QNetworkAccessManager does not return any data - qt

Question

I have come across bug that I am not able to see myself. After studing QT and stack sites I wrote following code:
void RateOfExchangeGetter::run(){
mRunning = true;
mAccessManager = new QNetworkAccessManager();
//connect(mAccessManager, SIGNAL(finished(QNetworkReply*)),
// this, SLOT(replyFinished(QNetworkReply*)));
while(mRunning){
QNetworkReply *reply;
for(SiteParser *parser: mSiteParsers){
QNetworkRequest request(QUrl("https://www.google.pl/"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");
qDebug() << "here";
reply = mAccessManager->get(request);
parser->setQNetworkReply(reply);
connect(reply, SIGNAL(readyRead()), parser, SLOT(slotReadyRead()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
parser, SLOT(slotError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
parser, SLOT(slotSslErrors(QList<QSslError>)));
}
//for test only ->
this->sleep(10);
QByteArray array = reply->read(50);
qDebug() << array;
}
}
As good eye might have already noticed - this code is placed in class that inherits QThread.
For some reason (that I cannot find) I can't receive any data from get function (I know that it is asynchronous), no signals are transmitted, and also after waiting 10 second there are no data available in read. I had also tried to get data via finished signal from QNetworkAccessManager itself but also nothing appeared.
Thanks to anyone who might have a clue what is happening.

Answer 1

You don't have an event loop in your thread's run() method, so there's no chance of any networking code working.
You should put all of your code into a QObject, and move it to a generic thread. The default implementation of QThread::run() spins an event loop.
I also don't see much reason for there to be more than one parser. You can simply let the QNetworkReply accumulate the response in its internal buffer until the request is finished - you can then parse it all in one go, obviating the need for parser state. Use the finished slot instead of readyRead. The readyRead slot only makes sense for large requests.
Below is how it might be done. Note the arming mechanism to prevent races between the worker thread and the main thread. You're guaranteed that the finishedAllRequests signal will be emitted exactly once after a call to arm(). Without this mechanism, the worker thread might be able to process all requests before the connect has a chance to run, and no signal will reach the recipient, or you might get multiple signals as the requests are processed before the next one is added.
#include <QCoreApplication>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QPointer>
#include <QSslError>
#include <QThread>
#include <QMetaMethod>
#include <QDebug>
class Parser : public QObject {
Q_OBJECT
bool m_armed;
QList<QNetworkReply*> m_replies;
QPointer<QNetworkAccessManager> m_manager;
QMetaMethod m_addRequestImpl, m_armImpl;
Q_SLOT void finished() {
QNetworkReply * reply = static_cast<QNetworkReply*>(sender());
Q_ASSERT(m_replies.contains(reply));
qDebug() << "reply" << reply << "is finished";
// ... use the data
m_replies.removeAll(reply);
if (m_armed && m_replies.isEmpty()) {
emit finishedAllRequests();
m_armed = false;
}
}
Q_SLOT void error(QNetworkReply::NetworkError) {
QNetworkReply * reply = static_cast<QNetworkReply*>(sender());
m_replies.removeAll(reply);
}
Q_SLOT void sslErrors(QList<QSslError>) {
QNetworkReply * reply = static_cast<QNetworkReply*>(sender());
m_replies.removeAll(reply);
}
Q_INVOKABLE void addRequestImpl(const QNetworkRequest & req) {
QNetworkReply * reply = m_manager->get(req);
connect(reply, SIGNAL(finished()), SLOT(finished()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
SLOT(error(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
SLOT(sslErrors(QList<QSslError>)));
m_replies << reply;
}
Q_INVOKABLE void armImpl() {
if (m_replies.isEmpty()) {
emit finishedAllRequests();
m_armed = false;
} else
m_armed = true;
}
static QMetaMethod method(const char * signature) {
return staticMetaObject.method(staticMetaObject.indexOfMethod(signature));
}
public:
// The API is fully thread-safe. The methods can be accessed from any thread.
explicit Parser(QNetworkAccessManager * nam, QObject * parent = 0) :
QObject(parent), m_armed(false), m_manager(nam),
m_addRequestImpl(method("addRequestImpl(QNetworkRequest)")),
m_armImpl(method("armImpl()"))
{}
void addRequest(const QNetworkRequest & req) {
m_addRequestImpl.invoke(this, Q_ARG(QNetworkRequest, req));
}
void arm() {
m_armImpl.invoke(this);
}
Q_SIGNAL void finishedAllRequests();
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QThread * thread = new QThread(&a);
thread->start();
QNetworkAccessManager mgr;
Parser parser(&mgr);
mgr.moveToThread(thread);
parser.moveToThread(thread);
for (int i = 0; i < 10; ++i) {
QNetworkRequest request(QUrl("https://www.google.pl/"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");
parser.addRequest(request);
}
thread->connect(&parser, SIGNAL(finishedAllRequests()), SLOT(quit()));
a.connect(thread, SIGNAL(finished()), SLOT(quit()));
parser.arm();
int rc = a.exec();
thread->wait();
delete thread; // Otherwise mgr's destruction would fail
return rc;
}
#include "main.moc"
Output:
reply QNetworkReplyHttpImpl(0x1011619e0) is finished
reply QNetworkReplyHttpImpl(0x101102260) is finished
reply QNetworkReplyHttpImpl(0x101041670) is finished
reply QNetworkReplyHttpImpl(0x1011023e0) is finished
reply QNetworkReplyHttpImpl(0x10102fa00) is finished
reply QNetworkReplyHttpImpl(0x101040090) is finished
reply QNetworkReplyHttpImpl(0x101163110) is finished
reply QNetworkReplyHttpImpl(0x10103af10) is finished
reply QNetworkReplyHttpImpl(0x10103e6b0) is finished
reply QNetworkReplyHttpImpl(0x101104c80) is finished