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Closure Stylesheets is an extension to CSS that adds variables, functions, conditionals, and mixins to standard CSS. The tool also supports minification, linting, RTL flipping, and CSS class renaming.

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/*
 * Copyright 2009 Google Inc.
 *
 * 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 com.google.common.css.compiler.passes;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.css.compiler.ast.CssTreeVisitor;
import com.google.common.reflect.Reflection;

import java.lang.reflect.InvocationTargetException;
import java.util.List;

/**
 * Dispatches to multiple {@code CssTreeVisitor}s. All {@code enter*} methods are called in the
 * order provided to the constructor and all {@code leave*} methods are called in the opposite
 * order.
 *
 * Because {@code enter*} methods' return value changes the visitor behavior, the value returned
 * by the last delegate is the one returned by the method.
 */
public class DelegatingVisitor {
    private DelegatingVisitor() {
    }

    /**
     * Creates a {@code DelegatingVisitor} from the given list of visitors. The list must have at
     * least one element.
     */
    public static CssTreeVisitor from(List originalVisitors) {
        Preconditions.checkArgument(!originalVisitors.isEmpty());
        if (originalVisitors.size() == 1) {
            return originalVisitors.get(0);
        }

        final ImmutableList visitors = ImmutableList.copyOf(originalVisitors);
        final ImmutableList reverseVisitors = visitors.reverse();
        return Reflection.newProxy(
                CssTreeVisitor.class,
                (proxy, method, args) -> {
                    try {
                        Object returnValue = null;
                        Iterable visitorsInOrderForMethod;
                        if (method.getName().startsWith("enter")) {
                            visitorsInOrderForMethod = visitors;
                        } else { // assume it's a leave* method
                            visitorsInOrderForMethod = reverseVisitors;
                        }
                        for (CssTreeVisitor visitor : visitorsInOrderForMethod) {
                            returnValue = method.invoke(visitor, args);
                        }
                        return returnValue;
                    } catch (InvocationTargetException e) {
                        throw e.getTargetException();
                    }
                });
    }

    /**
     * Creates a {@code DelegatingVisitor} from the given array of visitors. Changes to the array will
     * not be reflected after construction.
     */
    public static CssTreeVisitor from(CssTreeVisitor... visitors) {
        return from(ImmutableList.copyOf(visitors));
    }
}