    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.annotations;
    014
    015import org.apache.tapestry5.ioc.annotations.UseWith;
    016
    017import java.lang.annotation.*;
    018
    019import static org.apache.tapestry5.ioc.annotations.AnnotationUseContext.*;
    020
    021/**
    022 * Indicates that a method should only be evaluated once per request and the result cached.
    023 * Further calls to the method during the same request will return the cached result.
    024 * However, if the method's component occurs more than once within an enclosing component,
    025 * the cached results will be distinct for each occurrence.
    026 *
    027 * This annotation is commonly used on getters for component properties:
    028 * <pre>
    029 * &#064;Cached
    030 * Date getNow() {
    031 *     new Date();
    032 * }
    033 * </pre>
    034 * You may not apply &#064;Cached to void methods or methods with parameters.
    035 *
    036 * Note that this annotation is inheritance-safe; if a subclass calls a superclass method that
    037 * has &#064;Cached then the value the subclass method gets is the cached value.
    038 *
    039 * The watch parameter can be passed a binding expression which will be evaluated each time the method is called. The
    040 * method will then only be executed the first time it is called and after that only when the value of the binding
    041 * changes. This can be used, for instance, to have the method only evaluated once per iteration of a loop by setting
    042 * watch to the value or index of the loop.
    043 */
    044@Target(ElementType.METHOD)
    045@Retention(RetentionPolicy.RUNTIME)
    046@Documented
    047@UseWith({COMPONENT,MIXIN,PAGE})
    048public @interface Cached
    049{
    050    /**
    051     * The optional binding to watch (default binding prefix is "prop").
    052     */
    053    String watch() default "";
    054}