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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 freemarker.template;

import java.io.IOException;

/**
 * An interface that can be implemented by writers returned from
 * {@link TemplateTransformModel#getWriter(java.io.Writer, java.util.Map)}. The
 * methods on this
 * interfaces are callbacks that will be called by the template engine and that
 * give the writer a chance to better control the evaluation of the transform
 * body. The writer can instruct the engine to skip or to repeat body 
 * evaluation, and gets notified about exceptions that are thrown during the
 * body evaluation.
 */
public interface TransformControl {
    /**
     * Constant returned from {@link #afterBody()} that tells the
     * template engine to repeat transform body evaluation and feed
     * it again to the transform.
     */
    public static final int REPEAT_EVALUATION = 0;

    /**
     * Constant returned from {@link #afterBody()} that tells the
     * template engine to end the transform and close the writer.
     */
    public static final int END_EVALUATION = 1;
 
    /**
     * Constant returned from {@link #onStart()} that tells the
     * template engine to skip evaluation of the body.
     */
    public static final int SKIP_BODY = 0;
    
    /**
     * Constant returned from {@link #onStart()} that tells the
     * template engine to evaluate the body.
     */
    public static final int EVALUATE_BODY = 1;

    /**
     * Called before the body is evaluated for the first time.
     * @return 
     * 
     * {@code SKIP_BODY} if the transform wants to ignore the body. In this
     * case, only {@link java.io.Writer#close()} is called next and processing ends.
     * {@code EVALUATE_BODY} to normally evaluate the body of the transform
     * and feed it to the writer
     * 
     */
    public int onStart() throws TemplateModelException, IOException;
    
    /**
     * Called after the body has been evaluated.
     * @return
     * 
     * {@code END_EVALUATION} if the transformation should be ended.
     * {@code REPEAT_EVALUATION} to have the engine re-evaluate the 
     * transform body and feed it again to the writer.
     * 
     */
    public int afterBody() throws TemplateModelException, IOException;
    
    /**
     * Called if any exception occurs during the transform between the
     * {@link TemplateTransformModel#getWriter(java.io.Writer, java.util.Map)} call
     * and the {@link java.io.Writer#close()} call.
     * @param t the throwable that represents the exception. It can be any 
     * non-checked throwable, as well as {@link TemplateException} and 
     * {@link java.io.IOException}.
     * 
     * @throws Throwable is recommended that the methods rethrow the received 
     * throwable. If the method wants to throw another throwable, it should
     * either throw a non-checked throwable, or an instance of 
     * {@link TemplateException} and {@link java.io.IOException}. Throwing any
     * other checked exception will cause the engine to rethrow it as
     * a {@link java.lang.reflect.UndeclaredThrowableException}.
     */
    public void onError(Throwable t) throws Throwable;
}