shady-css-parser/src/shady-css/node-visitor.ts

A fast, small and flexible CSS parser.

/**
 * @license
 * Copyright (c) 2016 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at
 * http://polymer.github.io/LICENSE.txt The complete set of authors may be found
 * at http://polymer.github.io/AUTHORS.txt The complete set of contributors may
 * be found at http://polymer.github.io/CONTRIBUTORS.txt Code distributed by
 * Google as part of the polymer project is also subject to an additional IP
 * rights grant found at http://polymer.github.io/PATENTS.txt
 */

/**
 * Class that implements a visitor pattern for ASTs produced by the Parser.
 * Extend the NodeVisitor class to implement useful tree traversal operations
 * such as stringification.
 */
class NodeVisitor<Node extends{type: any}, ReturnValue> {
  private path_: Node[];
  /**
   * Create a NodeVisitor instance.
   */
  constructor() {
    this.path_ = [];
  }

  /**
   * A list of nodes that corresponds to the current path through an AST being
   * visited, leading to where the currently visited node will be found.
   */
  get path() {
    return this.path_;
  }

  /**
   * Visit a node. The visited node will be added to the `path` before it is
   * visited, and will be removed after it is visited. Nodes are "visited" by
   * calling a method on the NodeVisitor instance that matches the node's type,
   * if one is available on the NodeVisitor instance.
   * @param node The node to be visited.
   * @return The return value of the method visiting the node, if any.
   */
  visit(node: Node) {
    let result: ReturnValue|undefined;
    const callback: ((token: Node) => ReturnValue)|undefined =
        (this as any)[node.type];
    if (callback) {
      this.path_.push(node);
      result = (this as any)[node.type](node);
      this.path_.pop();
    }
    return result;
  }
}

export {NodeVisitor};