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<li class="toctree-l1"><a class="reference internal" href="../00_Prepare_everything/">Before You Start</a>
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<li class="toctree-l1 current"><a class="reference internal current" href="./">Concept of the Extractor</a>
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<li class="toctree-l2"><a class="reference internal" href="#the-collectorextractor-pattern">The Collector/Extractor Pattern</a>
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<li class="toctree-l3"><a class="reference internal" href="#usage-in-the-front-end">Usage in the Front End</a>
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<li class="toctree-l3"><a class="reference internal" href="#typical-implementation-of-a-single-data-extractor">Typical Implementation of a Single Data Extractor</a>
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<li class="toctree-l2"><a class="reference internal" href="#collectorextractor-pattern-for-lists">Collector/Extractor Pattern for Lists</a>
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<li class="toctree-l1"><a class="reference internal" href="../02_Concept_of_LinkHandler/">Concept of the LinkHandler</a>
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<h1 id="concept-of-the-extractor">Concept of the Extractor</h1>
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<h2 id="the-collectorextractor-pattern">The Collector/Extractor Pattern</h2>
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<p>Before you start coding your own service, you need to understand the basic concept of the extractor itself. There is a pattern
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you will find all over the code, called the <strong>extractor/collector</strong> pattern. The idea behind it is that
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the <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/Extractor.html">extractor</a>
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would produce fragments of data, and the collector would collect them and assemble that data into a readable format for the front end.
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The collector also controls the parsing process, and takes care of error handling. So, if the extractor fails at any
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point, the collector will decide whether or not it should continue parsing. This requires the extractor to be made out of
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multiple methods, one method for every data field the collector wants to have. The collectors are provided by NewPipe.
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You need to take care of the extractors.</p>
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<h3 id="usage-in-the-front-end">Usage in the Front End</h3>
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<p>A typical call for retrieving data from a website would look like this:</p>
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<pre><code class="language-java">Info info;
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try {
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// Create a new Extractor with a given context provided as parameter.
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Extractor extractor = new Extractor(some_meta_info);
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// Retrieves the data form extractor and builds info package.
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info = Info.getInfo(extractor);
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} catch(Exception e) {
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// handle errors when collector decided to break up extraction
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}
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</code></pre>
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<h3 id="typical-implementation-of-a-single-data-extractor">Typical Implementation of a Single Data Extractor</h3>
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<p>The typical implementation of a single data extractor, on the other hand, would look like this:</p>
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<pre><code class="language-java">class MyExtractor extends FutureExtractor {
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public MyExtractor(RequiredInfo requiredInfo, ForExtraction forExtraction) {
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super(requiredInfo, forExtraction);
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...
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}
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@Override
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public void fetch() {
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// Actually fetch the page data here
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}
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@Override
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public String someDataField()
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throws ExtractionException { //The exception needs to be thrown if something failed
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// get piece of information and return it
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}
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... // More datafields
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}
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</code></pre>
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<h2 id="collectorextractor-pattern-for-lists">Collector/Extractor Pattern for Lists</h2>
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<p>Information can be represented as a list. In NewPipe, a list is represented by an
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<a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/InfoItemsCollector.html">InfoItemsCollector</a>.
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An InfoItemsCollector will collect and assemble a list of <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/InfoItem.html">InfoItem</a>.
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For each item that should be extracted, a new Extractor must be created, and given to the InfoItemsCollector via <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/InfoItemsCollector.html#commit-E-">commit()</a>.</p>
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<p><img alt="InfoItemsCollector_objectdiagram.svg" src="../img/InfoItemsCollector_objectdiagram.svg" /></p>
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<p>If you are implementing a list in your service you need to implement an <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/Extractor.html">InfoItemExtractor</a>,
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that will be able to retrieve data for one and only one InfoItem. This extractor will then be <em>comitted</em> to the <strong>InfoItemsCollector</strong> that can collect the type of InfoItems you want to generate.</p>
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<p>A common implementation would look like this:</p>
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<pre><code>private SomeInfoItemCollector collectInfoItemsFromElement(Element e) {
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// See *Some* as something like Stream or Channel
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// e.g. StreamInfoItemsCollector, and ChannelInfoItemsCollector are provided by NP
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SomeInfoItemCollector collector = new SomeInfoItemCollector(getServiceId());
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for(final Element li : element.children()) {
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collector.commit(new InfoItemExtractor() {
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@Override
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public String getName() throws ParsingException {
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...
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}
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@Override
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public String getUrl() throws ParsingException {
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...
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}
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...
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}
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return collector;
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}
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</code></pre>
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<h2 id="listextractor">ListExtractor</h2>
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<p>There is more to know about lists:</p>
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<ol>
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<li>
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<p>When a streaming site shows a list of items, it usually offers some additional information about that list like its title, a thumbnail,
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and its creator. Such info can be called <strong>list header</strong>.</p>
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</li>
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<li>
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<p>When a website shows a long list of items it usually does not load the whole list, but only a part of it. In order to get more items you may have to click on a next page button, or scroll down.</p>
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</li>
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</ol>
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<p>Both of these Problems are fixed by the <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/ListExtractor.html">ListExtractor</a> which takes care about extracting additional metadata about the list,
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and by chopping down lists into several pages, so called <a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/ListExtractor.InfoItemsPage.html">InfoItemsPage</a>s.
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Each page has its own URL, and needs to be extracted separately.</p>
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<p>For extracting list header information a <code>ListExtractor</code> behaves like a regular extractor. For handling <code>InfoItemsPages</code> it adds methods
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such as:</p>
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<ul>
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<li><a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/ListExtractor.html#getInitialPage--">getInitialPage()</a>
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which will return the first page of InfoItems.</li>
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<li><a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/ListExtractor.html#getNextPageUrl--">getNextPageUrl()</a>
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If a second Page of InfoItems is available this will return the URL pointing to them.</li>
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<li><a href="https://teamnewpipe.github.io/NewPipeExtractor/javadoc/org/schabi/newpipe/extractor/ListExtractor.html#getPage-java.lang.String-">getPage()</a>
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returns a ListExtractor.InfoItemsPage by its URL which was retrieved by the <code>getNextPageUrl()</code> method of the previous page.</li>
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</ul>
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<p>The reason why the first page is handled special is because many Websites such as YouTube will load the first page of
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items like a regular web page, but all the others as an AJAX request.</p>
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<p>An InfoItemsPage itself has two constructors which take these parameters:
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- The <strong>InfoitemsCollector</strong> of the list that the page should represent
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- A <strong>nextPageUrl</strong> which represents the url of the following page (may be null if not page follows).
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- Optionally <strong>errors</strong> which is a list of Exceptions that may have happened during extracton.</p>
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<p>Here is a simplified reference implementation of a list extractor that only extracts pages, but not metadata:</p>
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<pre><code>class MyListExtractor extends ListExtractor {
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...
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private Document document;
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...
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public InfoItemsPage<SomeInfoItem> getPage(pageUrl)
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throws ExtractionException {
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SomeInfoItemCollector collector = new SomeInfoItemCollector(getServiceId());
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document = myFunctionToGetThePageHTMLWhatever(pageUrl);
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//remember this part from the simple list extraction
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for(final Element li : document.children()) {
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collector.commit(new InfoItemExtractor() {
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@Override
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public String getName() throws ParsingException {
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...
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}
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@Override
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public String getUrl() throws ParsingException {
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...
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}
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...
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}
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return new InfoItemsPage<SomeInfoItem>(collector, myFunctionToGetTheNextPageUrl(document));
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}
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public InfoItemsPage<SomeInfoItem> getInitialPage() {
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//document here got initialized by the fetch() function.
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return getPage(getTheCurrentPageUrl(document));
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}
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...
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}
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</code></pre>
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