnotationUseContext.*;
    020import org.apache.tapestry5.ioc.annotations.UseWith;
    021
    022/**
    023 * Indicates that a method should only be evaluated once per request and the result cached.
    024 * Further calls to the method during the same request will return the cached result.
    025 * However, if the method's component occurs more than once within an enclosing component,
    026 * the cached results will be distinct for each occurrence.
    027 * <p/>
    028 * This annotation is commonly used on getters for component properties:
    029 * <pre>
    030 * &#064;Cached
    031 * Date getNow() {
    032 *     new Date();
    033 * }
    034 * </pre>
    035 * You may not apply &#064;Cached to void methods or methods with parameters.
    036 * <p/>
    037 * Note that this annotation is inheritance-safe; if a subclass calls a superclass method that
    038 * has &#064;Cached then the value the subclass method gets is the cached value.
    039 * <p/>
    040 * The watch parameter can be passed a binding expression which will be evaluated each time the method is called. The
    041 * method will then only be executed the first time it is called and after that only when the value of the binding
    042 * changes. This can be used, for instance, to have the method only evaluated once per iteration of a loop by setting
    043 * watch to the value or index of the loop.
    044 */
    045@Target(ElementType.METHOD)
    046@Retention(RetentionPolicy.RUNTIME)
    047@Documented
    048@UseWith({COMPONENT,MIXIN,PAGE})
    049public @interface Cached
    050{
    051    /**
    052     * The optional binding to watch (default binding prefix is "prop").
    053     */
    054    String watch() default "";
    055}