/*
* 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 org.apache.flink.util;
import org.apache.flink.annotation.Internal;
/**
* This interface marks types as visitable during a traversal. The central method <i>accept(...)</i> contains the logic
* about how to invoke the supplied {@link Visitor} on the visitable object, and how to traverse further.
* <p>
* This concept makes it easy to implement for example a depth-first traversal of a tree or DAG with different types of
* logic during the traversal. The <i>accept(...)</i> method calls the visitor and then send the visitor to its children
* (or predecessors). Using different types of visitors, different operations can be performed during the traversal, while
* writing the actual traversal code only once.
*
* @see Visitor
*/
@Internal
public interface Visitable<T extends Visitable<T>> {
/**
* Contains the logic to invoke the visitor and continue the traversal.
* Typically invokes the pre-visit method of the visitor, then sends the visitor to the children (or predecessors)
* and then invokes the post-visit method.
* <p>
* A typical code example is the following:
* <pre>{@code
* public void accept(Visitor<Operator> visitor) {
* boolean descend = visitor.preVisit(this);
* if (descend) {
* if (this.input != null) {
* this.input.accept(visitor);
* }
* visitor.postVisit(this);
* }
* }
* }</pre>
*
* @param visitor The visitor to be called with this object as the parameter.
*
* @see Visitor#preVisit(Visitable)
* @see Visitor#postVisit(Visitable)
*/
void accept(Visitor<T> visitor);
}