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Groovy: A powerful, dynamic language for the JVM
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/*
 * Copyright 2008-2009 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package groovy.lang;

import org.codehaus.groovy.transform.GroovyASTTransformationClass;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Transforms an instance-style Groovy class or interface to become a static-style
 * conventional Groovy category.
 * 
 * Groovy categories are the original mechanism used
 * by Groovy when augmenting classes with new methods. Writing categories required
 * using a class writing style where all methods were static and an additional
 * self parameter was defined. The self parameter and static nature of the methods
 * disappeared once applied by Groovy's metaclass framework but some regarded
 * the writing style as a little noisy. This transformation allows you to write
 * your categories without the "apparent noise" but adds it back in during
 * compilation so that the classes appear as normal categories.
 * 
 * It might seem strange writing your class/object enhancements using a succinct
 * notation, then having "noise" added, then having the noise removed during
 * category application. If this worries you, then you may also like to consider
 * using Groovy's {@code ExpandoMetaClass} mechanism which avoids
 * the category definition altogether. If you already have an investment in
 * categories or like some of the other features which categories currently give you,
 * then read on.
 * 
 * The mechanics: during compilation, all methods are transformed to static ones with an additional
 * self parameter of the type you supply as the annotation parameter (the default type
 * for the self parameters is {@code Object} which might be more broad reaching than
 * you like so it is usually wise to specify a type).
 * Properties invoked using 'this' references are transformed so that
 * they are instead invoked on the additional self parameter and not on
 * the Category instance. (Remember that once the category is applied, the reverse
 * will occur and we will be back to conceptually having methods on the {@code this}
 * references again!)
 * 
 * Classes conforming to the conventional Groovy category conventions can be used
 * within {@code use} statements or mixed in at compile time with the {@code @Mixin}
 * transformation or at runtime with the {@code mixin} method on classes.
 * 
 * An example showing a {@code use} statement (allowing fine-grained application of
 * the category methods):
 *  * {@code @Category}(Integer)
 * class IntegerOps {
 *     def triple() {
 *         this * 3
 *     }
 * }
 *
 * use (IntegerOps) {
 *     assert 25.triple() == 75
 * }
 * 
 * Or, using the {@code @Mixin} flavor for compile-time "mixing in" of the methods:
 *  * {@code @Category}(List)
 * class Shuffler {
 *     def shuffle() {
 *         def result = new ArrayList(this)
 *         Collections.shuffle(result)
 *         result
 *     }
 * }
 *
 * {@code @Mixin}(Shuffler)
 * class Sentence extends ArrayList {
 *     Sentence(Collection initial) { super(initial) }
 * }
 *
 * def words = ["The", "quick", "brown", "fox"]
 * println new Sentence(words).shuffle()
 * // => [quick, fox, The, brown]       (order will vary)
 * 
 * Or, instead of using {@code @Mixin}, try "mixing in" your methods at runtime:
 *  * // ... as before ...
 *
 * class Sentence extends ArrayList {
 *     Sentence(Collection initial) { super(initial) }
 * }
 * Sentence.mixin Shuffler
 *
 * // ... as before ...
 * 
 *
 * @author Alex Tkachman
 */
@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.TYPE)
@GroovyASTTransformationClass("org.codehaus.groovy.transform.CategoryASTTransformation")
public @interface Category {
    Class value () default Object.class;
}