Qt FileDownloader class adaptation for threading

PusRob

Hello Everyone!

I'm new to threads and Qt in general, so this question will probably seem like trivial to someone, sorry for that. So let's get to the point: I'm trying to adapt the (Qt example downloader class) to be used in a separate thread as part of a virtual proxy, so I modified it slightly in the following manner:

// filedownloader.h
class FileDownloader : public QObject
{
 Q_OBJECT
 public:
  FileDownloader() {}
  explicit FileDownloader(const QUrl& Url, QObject *parent = 0);
  virtual ~FileDownloader();
  QByteArray downloadedData() const;

 signals:
  void downloaded();

 private slots:
  void startDownload();
  void fileDownloaded(QNetworkReply* pReply);

 private:
  QUrl imageUrl;
  QNetworkAccessManager m_WebCtrl;
  QByteArray m_DownloadedData;
};

// filedownloader.cpp
FileDownloader::FileDownloader(const QUrl& Url, QObject *parent) :
 QObject(parent), imageUrl{Url} {}

FileDownloader::~FileDownloader() { }

void FileDownloader::startDownload()
{
    connect(
     &m_WebCtrl, SIGNAL (finished(QNetworkReply*)),
     this, SLOT (fileDownloaded(QNetworkReply*))
     );

    QNetworkRequest request(imageUrl);
    m_WebCtrl.get(request);
// rest of the class definitions...
}

I moved the contents of the constructor to a separate "startDownload" function (which is also a slot) and I try to use this class in the following manner:

// trying to use downloader
 thread = new QThread();
 downloader = new FileDownloader(imageURL);
 downloader->moveToThread(thread);
 connect(thread, SIGNAL(started()), downloader, SLOT(startDownload()));
 connect(downloader, SIGNAL(downloaded()), this, SLOT(loadImage()));
 connect(downloader, SIGNAL(downloaded()), thread, SLOT(quit()));
 connect(downloader, SIGNAL(downloaded()), downloader, SLOT(deleteLater()));
 connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
 thread->start();

The problem is that it doesn't work. At runtime I get the following error message:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNetworkAccessManager(0x7fe54c008858), parent's thread is QThread(0x564bee0a4910), current thread is QThread(0x564bee38d7c0)

I went through it with the debugger, and the error is emitted while trying to execute the following line:

// in startDownload() function:
    m_WebCtrl.get(request);

Could somebody give some suggestions how to make this work? I thought all the functions of this class would now be executed in a separate thread.

Thanks in advance.

Christian Ehrlicher

Since all QNetworkManager actions are already async in a separate thread there is no need for another thread here at all.