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1. Implement a class that can turn a TextTemplate into a ResourceReference

TextTemplateResourceReference.java
import org.apache.wicket.IClusterable;
import org.apache.wicket.Resource;
import org.apache.wicket.ResourceReference;
import org.apache.wicket.model.IModel;
import org.apache.wicket.util.resource.IResourceStream;
import org.apache.wicket.util.resource.StringResourceStream;
import org.apache.wicket.util.template.PackagedTextTemplate;
import org.apache.wicket.util.template.TextTemplate;
import org.apache.wicket.util.time.Time;

import java.util.Map;

/**
 * A class which adapts a {@link PackagedTextTemplate} to a {@link ResourceReference}.
 * 
 * @author James Carman
 */
public class TextTemplateResourceReference extends ResourceReference implements IClusterable
{
//**********************************************************************************************************************
// Fields
//**********************************************************************************************************************

    private static final long serialVersionUID = 1L;
    private final TextTemplate textTemplate;
    private final IModel<Map<String,Object>> variablesModel;

//**********************************************************************************************************************
// Constructors
//**********************************************************************************************************************

    /**
     * Creates a resource reference to a {@link PackagedTextTemplate}.
     *
     * @param scope the <code>Class</code> to be used for retrieving the classloader for loading the
	 *            <code>PackagedTextTemplate</code>
     * @param fileName the file name
     * @param variablesModel the template variables as a model
     */
    public TextTemplateResourceReference(final Class<?> scope, final String fileName, IModel<Map<String,Object>> variablesModel)
    {
        super(scope, fileName);
        this.textTemplate = new PackagedTextTemplate(scope, fileName);
        this.variablesModel = variablesModel;
    }

    /**
     * Creates a resource reference to a {@link PackagedTextTemplate}.
     *
     * @param scope the <code>Class</code> to be used for retrieving the classloader for loading the
	 *            <code>PackagedTextTemplate</code>
     * @param fileName the file name
     * @param contentType the mime type of this resource, such as "<code>image/jpeg</code>" or "<code>text/html</code>"
     * @param variablesModel the template variables as a model
     */
    public TextTemplateResourceReference(final Class<?> scope, final String fileName, final String contentType, IModel<Map<String,Object>> variablesModel)
    {
        super(scope, fileName);
        this.textTemplate = new PackagedTextTemplate(scope, fileName, contentType);
        this.variablesModel = variablesModel;
    }

    /**
     * Creates a resource reference to a {@link PackagedTextTemplate}.
     * 
     * @param scope the <code>Class</code> to be used for retrieving the classloader for loading the
	 *            <code>PackagedTextTemplate</code>
     * @param fileName the file name
     * @param contentType the mime type of this resource, such as "<code>image/jpeg</code>" or "<code>text/html</code>"
     * @param encoding the file's encoding, for example, "<code>UTF-8</code>"
     * @param variablesModel the template variables as a model
     */
    public TextTemplateResourceReference(final Class<?> scope, final String fileName, final String contentType, final String encoding, IModel<Map<String,Object>> variablesModel)
    {
        super(scope, fileName);
        this.textTemplate = new PackagedTextTemplate(scope, fileName, contentType, encoding);
        this.variablesModel = variablesModel;
    }

//**********************************************************************************************************************
// Other Methods
//**********************************************************************************************************************

    /**
     * Creates a new resource which returns the interpolated value of the text template.
     * @return a new resource which returns the interpolated value of the text template
     */
    protected Resource newResource()
    {
        return new Resource()
        {
            private static final long serialVersionUID = 1L;

            public IResourceStream getResourceStream()
            {
                final String stringValue = textTemplate.asString(variablesModel.getObject());
                variablesModel.detach(); // We're done with the model so detach it!
                return new StringResourceStream(stringValue, textTemplate.getContentType());
            }
        };
    }

    /**
     * Returns the last modified time of the {@link PackagedTextTemplate} itself.
     * @return the last modified time of the {@link PackagedTextTemplate} itself
     */
    public Time lastModifiedTime()
    {
        return textTemplate.lastModifiedTime();
    }
}

2. Use the new resource reference class to create a StyleSheetReference

MyPage.java
import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.resources.StyleSheetReference;
import org.apache.wicket.model.LoadableDetachableModel;

import java.util.HashMap;
import java.util.Map;

/**
 * @author James Carman
 */
public class MyPage extends WebPage
{
    public MyPage()
    {
        add(new StyleSheetReference("stylesheet", new TextTemplateResourceReference(MyPage.class, "style.css", "text/css", new LoadableDetachableModel<Map<String,Object>>()
        {
            private static final long serialVersionUID = 1L;

            public Map<String,Object> load()
            {
                final Map<String,Object> vars = new HashMap<String,Object>();
                vars.put("color1", "#0000ff");
                return vars;
            }
        })));
    }
}

3. Include the stylesheet link in your markup

MyPage.html
<html xmlns:wicket="http://wicket.apache.org">
<wicket:head>
    <link wicket:id="stylesheet"/>
</wicket:head>
<body>
Stuff goes here!
</body>
</html>

4. Now create the css file that uses the variables (place it next to MyPage.java)

style.css
body 
{
  background: ${color1};
}
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