Using QThread and QNetworkAccessManager

jeremy_c

I am trying to make my desktop app execute some Internet lookups in the background and planned on having the background thread send a signal to my main thread when completed w/the data found or network error. I am starting the thread just fine but it never seems to execute. The output is:

@Running QrzCallbook thread
QUrl( "http://localhost/xml?callsign=AA1AA" )@

The header is:

@#ifndef QRZCALLBOOK_H
#define QRZCALLBOOK_H

#include <QThread>
#include <QUrl>
#include <QNetworkAccessManager>
#include <QNetworkReply>

class QrzCallbook : public QThread
{
Q_OBJECT

public:
explicit QrzCallbook(QObject *parent = 0) : QThread(parent) {}

virtual void lookup(QString call) {
    callsign = call;
    start();
}

virtual void run();

signals:
void stationData(const QString& name);
void networkError(const QString errorMessage);
void parseError(const QString errorMessage);

private slots:
void downloadFinished();

private:
QNetworkAccessManager *netAccessMgr;
QNetworkReply *netReply;
QString callsign;
};

#endif // QRZCALLBOOK_H@

The class file is:

@#include <QNetworkRequest>

#include "SmartLog.h"
#include "QrzCallbook.h"

void QrzCallbook::run()
{
qDebug() << "Running QrzCallbook thread";

QUrl thisUrl("http://localhost/xml");
thisUrl.addQueryItem("callsign", callsign);

qDebug() << thisUrl;

netAccessMgr = new QNetworkAccessManager;
netReply = netAccessMgr->get(QNetworkRequest(thisUrl));
connect(netReply, SIGNAL(finished()), this, SLOT(downloadFinished()));

}

void QrzCallbook::downloadFinished()
{
qDebug() << "Download has finished";
netAccessMgr->deleteLater();
netReply->deleteLater();

if (netReply->error()) {
    qDebug() << netReply->errorString();
    emit networkError(netReply->errorString());
    return;
}

qDebug() << netReply->readAll();

emit stationData("Not Implemented");

}
@

Any thoughts on why the downloadFinished() slot is never run?

Jeremy

jeremy_c

I think I understand what's going on here. A request is made to download a web page, signals/slots are connected and then my thread terminates before the results are returned, thus I never get a download.

How do I tell my thread to remain active until a download is complete?

Felix

why do you want to use threads for that? in qnetworkaccessmanager all requests are asynchronus already

jeremy_c

I originally had it to not use a thread and when doing a lookup the GUI hangs for about 1 1/2 seconds when the action is executed. With this particular action if call server #1 doesn't have the callsign on file then a subsequent request will go out to another callbook server. That created another 1 1/2 second GUI freeze. I am not certain where the "freeze" occurs at. Here is my non-threaded code output:

@Running QrzCallbook thread: 0
Requesting: 0
Done Requesting: 1
Download has finished: 1765
Reading data: 1765
Done reading: 1765@

There is no delay in the "run" method, nor any in the "downloadFinished" method but the GUI hangs. By hang I mean I cannot use my arrow keys to move around in the field, tab ceases to work and clicking with my mouse on any new field does not function for about 1 1/2 seconds. The event is triggered in a slot responding to a QLineEdit's textChanged signal.

Here is the non-threaded source .h:

@class QrzCallbook : public QObject
{
Q_OBJECT

public:
explicit QrzCallbook(QObject *parent = 0) : QObject(parent) {}

virtual void lookup(QString call) {
    callsign = call;
    run();
}

virtual void run();

signals:
void stationData(const QString& name);
void networkError(const QString errorMessage);
void parseError(const QString errorMessage);

private slots:
void downloadFinished();

private:
QNetworkAccessManager *netAccessMgr;
QNetworkReply *netReply;
QString callsign;
QTime timer;
};@

And the non-threaded .cpp:

@void QrzCallbook::run()
{
timer.start();

qDebug() << "Running QrzCallbook thread: " << timer.elapsed();

QUrl thisUrl("http://localhost/xml");

qDebug() << "Requesting: " << timer.elapsed();
netAccessMgr = new QNetworkAccessManager;
netReply = netAccessMgr->get(QNetworkRequest(thisUrl));
connect(netReply, SIGNAL(finished()), this, SLOT(downloadFinished()));
qDebug() << "Done Requesting: " << timer.elapsed();

}

void QrzCallbook::downloadFinished()
{
qDebug() << "Download has finished: " << timer.elapsed();
netAccessMgr->deleteLater();
netReply->deleteLater();

if (netReply->error()) {
    qDebug() << netReply->errorString();
    emit networkError(netReply->errorString());
    return;
}

qDebug() << "Reading data: " << timer.elapsed();
QByteArray data = netReply->readAll();
qDebug() << "Done reading: " << timer.elapsed();

emit stationData("Not Implemented");

}@

So... to answer your question, I am trying to get rid of the 1 1/2 second delay that occurs.

Jeremy

hasnain

try placing your connect line in constructor

jeremy_c

Ah, I got it. I didn't include an event loop in my thread. Simply adding exec(); to the end of my run() method caused everything to work like a charm.

jeremy_c

Oh, also one more thing. I did add an exit() to my downloadFinished slot. Further, this has caused my main gui to remain responsive. There is no 1 1/2 second pause when the download is taking place.