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Open HTML to PDF is a CSS 2.1 renderer written in Java. This artifact contains the core rendering and layout code.

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/*
 * {{{ header & license
 * Copyright (c) 2006 Wisconsin Court System
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 2.1
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 * }}}
 */
package com.openhtmltopdf.css.extend;

import org.w3c.dom.Element;

import com.openhtmltopdf.css.parser.FSFunction;
import com.openhtmltopdf.layout.LayoutContext;
import com.openhtmltopdf.render.InlineText;
import com.openhtmltopdf.render.RenderingContext;

/**
 * Interface for objects which implement a function which creates content
 * (e.g. counter(pages))
 */
public interface ContentFunction {
    /**
     * Whether or not the function value can change at render time.  If true,
     * {@link #calculate(LayoutContext, FSFunction)} will be called.  If false,
     * {@link #calculate(RenderingContext, FSFunction, InlineText)} will be called.
     */
    public boolean isStatic();
    
    public String calculate(LayoutContext c, FSFunction function);
    public String calculate(RenderingContext c, FSFunction function, InlineText text);
    
    /**
     * If a function value can change at render time (i.e. {@link #isStatic()} returns false)
     * use this text as an approximation at layout.
     */
    public String getLayoutReplacementText();

    /**
     * Some content functions can provide a better replacement text after
     * boxing but before layout (ie. target-text).
     */
    default public String getPostBoxingLayoutReplacementText(
            LayoutContext c, Element element, FSFunction fsFunction) {
        return getLayoutReplacementText();
    }

    public boolean canHandle(LayoutContext c, FSFunction function);

    /**
     * Is the calculate method going to return valid results if called
     * before or during layout as opposed to at render time.
     */
    default public boolean isCalculableAtLayout() {
        return false;
    }
}