/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.runtime.tree;

import org.antlr.v4.runtime.ParserRuleContext;
import org.antlr.v4.runtime.RuleContext;

public class ParseTreeWalker {
    public static final ParseTreeWalker DEFAULT = new ParseTreeWalker();

    public void walk(ParseTreeListener listener, ParseTree t) {
		if ( t instanceof ErrorNode) {
			listener.visitErrorNode((ErrorNode)t);
			return;
		}
		else if ( t instanceof TerminalNode) {
			listener.visitTerminal((TerminalNode)t);
			return;
		}
		RuleNode r = (RuleNode)t;
        enterRule(listener, r);
        int n = r.getChildCount();
        for (int i = 0; i<n; i++) {
            walk(listener, r.getChild(i));
        }
		exitRule(listener, r);
    }

	
The discovery of a rule node, involves sending two events: the generic ParseTreeListener.enterEveryRule and a RuleContext-specific event. First we trigger the generic and then the rule specific. We to them in reverse order upon finishing the node. /**
	 * The discovery of a rule node, involves sending two events: the generic
	 * {@link ParseTreeListener#enterEveryRule} and a
	 * {@link RuleContext}-specific event. First we trigger the generic and then
	 * the rule specific. We to them in reverse order upon finishing the node.
	 */
    protected void enterRule(ParseTreeListener listener, RuleNode r) {
		ParserRuleContext ctx = (ParserRuleContext)r.getRuleContext();
		listener.enterEveryRule(ctx);
		ctx.enterRule(listener);
    }

    protected void exitRule(ParseTreeListener listener, RuleNode r) {
		ParserRuleContext ctx = (ParserRuleContext)r.getRuleContext();
		ctx.exitRule(listener);
		listener.exitEveryRule(ctx);
    }
}