Web crawler app using QWebPage



  • Hello all,

    I'm trying to create a web crawler app that should get the URL from user input, connect to that web-page and search for some expression(a string probably) on that page.

    I came here looking for tips on how to do this.

    I looked on this site and google for examples but only found how to make a Web Browser in Qt, how to go back, change pages etc...
    I don't need to show that page in my app, just to search it, and count occurrences of a string, maybe print out those sentences etc.

    I think QWebPage can do the trick because it has findText() function implemented. But, todays pages are not written exclusively in HTML (or some other thing), and are mostly a combo of few things...
    How do I download entire web page to some buffer or something, and then search it?

    Thanks in advance!


  • Moderators

    Hi @jelicicm,
    You are on the right track with the usage of QWebPage but I think that would not be alone enough. Web crawlers try to crawl through every link it finds on a webpage. To implement as such you will need to parse the HTML (basically the dom elements) which you will receive when the page loading finishes. To parse an HTML, Qt has a nice API called QWebElement which will ease your work in helping to extract the elements which you need.

    How do I download entire web page to some buffer or something, and then search it?

    Usually you should retrieve the web page after the loading finishes. QWebView loads the page and fires a signal when the loading finishes. Use it, connect a slot to loadFinished signal and retrieved the loaded page's contents. Typically like this:

    void MyClass::onLoadFinished(bool)
    {
        QString page_content = webView->page()->mainFrame()->toHtml(); 
        //page = QWebPage, mainFrame = QWebFrame
    }
    

  • Moderators

    Hi again @jelicicm,
    I completely missed that you don't want to show the web page. The above example requires you to use QWebView.
    Well, In that case you can use QNetworkAccessManager to download a page. It too has a finished signal. Connect to it and retrieve the contents using QNetworkReply::readAll(). Then the usual stuff of parsing the HTML (an HTML is an XML). To do it you can use either QXmlStreamReader or QDomDocument. Each one has advantages of its own.



  • @p3c0 Thank you for your reply... both of them!

    I have used QNetworkAccessManager once before, so I have some experience with it.

    I guess I could put QNetworkReply::readAll(), and load it in QXmlStreamReader object?

    So far, I have this>

    void MainWindow::on_crawl_clicked() //push button click
    {
    QXmlStreamReader *xml = new QXmlStreamReader();
        ui->plainTextEdit->clear();
        QUrl URL(ui->url->text());
        Downloader *d = new Downloader(this);
        QNetworkRequest req(URL);
        QObject::connect(d,SIGNAL(dloadend(QXmlStreamReader*)),this,SLOT(print(QXmlStreamReader*)));
        QObject::connect(d,SIGNAL(dloadend(QXmlStreamReader*)),d,SLOT(deleteLater()));
    }
    
    void MainWindow::print(QXmlStreamReader *xml)  //it never prints out "debug", so it never gets to here...
    {
        ui->plainTextEdit->appendPlainText("debug");
        ui->plainTextEdit->appendPlainText(xml->text().toString());
    }
    
    void Downloader::doDownload(QNetworkRequest req)
    {
        manager = new QNetworkAccessManager(this);
    
        connect(manager, SIGNAL(finished(QNetworkReply*)),this,SLOT(replyFinished(QNetworkReply*)));
    
        manager -> get(req);
    }
    
    void Downloader::replyFinished (QNetworkReply *reply)
    {
        QXmlStreamReader *buffer = new QXmlStreamReader();
        if(reply->error()) {
            qDebug() << "ERROR!";
            qDebug() << reply->errorString();
            reply->deleteLater();
            manager->deleteLater();
            emit err();
        }
        else
    
        {
            buffer->addData(reply->readAll());
    
            reply->deleteLater();
            manager->deleteLater();
    
            emit dloadend(buffer);
    
    }
    

    Do you maybe see what am I doing wrong?


  • Moderators

    @jelicicm I guess the order of deletion is causing the problem.
    Also where are you calling doDownload ?
    Does it get into replyFinished slot i.e in else part ? I would create QXmlStreamReader object in the else part i.e when it succeeds.



  • @p3c0 said:Then the usual stuff of parsing the HTML (an HTML is an XML). To do it you can use either >QXmlStreamReader or QDomDocument. Each one has advantages of it own.

    Well, HTML is not necessarily XML, if you're lucky, the downloaded page is XHTML and it's a win. But if the page is HTML where tags are either not nested correctly nor properly closed, your XML parser will most probably yell at you that the document contains syntax errors I'm afraid...



  • @p3c0 I can't believe this, I never called doDownload(), and I wonder why it doesn't download. Stupid, stupid..

    I created a QFile, and managed to save some page to it. When I open it, everything looks good, and it seems to me that the download works fine. I saved it as .html, and managed to open it nicely.

    However, I'm still not sure that my QXmlStreamReader buffer object is good. I tried printing all of it withui->plainTextEdit->appendPlainText(xml->text().toString());, but after "debug" nothing gets printed?!


  • Moderators

    @jelicicm Is the print slot called ?



  • @p3c0 Yes, I have a line that just prints "debug" in my PlainTextEdit field, and that gets printed, but, after that, I want to printout what I have downloaded, and it doesn't work.


  • Moderators

    @jelicicm Ok can you make sure it works in replyFinished else part ? Try the same with buffer.



  • @p3c0 I did this in my replyFinished, else part>

            buffer->addData(reply->readAll());
    
            while(!buffer->atEnd()) {
                qDebug()<<buffer->readNext();
            }
    

    What I get are some weird numbers, that I don't understand..
    My qDebug output looks like this

    2
    8
    4
    6
    4
    6
    4
    6
    5
    6
    4
    6
    4
    6
    4
    6
    4
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    5
    6
    4
    1
    

    Web page I was downloading> http://www.bbc.com/future/story/20150604-the-bravest-walks-ever-taken


  • Moderators

    @jelicicm It prints a TokenType and not the text. Since TokenType is an enum what you see are its values. Also text() will not so the same.
    You can try to print errorString() to see if there are any errors.
    Since readNext prints something it means the there's data in it.
    There are few examples on forums where you can see how to parse the XML using QXmlStreamReader. I would also suggest you to look for htmlinfo example under <QtDir>/xml/htmlinfo installed on your system. Another example would be this. But a more close example will be htmlinfo.

    Edited



  • Wow @p3c0 this really helped a lot! Thank you very much!

    Currently I have>

    void MainWindow::print(QXmlStreamReader *reader)
    {
        int paragraphCount = 0;
        QStringList links;
        QString title;
        QString text;
    
        while (!reader->atEnd()) {
            reader->readNext();
            /*text = reader->readElementText();
            if(text.contains("some text")) {
                qDebug()<<"found text\n";
            }*/
            if (reader->isStartElement()) {
                if (reader->name() == "title")
                    title = reader->readElementText();
                else if(reader->name() == "a")
                    links.append(reader->attributes().value("href").toString());
                else if(reader->name() == "p")
                    ++paragraphCount;
            }
        }
        if (reader->hasError()) {
            ui->plainTextEdit->appendPlainText( "  The HTML file isn't well-formed: " + reader->errorString()+"\n");
            return;
        }
    
        qDebug()<<"Title: "<<title;
        qDebug()<<"Paragraph count: "<<paragraphCount;
        qDebug()<<"No of links: "<<links.size();
        qDebug()<<"One link: "<<links[3];
    }
    

    And, this works perfectly!

    However, when I uncomment reader->readElementText();, I always get an error from little lower in the code.

    What I'm trying to do is to search for some text in the web page, and I guess that should be done with this function, but I can't get it to work.


  • Moderators

    @jelicicm As the readElementText doc states:

    Convenience function to be called in case a StartElement was read. Reads until the corresponding EndElement and returns all text in-between.

    It should be used only when StartElement is encountered. Your commented code doesn't do that. The next code shows how it should be done.



  • @p3c0 In the meantime I discovered that the problem is in QString text = reader->readElementText();

    I just did:

    while (!reader->atEnd()) {
            reader->readNext();
    
            if (reader->isStartElement()) {
                text = reader->readElementText();
                qDebug()<<text;
    }
    
        if (reader->hasError()) {
            ui->plainTextEdit->appendPlainText( "  The HTML file isn't well-formed: " + reader->errorString()+"\n");
            return;
        }
    

    And, it always prints out: The HTML file isn't well-formed: Expected character data.
    qDebug just prints
    "

    "
    It doesnt even make a difference if I put this in or not.

    if (reader->name() == "title")
                    title = reader->readElementText();
                else if(reader->name() == "a")
                    links.append(reader->attributes().value("href").toString());
                else if(reader->name() == "p")
                    ++paragraphCount;
    

    For a moment I thought that reader is emptied after reading text, but thats not it.


  • Moderators

    @jelicicm After some analysis I'm to unsure about the detailed working of it.
    However QXmlStreamReader can help you in extracting the links which you can probably use in your web crawler implemention as shown in that example.
    Also to implement something as simple as searching you can instead resort to QTextStream. Set the html content as byte array to it. The iterate over it, extract the line and check if the particular word exists in it using QString::contains.


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