Blocking a Qt application during downloading a short file

Question

I'm writing an application using Qt4.

I need to download a very short text file from a given http address.

The file is short and is needed for my app to be able to continue, so I would like to make sure the download is blocking (or will timeout after a few seconds if the file in not found/not available).

I wanted to use QHttp::get(), but this is a non-blocking method.

I thought I could use a thread : my app would start it, and wait for it to finish. The thread would handle the download and quit when the file is downloaded or after a timeout.

But I cannot make it work :

class JSHttpGetterThread : public QThread
{
  Q_OBJECT

public:
  JSHttpGetterThread(QObject* pParent = NULL);
  ~JSHttpGetterThread();

  virtual void run()
  {
    m_pHttp = new QHttp(this);
    connect(m_pHttp, SIGNAL(requestFinished(int, bool)), this, SLOT(onRequestFinished(int, bool)));

    m_pHttp->setHost("127.0.0.1");
    m_pHttp->get("Foo.txt", &m_GetBuffer);
    exec();
  }

  const QString& getDownloadedFileContent() const
  {
    return m_DownloadedFileContent;
  }

private:
  QHttp* m_pHttp;

  QBuffer m_GetBuffer;
  QString m_DownloadedFileContent;

private slots:
  void onRequestFinished(int Id, bool Error)
  {
    m_DownloadedFileContent = "";
    m_DownloadedFileContent.append(m_GetBuffer.buffer());
  }
};

In the method creating the thread to initiate the download, here is what I'm doing :

JSHttpGetterThread* pGetter = new JSHttpGetterThread(this);
pGetter->start();
pGetter->wait();

But that doesn't work and my app keeps waiting. It looks lit the slot 'onRequestFinished' is never called.

Any idea ?

Is there a better way to do what I'm trying to do ?

Answer 1

A little late but: Do not use these wait loops, the correct way is to use the done() signal from QHttp.

The requestFinished signal from what I have seen is just for when your application has finished the request, the data may still be on its way down.

You do not need a new thread, just setup the qhttp:

httpGetFile= new QHttp();
connect(httpGetFile, SIGNAL(done(bool)), this, SLOT(processHttpGetFile(bool)));

Also do not forget to flush the file in processHttpGetFile as it might not all be on the disk.

Answer 2

Instead of using a thread you can just go into a loop which calls processEvents:

while (notFinished) {
   qApp->processEvents(QEventLoop::WaitForMore | QEventLoop::ExcludeUserInput);
}

Where notFinished is a flag which can be set from the onRequestFinished slot.

The ExcludeUserInput will ensure that GUI related events are ignored while waiting.