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Pax Wicket Service is an OSGi extension of the Wicket framework, allowing for dynamic loading and unloading of Wicket components and pageSources.
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/*
 * 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.wicket.behavior;

import org.apache.wicket.AttributeModifier;
import org.apache.wicket.model.IModel;
import org.apache.wicket.util.string.AppendingStringBuffer;
import org.apache.wicket.util.string.Strings;

/**
 * AttributeModifier that appends the given value, rather than replace it. This is especially useful
 * for adding CSS classes to markup elements, or adding JavaScript snippets to existing element
 * handlers.
 * 
 *  *     <a href="#" wicket:id="foo" class="link" onmouseover="doSomething()">
 * 
 * 
 * can be modified with these AttributeAppenders:
 * 
 *  * link.add(new AttributeAppender("class", new Model<String>("hot"), " "));
 * link.add(new AttributeAppender("onmouseover", new Model<String>("foo();return false;"), ";"));
 * 
 * 
 * this will result in the following markup:
 * 
 *  *     <a href="#" wicket:id="foo" class="link hot" onmouseover="doSomething();foo();return false;">
 * 
 * 
 * @author Martijn Dashorst
 */
public class AttributeAppender extends AttributeModifier
{
	/** For serialization. */
	private static final long serialVersionUID = 1L;

	/**
	 * Separates the existing attribute value and the append value.
	 */
	private final String separator;

	/**
	 * Creates an AttributeModifier that appends the appendModel's value to the current value of the
	 * attribute, and will add the attribute when addAttributeIfNotPresent is true.
	 * 
	 * @param attribute
	 *            the attribute to append the appendModels value to
	 * @param addAttributeIfNotPresent
	 *            when true, adds the attribute to the tag
	 * @param appendModel
	 *            the model supplying the value to append
	 * @param separator
	 *            the separator string, comes between the original value and the append value
	 */
	public AttributeAppender(String attribute, boolean addAttributeIfNotPresent,
		IModel appendModel, String separator)
	{
		super(attribute, addAttributeIfNotPresent, appendModel);
		this.separator = separator;
	}

	/**
	 * Creates an AttributeModifier that appends the appendModel's value to the current value of the
	 * attribute, and will add the attribute when it is not there already.
	 * 
	 * @param attribute
	 *            the attribute to append the appendModels value to
	 * @param appendModel
	 *            the model supplying the value to append
	 * @param separator
	 *            the separator string, comes between the original value and the append value
	 */
	public AttributeAppender(String attribute, IModel appendModel, String separator)
	{
		super(attribute, true, appendModel);
		this.separator = separator;
	}

	/**
	 * @see org.apache.wicket.AttributeModifier#newValue(java.lang.String, java.lang.String)
	 */
	@Override
	protected String newValue(String currentValue, String appendValue)
	{
		final int appendValueLen = (appendValue == null) ? 0 : appendValue.length();

		final AppendingStringBuffer sb;
		if (currentValue == null)
		{
			sb = new AppendingStringBuffer(appendValueLen + separator.length());
		}
		else
		{
			sb = new AppendingStringBuffer(currentValue.length() + appendValueLen +
				separator.length());
			sb.append(currentValue);
		}

		// if the current value or the append value is empty, the separator is
		// not needed.
		if (!Strings.isEmpty(currentValue) && !Strings.isEmpty(appendValue))
		{
			sb.append(separator);
		}

		// only append the value when it is not empty.
		if (!Strings.isEmpty(appendValue))
		{
			sb.append(appendValue);
		}
		return sb.toString();
	}
}