NewPipeExtractor/extractor/src/main/java/org/schabi/newpipe/extractor/ListExtractor.java

package org.schabi.newpipe.extractor;

import org.schabi.newpipe.extractor.exceptions.ExtractionException;
import org.schabi.newpipe.extractor.linkhandler.ListLinkHandler;

import javax.annotation.Nonnull;
import java.io.IOException;
import java.util.Collections;
import java.util.List;

/**
 * Base class to extractors that have a list (e.g. playlists, users).
 */
public abstract class ListExtractor<R extends InfoItem> extends Extractor {

    public ListExtractor(StreamingService service, ListLinkHandler linkHandler) {
        super(service, linkHandler);
    }

    /**
     * A {@link InfoItemsPage InfoItemsPage} corresponding to the initial page where the items are from the initial request and
     * the nextPageUrl relative to it.
     *
     * @return a {@link InfoItemsPage} corresponding to the initial page
     */
    @Nonnull
    public abstract InfoItemsPage<R> getInitialPage() throws IOException, ExtractionException;

    /**
     * Returns an url that can be used to get the next page relative to the initial one.
     * <p>Usually, these links will only work in the implementation itself.</p>
     *
     * @return an url pointing to the next page relative to the initial page
     * @see #getPage(String)
     */
    public abstract String getNextPageUrl() throws IOException, ExtractionException;

    /**
     * Get a list of items corresponding to the specific requested page.
     *
     * @param pageUrl any page url got from the exclusive implementation of the list extractor
     * @return a {@link InfoItemsPage} corresponding to the requested page
     * @see #getNextPageUrl()
     * @see InfoItemsPage#getNextPageUrl()
     */
    public abstract InfoItemsPage<R> getPage(final String pageUrl) throws IOException, ExtractionException;

    public boolean hasNextPage() throws IOException, ExtractionException {
        final String nextPageUrl = getNextPageUrl();
        return nextPageUrl != null && !nextPageUrl.isEmpty();
    }

    @Override
    public ListLinkHandler getLinkHandler() {
        return (ListLinkHandler) super.getLinkHandler();
    }

    /*//////////////////////////////////////////////////////////////////////////
    // Inner
    //////////////////////////////////////////////////////////////////////////*/

    /**
     * A class that is used to wrap a list of gathered items and eventual errors, it
     * also contains a field that points to the next available page ({@link #nextPageUrl}).
     */
    public static class InfoItemsPage<T extends InfoItem> {
        private static final InfoItemsPage<InfoItem> EMPTY =
                new InfoItemsPage<>(Collections.<InfoItem>emptyList(), "", Collections.<Throwable>emptyList());

        /**
         * A convenient method that returns a representation of an empty page.
         *
         * @return a type-safe page with the list of items and errors empty and the nextPageUrl set to an empty string.
         */
        public static <T extends InfoItem> InfoItemsPage<T> emptyPage() {
            //noinspection unchecked
            return (InfoItemsPage<T>) EMPTY;
        }


        /**
         * The current list of items of this page
         */
        private final List<T> itemsList;

        /**
         * Url pointing to the next page relative to this one
         *
         * @see ListExtractor#getPage(String)
         */
        private final String nextPageUrl;

        /**
         * Errors that happened during the extraction
         */
        private final List<Throwable> errors;

        public InfoItemsPage(InfoItemsCollector<T, ?> collector, String nextPageUrl) {
            this(collector.getItems(), nextPageUrl, collector.getErrors());
        }

        public InfoItemsPage(List<T> itemsList, String nextPageUrl, List<Throwable> errors) {
            this.itemsList = itemsList;
            this.nextPageUrl = nextPageUrl;
            this.errors = errors;
        }

        public boolean hasNextPage() {
            return nextPageUrl != null && !nextPageUrl.isEmpty();
        }

        public List<T> getItems() {
            return itemsList;
        }

        public String getNextPageUrl() {
            return nextPageUrl;
        }

        public List<Throwable> getErrors() {
            return errors;
        }
    }

}