/*
 * 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.extensions.ajax.markup.html;

import java.time.Duration;
import java.util.List;
import java.util.Optional;
import org.apache.wicket.Component;
import org.apache.wicket.ajax.AbstractAjaxTimerBehavior;
import org.apache.wicket.ajax.AbstractDefaultAjaxBehavior;
import org.apache.wicket.ajax.AjaxRequestTarget;
import org.apache.wicket.markup.html.basic.Label;
import org.apache.wicket.markup.html.panel.Panel;
import org.apache.wicket.model.IModel;
import org.apache.wicket.request.IRequestHandler;
import org.apache.wicket.request.cycle.RequestCycle;
import org.apache.wicket.request.handler.resource.ResourceReferenceRequestHandler;
import org.apache.wicket.util.lang.Comparators;
import org.apache.wicket.util.visit.IVisit;
import org.apache.wicket.util.visit.IVisitor;

/**
 * A panel which load lazily a single content component. This can be used if you have a
 * component that is pretty heavy in creation and you first want to show the user the page and
 * then replace the panel when it is ready.
 * <p>
 * This panel will wait with adding the content until {@link #isContentReady()} returns
 * {@code true}. It will poll using an {@link AbstractAjaxTimerBehavior} that is installed on the page. When the
 * component is replaced, the timer stops. When you have multiple {@code AjaxLazyLoadPanel}s on the
 * same page, only one timer is used and all panels piggyback on this single timer.
 * <p> 
 * This component will also replace the contents when a normal request comes through and the
 * content is ready.
 * 
 * @since 1.3
 */
public abstract class AjaxLazyLoadPanel<T extends Component> extends Panel
{
        private static final long serialVersionUID = 1L;

        /**
         * The component id which will be used to load the lazily loaded component.
         */
        private static final String CONTENT_ID = "content";

        private boolean loaded;

        /**
         * Constructor
         * 
         * @param id
         */
        public AjaxLazyLoadPanel(final String id)
        {
                this(id, null);
        }

        /**
         * Constructor
         * 
         * @param id
         * @param model
         */
        public AjaxLazyLoadPanel(final String id, final IModel<?> model)
        {
                super(id, model);

                setOutputMarkupId(true);
        }

        /**
         * Determines that the content we're waiting for is ready, typically used in polling background
         * threads for their result. Override this to implement your own check.
         * <p>
         * This default implementation returns {@code true}, i.e. assuming the content is ready immediately.
         * 
         * @return whether the actual content is ready
         */
        protected boolean isContentReady()
        {
                return true;
        }

        /**
         * Create a loading component shown instead of the actual content until it is {@link #isContentReady()}.
         * 
         * @param id
         *            The components id
         * @return The component to show while the real content isn't ready yet
         */
        public Component getLoadingComponent(final String id)
        {
                IRequestHandler handler = new ResourceReferenceRequestHandler(
                        AbstractDefaultAjaxBehavior.INDICATOR);
                return new Label(id,
                        "<img alt=\"Loading...\" src=\"" + RequestCycle.get().urlFor(handler) + "\"/>")
                                .setEscapeModelStrings(false);
        }

        /**
         * Factory method for creating the lazily loaded content that replaces the loading component after
         * {@link #isContentReady()} returns {@code true}. You may call setRenderBodyOnly(true)
         * on this component if you need the body only.
         * 
         * @param markupId
         *            The components markupid.
         * @return the content to show after {@link #isContentReady()}
         */
        public abstract T getLazyLoadComponent(String markupId);

        /**
         * Called after the loading component was replaced with the lazy loaded content.
         * <p>
         * This default implementation does nothing.
         *
         * @param content
         *            The lazy loaded content
         * @param target
         *            optional Ajax request handler
         */
        protected void onContentLoaded(T content, Optional<AjaxRequestTarget> target)
        {
        }

        /**
         * Installs a page-global timer if not already present.
         */
        @Override
        protected void onBeforeRender()
        {
                super.onBeforeRender();

                if (loaded == false) {
                        initTimer();
                }
        }

        /**
         * Initialize a timer - default implementation installs an {@link AbstractAjaxTimerBehavior} on the page,
         * if it is not already present.
         */
        protected void initTimer()
        {
                // when the timer is not yet installed add it
                List<AjaxLazyLoadTimer> behaviors = getPage().getBehaviors(AjaxLazyLoadTimer.class);
                if (behaviors.isEmpty()) {
                        AbstractAjaxTimerBehavior timer = new AjaxLazyLoadTimer();
                        getPage().add(timer);
                        
                        getRequestCycle().find(AjaxRequestTarget.class).ifPresent(target -> {
                                // the timer will not be rendered, so restart it immediately on the Ajax target
                                timer.restart(target);
                        });
                }
        }

        @Override
        protected void onConfigure()
        {
                super.onConfigure();

                if (get(CONTENT_ID) == null) {
                        add(getLoadingComponent(CONTENT_ID));
                }
        }

        /**
         * Get the preferred interval for updates.
         * <p>
         * Since all LazyLoadingPanels on a page share the same Ajax timer, its update interval
         * is derived from the minimum of all panel's update intervals.
         * 
         * @return update interval, must not be {@code null}
         */
        protected Duration getUpdateInterval() {
                return Duration.ofSeconds(1);
        }

        /**
         * Check whether the content is loaded.
         * <p>
         * If not loaded already and the content is ready, replaces the lazy loading component with 
         * the lazily loaded content. 
         * 
         * @return {@code true} if content is loaded
         * 
         * @see #isContentReady()
         */
        protected final boolean isLoaded() {
                if (loaded == false)
                {
                        if (isContentReady())
                        {
                                loaded = true;

                                // create the lazy load component
                                T content = getLazyLoadComponent(CONTENT_ID);

                                // replace the loading component with the new component
                                // note: use addOrReplace(), since onConfigure() might not have been called yet 
                                AjaxLazyLoadPanel.this.addOrReplace(content);

                                Optional<AjaxRequestTarget> target = getRequestCycle().find(AjaxRequestTarget.class);

                                // notify our subclasses of the updated component
                                onContentLoaded(content, target);

                                // repaint our selves if there's an AJAX request in play, otherwise let the page
                                // redraw itself
                                target.ifPresent(t -> t.add(AjaxLazyLoadPanel.this));
                        }
                }
                
                return loaded;
        }

        /**
         * The AJAX timer for updating the AjaxLazyLoadPanel. Is designed to be a page-local singleton
         * running as long as LazyLoadPanels are still loading.
         * 
         * @see AjaxLazyLoadPanel#isLoaded()
         */
        static class AjaxLazyLoadTimer extends AbstractAjaxTimerBehavior
        {
                private static final long serialVersionUID = 1L;

                public AjaxLazyLoadTimer()
                {
                        super(Duration.ofSeconds(1));
                }

                @Override
                protected void onTimer(AjaxRequestTarget target)
                {
                        load(target);
                }

                public void load(AjaxRequestTarget target)
                {
                        setUpdateInterval(Duration.ofMillis(Long.MAX_VALUE));
                        
                        getComponent().getPage().visitChildren(AjaxLazyLoadPanel.class, new IVisitor<AjaxLazyLoadPanel<?>, Void>()
                        {
                                @Override
                                public void component(AjaxLazyLoadPanel<?> panel, IVisit<Void> visit)
                                {
                                        if (panel.isVisibleInHierarchy() && panel.isLoaded() == false) {
                                                Duration updateInterval = panel.getUpdateInterval();
                                                if (getUpdateInterval() == null) {
                                                        throw new IllegalArgumentException("update interval must not ben null");
                                                }
                                                
                                                setUpdateInterval(Comparators.min(getUpdateInterval(), updateInterval));
                                        }                                               
                                }
                        });

                        // all panels have completed their replacements, we can stop the timer
                        if (Duration.ofMillis(Long.MAX_VALUE).equals(getUpdateInterval()))
                        {
                                stop(target);
                                
                                getComponent().remove(this);
                        }
                }
        }
}