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JSR-000372: JavaServer(TM) Faces 2.3 API
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/*
 * Copyright (c) 1997, 2018 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0, which is available at
 * http://www.eclipse.org/legal/epl-2.0.
 *
 * This Source Code may also be made available under the following Secondary
 * Licenses when the conditions for such availability set forth in the
 * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
 * version 2 with the GNU Classpath Exception, which is available at
 * https://www.gnu.org/software/classpath/license.html.
 *
 * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
 */

package javax.faces.view;

import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

import javax.faces.component.UIComponent;
import javax.faces.component.UIImportConstants;
import javax.faces.component.UIViewAction;
import javax.faces.component.UIViewParameter;
import javax.faces.component.UIViewRoot;
import javax.faces.context.FacesContext;

/**
 *  ViewMetadata is
 * reponsible for extracting and providing view parameter metadata from
 * VDL views.  Because {@link ViewDeclarationLanguage#getViewMetadata}
 * is required to return null for Jakarta Server Pages views and
 * non-null for views authored in Facelets for Jakarta Server Faces 2, this
 * specification only applies to Facelets for Jakarta Server Faces 2.  
 *
 * @since 2.0
 */
public abstract class ViewMetadata {


    /**
     * Get the view id
     * 
     * @return the view ID for which this ViewMetadata instance
     *  was created
     */
    public abstract String getViewId();


    /**
     *  Creates a new {@link UIViewRoot}
     * containing only view parameter metadata.  The processing of
     * building this UIViewRoot with metadata should not
     * cause any events to be published to the application.  The
     * implementation must call {@link FacesContext#setProcessingEvents}
     * passing false as the argument, at the beginning of
     * the method, and pass true to the same method at the
     * end.  The implementation must ensure that this happens regardless
     * of ant exceptions that may be thrown.
     * 
     * 
     *  Take note a compliant implementation has to ensure that:
     * 
     * 
     *  
     *   the new UIViewRoot must be set as the FacesContext's viewRoot 
     *   before applying the tag handlers, restoring the old FacesContext 
     *   in a finally block.
     *  
     *  
     *   The contents of the current UIViewRoot's ViewMap must be copied 
     *   to the ViewMap of the new UIViewRoot before applying the tag handlers.
     *  
     *  
     *   The {@link UIImportConstants} must be processed after applying the tag handlers.
     *  
     * 
     *
     * @param context the {@link FacesContext} for the current request
     * @return a UIViewRoot containing only view parameter metadata
     *  (if any)
     */
    public abstract UIViewRoot createMetadataView(FacesContext context);


    /**
     *  Utility method to extract view
     * metadata from the provided {@link UIViewRoot}.  
     *
     * @param root the {@link UIViewRoot} from which the metadata will
     * be extracted.
     *
     * @return a Collection of {@link UIViewParameter}
     * instances.  If the view has no metadata, the collection will be
     * empty.
     */
    public static Collection getViewParameters(UIViewRoot root) {
        return getMetadataChildren(root, UIViewParameter.class);
    }

    /**
     *  Utility method to extract view
     * metadata from the provided {@link UIViewRoot}.  
     *
     * @param root the {@link UIViewRoot} from which the metadata will
     * be extracted.
     *
     * @return a Collection of {@link UIViewAction}
     * instances.  If the view has no metadata, the collection will be
     * empty.
     */
    public static Collection getViewActions(UIViewRoot root) {
        return getMetadataChildren(root, UIViewAction.class);
    }

    /**
     * Utility method to extract view metadata from the provided {@link UIViewRoot}.
     *
     * @param root The {@link UIViewRoot} from which the metadata will be extracted.
     *
     * @return A Collection of {@link UIImportConstants} instances.
     * If the view has no metadata, the collection will be empty.
     */
    public static Collection getImportConstants(UIViewRoot root) {
        return getMetadataChildren(root, UIImportConstants.class);
    }

    /**
     * Utility method to determine if the 
     * the provided {@link UIViewRoot} has metadata.  The default implementation will 
     * return true if the provided {@code UIViewRoot} has a facet 
     * named {@link UIViewRoot#METADATA_FACET_NAME} and that facet has children.
     * It will return  false otherwise.
     *
     * @param root the {@link UIViewRoot} from which the metadata will
     * be extracted from
     *
     * @return true if the view has metadata, false otherwise.
     */
    public static boolean hasMetadata(UIViewRoot root) {    
        return getMetadataFacet(root).map(m -> m.getChildCount() > 0).orElse(false);
    }

    @SuppressWarnings("unchecked")
    private static  List getMetadataChildren(UIViewRoot root, Class type) {
        return (List) getMetadataFacet(root).map(m -> m.getChildren()).orElseGet(Collections::emptyList)
                                               .stream().filter(c -> type.isInstance(c)).collect(Collectors.toList());
    }

    private static Optional getMetadataFacet(UIViewRoot root) {
        return Optional.ofNullable(root.getFacet(UIViewRoot.METADATA_FACET_NAME));
    }

}