From http://javaevangelist.blogspot.com/2015/02/jsf-2x-tip-of-day-implementing.html

ViewMapListenerImpl.java

package com.bluelotussoftware;

import com.sun.faces.application.view.ViewScopeManager;
import java.util.Collections;
import java.util.Map;
import javax.faces.component.TransientStateHelper;
import javax.faces.component.UIViewRoot;
import javax.faces.context.ExternalContext;
import javax.faces.context.FacesContext;
import javax.faces.event.AbortProcessingException;
import javax.faces.event.PreDestroyViewMapEvent;
import javax.faces.event.SystemEvent;
import javax.faces.event.ViewMapListener;

/**
 * This class is an implementation of {@link ViewMapListener} which listens for
 * {@link SystemEvent} on the {@link UIViewRoot}. If the event is a
 * {@link PreDestroyViewMapEvent}, the listener will clear the session map of
 * the {@link ViewScopeManager#VIEW_MAP_ID} associated with the current view.
 *
 * @author John Yeary <jyeary@bluelotussoftware.com>
 * @version 1.0
 * @see ExternalContext#getSessionMap()
 * @see ViewScopeManager#ACTIVE_VIEW_MAPS
 * @see ViewScopeManager#VIEW_MAP_ID
 * @see TransientStateHelper#getTransient(java.lang.Object)
 */
public class ViewMapListenerImpl implements ViewMapListener {

    /**
     * {@inheritDoc}
     */
    @Override
    public void processEvent(SystemEvent event) throws AbortProcessingException {
        if (event instanceof PreDestroyViewMapEvent) {
            PreDestroyViewMapEvent pdvme = (PreDestroyViewMapEvent) event;
            UIViewRoot root = (UIViewRoot) pdvme.getComponent();
            FacesContext fctx = FacesContext.getCurrentInstance();
            String key = (String) root.getTransientStateHelper().getTransient(ViewScopeManager.VIEW_MAP_ID);
            System.out.println("Found " + ViewScopeManager.VIEW_MAP_ID + ": " + key);
            Map<String, Object> viewMaps = (Map<String, Object>) fctx.getExternalContext().getSessionMap().get(ViewScopeManager.ACTIVE_VIEW_MAPS);
            Map<String, Object> map = Collections.synchronizedMap(viewMaps);
            synchronized (map) {
                System.out.println("Views in session map: " + map.keySet().toString());
                map.remove(key);
                System.out.println("View removed from " + ViewScopeManager.ACTIVE_VIEW_MAPS);
                System.out.println("The remaining views are: " + map.keySet().toString());
            }
        }
    }

    /**
     * {@inheritDoc}
     *
     */
    @Override
    public boolean isListenerForSource(Object source) {
        return source instanceof UIViewRoot;
    }

}

faces-config.xml

<?xml version='1.0' encoding='UTF-8'?>
<faces-config version="2.0"
              xmlns="http://java.sun.com/xml/ns/javaee"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-facesconfig_2_0.xsd">
    <application>
        <system-event-listener>
            <system-event-listener-class>com.bluelotussoftware.ViewMapListenerImpl</system-event-listener-class>
            <system-event-class>javax.faces.event.PreDestroyViewMapEvent</system-event-class>
            <source-class>javax.faces.component.UIViewRoot</source-class>
        </system-event-listener>
    </application>
</faces-config>