Introduce FeedExtractor making fetching from dedicated feeds possible

YouTube, for example, has a dedicated feed which was built to be used
like this.

extractor/src/main/java/org/schabi/newpipe/extractor/StreamingService.java

@@ -7,6 +7,7 @@ import org.schabi.newpipe.extractor.channel.ChannelExtractor;
 import org.schabi.newpipe.extractor.comments.CommentsExtractor;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
+import org.schabi.newpipe.extractor.feed.FeedExtractor;
 import org.schabi.newpipe.extractor.kiosk.KioskList;
 import org.schabi.newpipe.extractor.linkhandler.LinkHandler;
 import org.schabi.newpipe.extractor.linkhandler.LinkHandlerFactory;
@@ -24,6 +25,8 @@ import org.schabi.newpipe.extractor.stream.StreamExtractor;
 import org.schabi.newpipe.extractor.subscription.SubscriptionExtractor;
 import org.schabi.newpipe.extractor.suggestion.SuggestionExtractor;
 
+import javax.annotation.Nullable;
+
 /*
  * Copyright (C) Christian Schabesberger 2018 <chris.schabesberger@mailbox.org>
  * StreamingService.java is part of NewPipe.
@@ -65,7 +68,7 @@ public abstract class StreamingService {
         public String getName() {
             return name;
         }
-        
+
         public List<MediaCapability> getMediaCapabilities() {
             return mediaCapabilities;
         }
@@ -116,7 +119,7 @@ public abstract class StreamingService {
     public String toString() {
         return serviceId + ":" + serviceInfo.getName();
     }
-    
+
     public abstract String getBaseUrl();
 
     /*//////////////////////////////////////////////////////////////////////////
@@ -173,6 +176,19 @@ public abstract class StreamingService {
      */
     public abstract SubscriptionExtractor getSubscriptionExtractor();
 
+    /**
+     * This method decides which strategy will be chosen to fetch the feed. In YouTube, for example, a separate feed
+     * exists which is lightweight and made specifically to be used like this.
+     * <p>
+     * In services which there's no other way to retrieve them, null should be returned.
+     *
+     * @return a {@link FeedExtractor} instance or null.
+     */
+    @Nullable
+    public FeedExtractor getFeedExtractor(String url) throws ExtractionException {
+        return null;
+    }
+
     /**
      * Must create a new instance of a KioskList implementation.
      * @return a new KioskList instance
@@ -258,7 +274,7 @@ public abstract class StreamingService {
         }
         return getCommentsExtractor(llhf.fromUrl(url));
     }
-    
+
     /*//////////////////////////////////////////////////////////////////////////
     // Utils
     //////////////////////////////////////////////////////////////////////////*/

extractor/src/main/java/org/schabi/newpipe/extractor/feed/FeedExtractor.java

@@ -0,0 +1,17 @@
+package org.schabi.newpipe.extractor.feed;
+
+import org.schabi.newpipe.extractor.ListExtractor;
+import org.schabi.newpipe.extractor.StreamingService;
+import org.schabi.newpipe.extractor.linkhandler.ListLinkHandler;
+import org.schabi.newpipe.extractor.stream.StreamInfoItem;
+
+/**
+ * This class helps to extract items from lightweight feeds that the services may provide.
+ * <p>
+ * YouTube is an example of a service that has this alternative available.
+ */
+public abstract class FeedExtractor extends ListExtractor<StreamInfoItem> {
+    public FeedExtractor(StreamingService service, ListLinkHandler listLinkHandler) {
+        super(service, listLinkHandler);
+    }
+}

extractor/src/main/java/org/schabi/newpipe/extractor/feed/FeedInfo.java

@@ -0,0 +1,52 @@
+package org.schabi.newpipe.extractor.feed;
+
+import org.schabi.newpipe.extractor.ListExtractor.InfoItemsPage;
+import org.schabi.newpipe.extractor.ListInfo;
+import org.schabi.newpipe.extractor.NewPipe;
+import org.schabi.newpipe.extractor.StreamingService;
+import org.schabi.newpipe.extractor.exceptions.ExtractionException;
+import org.schabi.newpipe.extractor.stream.StreamInfoItem;
+import org.schabi.newpipe.extractor.utils.ExtractorHelper;
+
+import java.io.IOException;
+import java.util.List;
+
+public class FeedInfo extends ListInfo<StreamInfoItem> {
+
+    public FeedInfo(int serviceId, String id, String url, String originalUrl, String name, List<String> contentFilter, String sortFilter) {
+        super(serviceId, id, url, originalUrl, name, contentFilter, sortFilter);
+    }
+
+    public static FeedInfo getInfo(String url) throws IOException, ExtractionException {
+        return getInfo(NewPipe.getServiceByUrl(url), url);
+    }
+
+    public static FeedInfo getInfo(StreamingService service, String url) throws IOException, ExtractionException {
+        final FeedExtractor extractor = service.getFeedExtractor(url);
+
+        if (extractor == null) {
+            throw new IllegalArgumentException("Service \"" + service.getServiceInfo().getName() + "\" doesn't support FeedExtractor.");
+        }
+
+        extractor.fetchPage();
+        return getInfo(extractor);
+    }
+
+    public static FeedInfo getInfo(FeedExtractor extractor) throws IOException, ExtractionException {
+        extractor.fetchPage();
+
+        final int serviceId = extractor.getServiceId();
+        final String id = extractor.getId();
+        final String url = extractor.getUrl();
+        final String originalUrl = extractor.getOriginalUrl();
+        final String name = extractor.getName();
+
+        final FeedInfo info = new FeedInfo(serviceId, id, url, originalUrl, name, null, null);
+
+        final InfoItemsPage<StreamInfoItem> itemsPage = ExtractorHelper.getItemsPageOrLogError(info, extractor);
+        info.setRelatedItems(itemsPage.getItems());
+        info.setNextPageUrl(itemsPage.getNextPageUrl());
+
+        return info;
+    }
+}

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeService.java

@@ -12,6 +12,7 @@ import org.schabi.newpipe.extractor.StreamingService;
 import org.schabi.newpipe.extractor.channel.ChannelExtractor;
 import org.schabi.newpipe.extractor.comments.CommentsExtractor;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
+import org.schabi.newpipe.extractor.feed.FeedExtractor;
 import org.schabi.newpipe.extractor.kiosk.KioskExtractor;
 import org.schabi.newpipe.extractor.kiosk.KioskList;
 import org.schabi.newpipe.extractor.linkhandler.LinkHandler;
@@ -24,14 +25,7 @@ import org.schabi.newpipe.extractor.localization.ContentCountry;
 import org.schabi.newpipe.extractor.localization.Localization;
 import org.schabi.newpipe.extractor.playlist.PlaylistExtractor;
 import org.schabi.newpipe.extractor.search.SearchExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeChannelExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeCommentsExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubePlaylistExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeSearchExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeStreamExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeSubscriptionExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeSuggestionExtractor;
-import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeTrendingExtractor;
+import org.schabi.newpipe.extractor.services.youtube.extractors.*;
 import org.schabi.newpipe.extractor.services.youtube.linkHandler.YoutubeChannelLinkHandlerFactory;
 import org.schabi.newpipe.extractor.services.youtube.linkHandler.YoutubeCommentsLinkHandlerFactory;
 import org.schabi.newpipe.extractor.services.youtube.linkHandler.YoutubePlaylistLinkHandlerFactory;
@@ -42,6 +36,8 @@ import org.schabi.newpipe.extractor.stream.StreamExtractor;
 import org.schabi.newpipe.extractor.subscription.SubscriptionExtractor;
 import org.schabi.newpipe.extractor.suggestion.SuggestionExtractor;
 
+import javax.annotation.Nonnull;
+
 /*
  * Created by Christian Schabesberger on 23.08.15.
  *
@@ -72,7 +68,7 @@ public class YoutubeService extends StreamingService {
     public String getBaseUrl() {
         return "https://youtube.com";
     }
-    
+
     @Override
     public LinkHandlerFactory getStreamLHFactory() {
         return YoutubeStreamLinkHandlerFactory.getInstance();
@@ -147,6 +143,12 @@ public class YoutubeService extends StreamingService {
         return new YoutubeSubscriptionExtractor(this);
     }
 
+    @Nonnull
+    @Override
+    public FeedExtractor getFeedExtractor(final String channelUrl) throws ExtractionException {
+        return new YoutubeFeedExtractor(this, getChannelLHFactory().fromUrl(channelUrl));
+    }
+
     @Override
     public ListLinkHandlerFactory getCommentsLHFactory() {
         return YoutubeCommentsLinkHandlerFactory.getInstance();

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeChannelExtractor.java

@@ -46,7 +46,6 @@ import java.io.IOException;
 @SuppressWarnings("WeakerAccess")
 public class YoutubeChannelExtractor extends ChannelExtractor {
     /*package-private*/ static final String CHANNEL_URL_BASE = "https://www.youtube.com/channel/";
-    private static final String CHANNEL_FEED_BASE = "https://www.youtube.com/feeds/videos.xml?channel_id=";
     private static final String CHANNEL_URL_PARAMETERS = "/videos?view=0&flow=list&sort=dd&live_view=10000";
 
     private Document doc;
@@ -130,7 +129,7 @@ public class YoutubeChannelExtractor extends ChannelExtractor {
     @Override
     public String getFeedUrl() throws ParsingException {
         try {
-            return CHANNEL_FEED_BASE + getId();
+            return YoutubeParsingHelper.getFeedUrlFrom(getId());
         } catch (Exception e) {
             throw new ParsingException("Could not get feed url", e);
         }

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeFeedExtractor.java

@@ -0,0 +1,82 @@
+package org.schabi.newpipe.extractor.services.youtube.extractors;
+
+import org.jsoup.Jsoup;
+import org.jsoup.nodes.Document;
+import org.jsoup.nodes.Element;
+import org.jsoup.select.Elements;
+import org.schabi.newpipe.extractor.ListExtractor;
+import org.schabi.newpipe.extractor.StreamingService;
+import org.schabi.newpipe.extractor.downloader.Downloader;
+import org.schabi.newpipe.extractor.downloader.Response;
+import org.schabi.newpipe.extractor.exceptions.ExtractionException;
+import org.schabi.newpipe.extractor.feed.FeedExtractor;
+import org.schabi.newpipe.extractor.linkhandler.ListLinkHandler;
+import org.schabi.newpipe.extractor.services.youtube.linkHandler.YoutubeParsingHelper;
+import org.schabi.newpipe.extractor.stream.StreamInfoItem;
+import org.schabi.newpipe.extractor.stream.StreamInfoItemsCollector;
+
+import javax.annotation.Nonnull;
+import java.io.IOException;
+
+public class YoutubeFeedExtractor extends FeedExtractor {
+    public YoutubeFeedExtractor(StreamingService service, ListLinkHandler linkHandler) {
+        super(service, linkHandler);
+    }
+
+    private Document document;
+
+    @Override
+    public void onFetchPage(@Nonnull Downloader downloader) throws IOException, ExtractionException {
+        final String channelIdOrUser = getLinkHandler().getId();
+        final String feedUrl = YoutubeParsingHelper.getFeedUrlFrom(channelIdOrUser);
+
+        final Response response = downloader.get(feedUrl);
+        document = Jsoup.parse(response.responseBody());
+    }
+
+    @Nonnull
+    @Override
+    public ListExtractor.InfoItemsPage<StreamInfoItem> getInitialPage() {
+        final Elements entries = document.select("feed > entry");
+        final StreamInfoItemsCollector collector = new StreamInfoItemsCollector(getServiceId());
+
+        for (Element entryElement : entries) {
+            collector.commit(new YoutubeFeedInfoItemExtractor(entryElement));
+        }
+
+        return new InfoItemsPage<>(collector, null);
+    }
+
+    @Nonnull
+    @Override
+    public String getId() {
+        return document.getElementsByTag("yt:channelId").first().text();
+    }
+
+    @Nonnull
+    @Override
+    public String getUrl() {
+        return document.select("feed > author > uri").first().text();
+    }
+
+    @Nonnull
+    @Override
+    public String getName() {
+        return document.select("feed > author > name").first().text();
+    }
+
+    @Override
+    public String getNextPageUrl() {
+        return null;
+    }
+
+    @Override
+    public InfoItemsPage<StreamInfoItem> getPage(String pageUrl) {
+        return null;
+    }
+
+    @Override
+    public boolean hasNextPage() {
+        return false;
+    }
+}

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeFeedInfoItemExtractor.java

@@ -0,0 +1,94 @@
+package org.schabi.newpipe.extractor.services.youtube.extractors;
+
+import org.jsoup.nodes.Element;
+import org.schabi.newpipe.extractor.exceptions.ParsingException;
+import org.schabi.newpipe.extractor.localization.DateWrapper;
+import org.schabi.newpipe.extractor.stream.StreamInfoItemExtractor;
+import org.schabi.newpipe.extractor.stream.StreamType;
+
+import javax.annotation.Nullable;
+import java.text.ParseException;
+import java.text.SimpleDateFormat;
+import java.util.Calendar;
+import java.util.Date;
+import java.util.TimeZone;
+
+public class YoutubeFeedInfoItemExtractor implements StreamInfoItemExtractor {
+    private final Element entryElement;
+
+    public YoutubeFeedInfoItemExtractor(Element entryElement) {
+        this.entryElement = entryElement;
+    }
+
+    @Override
+    public StreamType getStreamType() {
+        // It is not possible to determine the stream type using the feed endpoint.
+        // All entries are considered a video stream.
+        return StreamType.VIDEO_STREAM;
+    }
+
+    @Override
+    public boolean isAd() {
+        return false;
+    }
+
+    @Override
+    public long getDuration() {
+        // Not available when fetching through the feed endpoint.
+        return -1;
+    }
+
+    @Override
+    public long getViewCount() {
+        return Long.parseLong(entryElement.getElementsByTag("media:statistics").first().attr("views"));
+    }
+
+    @Override
+    public String getUploaderName() {
+        return entryElement.select("author > name").first().text();
+    }
+
+    @Override
+    public String getUploaderUrl() {
+        return entryElement.select("author > uri").first().text();
+    }
+
+    @Nullable
+    @Override
+    public String getTextualUploadDate() {
+        return entryElement.getElementsByTag("published").first().text();
+    }
+
+    @Nullable
+    @Override
+    public DateWrapper getUploadDate() throws ParsingException {
+        final Date date;
+        try {
+            final SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss+00:00");
+            dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
+            date = dateFormat.parse(getTextualUploadDate());
+        } catch (ParseException e) {
+            throw new ParsingException("Could not parse date (\"" + getTextualUploadDate() + "\")", e);
+        }
+
+        final Calendar calendar = Calendar.getInstance();
+        calendar.setTime(date);
+
+        return new DateWrapper(calendar);
+    }
+
+    @Override
+    public String getName() {
+        return entryElement.getElementsByTag("title").first().text();
+    }
+
+    @Override
+    public String getUrl() {
+        return entryElement.getElementsByTag("link").first().attr("href");
+    }
+
+    @Override
+    public String getThumbnailUrl() {
+        return entryElement.getElementsByTag("media:thumbnail").first().attr("url");
+    }
+}

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/linkHandler/YoutubeParsingHelper.java

@@ -38,6 +38,9 @@ public class YoutubeParsingHelper {
     private YoutubeParsingHelper() {
     }
 
+    private static final String FEED_BASE_CHANNEL_ID = "https://www.youtube.com/feeds/videos.xml?channel_id=";
+    private static final String FEED_BASE_USER = "https://www.youtube.com/feeds/videos.xml?user=";
+
     private static final String[] RECAPTCHA_DETECTION_SELECTORS = {
             "form[action*=\"/das_captcha\"]",
             "input[name*=\"action_recaptcha_verify\"]"
@@ -118,6 +121,16 @@ public class YoutubeParsingHelper {
                 + Long.parseLong(seconds);
     }
 
+    public static String getFeedUrlFrom(final String channelIdOrUser) {
+        if (channelIdOrUser.startsWith("user/")) {
+            return FEED_BASE_USER + channelIdOrUser.replace("user/", "");
+        } else if (channelIdOrUser.startsWith("channel/")) {
+            return FEED_BASE_CHANNEL_ID + channelIdOrUser.replace("channel/", "");
+        } else {
+            return FEED_BASE_CHANNEL_ID + channelIdOrUser;
+        }
+    }
+
     public static Calendar parseDateFrom(String textualUploadDate) throws ParsingException {
         Date date;
         try {

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/YoutubeFeedExtractorTest.java

@@ -0,0 +1,72 @@
+package org.schabi.newpipe.extractor.services.youtube;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.schabi.newpipe.DownloaderTestImpl;
+import org.schabi.newpipe.extractor.NewPipe;
+import org.schabi.newpipe.extractor.exceptions.ParsingException;
+import org.schabi.newpipe.extractor.services.BaseListExtractorTest;
+import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeFeedExtractor;
+
+import static org.junit.Assert.*;
+import static org.schabi.newpipe.extractor.ServiceList.YouTube;
+import static org.schabi.newpipe.extractor.services.DefaultTests.defaultTestRelatedItems;
+
+public class YoutubeFeedExtractorTest {
+    public static class Kurzgesagt implements BaseListExtractorTest {
+        private static YoutubeFeedExtractor extractor;
+
+        @BeforeClass
+        public static void setUp() throws Exception {
+            NewPipe.init(DownloaderTestImpl.getInstance());
+            extractor = (YoutubeFeedExtractor) YouTube
+                    .getFeedExtractor("https://www.youtube.com/user/Kurzgesagt");
+            extractor.fetchPage();
+        }
+
+        /*//////////////////////////////////////////////////////////////////////////
+        // Extractor
+        //////////////////////////////////////////////////////////////////////////*/
+
+        @Test
+        public void testServiceId() {
+            assertEquals(YouTube.getServiceId(), extractor.getServiceId());
+        }
+
+        @Test
+        public void testName() {
+            String name = extractor.getName();
+            assertTrue(name, name.startsWith("Kurzgesagt"));
+        }
+
+        @Test
+        public void testId() {
+            assertEquals("UCsXVk37bltHxD1rDPwtNM8Q", extractor.getId());
+        }
+
+        @Test
+        public void testUrl() {
+            assertEquals("https://www.youtube.com/channel/UCsXVk37bltHxD1rDPwtNM8Q", extractor.getUrl());
+        }
+
+        @Test
+        public void testOriginalUrl() throws ParsingException {
+            assertEquals("https://www.youtube.com/user/Kurzgesagt", extractor.getOriginalUrl());
+        }
+
+        /*//////////////////////////////////////////////////////////////////////////
+        // ListExtractor
+        //////////////////////////////////////////////////////////////////////////*/
+
+        @Test
+        public void testRelatedItems() throws Exception {
+            defaultTestRelatedItems(extractor, YouTube.getServiceId());
+        }
+
+        @Test
+        public void testMoreRelatedItems() {
+            assertFalse(extractor.hasNextPage());
+            assertNull(extractor.getNextPageUrl());
+        }
+    }
+}