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.