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TBEL is a powerful expression language for ThingsBoard platform user-defined functions. Original implementation is based on MVEL.

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/**
 * MVEL 2.0
 * Copyright (C) 2007 The Codehaus
 * Mike Brock, Dhanji Prasanna, John Graham, Mark Proctor
 *
 * 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 org.mvel2.integration;

import org.mvel2.ast.ASTNode;

/**
 * An interceptor can be used to decorate functionality into an expression or to hook into external functionality, such
 * as to log an event or fire some other event.
 *
 * @author Christopher Brock
 */
public interface Interceptor {
  public static final int NORMAL_FLOW = 0;
  public static final int SKIP = 1;
  public static final int END = 2;

  /**
   * This method is executed before the wrapped statement.
   *
   * @param node    The ASTNode wrapped by the interceptor
   * @param factory The variable factory
   * @return The response code.  Should return 0.
   */
  public int doBefore(ASTNode node, VariableResolverFactory factory);

  /**
   * This method is called after the wrapped statement has completed.  A copy of the top-value of the execution
   * stack is also availablehere.
   *
   * @param exitStackValue The value on the top of the stack after executing the statement.
   * @param node           The ASTNode wrapped by the interceptor
   * @param factory        The variable factory
   * @return The response code.  Should return 0.
   */
  public int doAfter(Object exitStackValue, ASTNode node, VariableResolverFactory factory);
}