[Solved] Cannot download using QNetworkAccessManager
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Hello. QNetworkAccessManager proved to be a pretty tough task. I have been fighting it for more than a week already... The following code is not working. It does not give any errors but it does not load the file either.
Downloader.h
[code]
#ifndef DOWNLOADER_H
#define DOWNLOADER_H#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QUrl>
#include <QDateTime>
#include <QFile>
#include <QDebug>class Downloader : public QObject
{
Q_OBJECT
public:
explicit Downloader(QObject *parent = 0);
void doDownload();
QString getData();signals:
public slots:
void replyFinished (QNetworkReply *reply);private:
QNetworkAccessManager *manager;
QByteArray data;
QString dataFinal;
};#endif // DOWNLOADER_H
[/code]Downloader.cpp
[code]
#include "Downloader.h"Downloader::Downloader(QObject *parent) :
QObject(parent)
{
}void Downloader::doDownload()
{
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(replyFinished(QNetworkReply*)));
manager->get(QNetworkRequest(QUrl ("ftp://someftp.com/somefile.TXT")));
}void Downloader::replyFinished (QNetworkReply *reply)
{
if(reply->error())
{
qDebug() << "ERROR!";
qDebug() << reply->errorString();
}
else
{
data = reply->readAll();
QString dataStr(data);
dataFinal = dataStr;
reply->deleteLater();
}
}QString Downloader::getData()
{
return dataFinal;
}
[/code]mainWindow.cpp
[code]
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "Downloader.h"MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}MainWindow::~MainWindow()
{
delete ui;
}void MainWindow::on_pushButton_clicked()
{
Downloader d;
d.doDownload();QString metarData; metarData = d.getData(); ui->plainTextEdit->setPlainText(metarData);}
[/code]Thank you so much in advance!
Igor
-
You are reading the data too soon. QNAM is asynchronous. You need to wait for the reply to come, before you can read it.
-
Hm, thanks! Too bad the examples that I have seen omit that. I will search for the solution.
Again, thanks a lot!EDIT:
So I have found that I need to have
[code]
QEventLoop loop;
loop.exec();
[/code]Can you prompt please where exactly it goes in the code?
Thanks!
-
They don't. You are simply doing it wrong in your MainWindow. Downloader class is done correctly.
-
@
Downloader d;
d.doDownload();QString metarData;
// This is WAY too soon!
metarData = d.getData();
@instead of getting the data in the very same method, you need to wait until the reply comes. Either use a forever loop (ugly solution!), or, just like you are doing in Downloader: respond to replyFinished().
-
sierdzio,
Sorry, I am trying to understand what you mean by "like you are doing in Downloader: respond to replyFinished()."
I have been searching more on this and another suggestion that I have come across with was to put the d object on the heap. I changed it to:
[code]
Downloader* d = new Downloader;
d->doDownload();QString metarData; metarData = d->getData(); ui->plainTextEdit->setPlainText(metarData);[/code]
Still no luck... Sorry for being so n00b.. ))
-
When the button is clicked, only send the request (doDownload). Then getData in a slot connected to finished(). You need to refactor the code bit. Here is a small helper:
@
// Downloader
signals:
void downloadFinished(Downloader *d);
...
void Downloader::replyFinished (QNetworkReply *reply) {
...
emit downloadFinished(this);
}// MainWindow
void MainWindow::on_pushButton_clicked()
{
Downloader *d = new Downloader(this);
connect (d, SIGNAL(downloadFinished(Downloader *)), this, SLOT(someSlot(Downloader *)));
d->doDownload();
...
}
...
void MainWindow::someSlot(Downloader *down)
{
QString metarData;
metarData = down->getData();
// Optional:
down->deleteLater();
}
@ -
sierdzio. You do belong to the Hall of Fame for your support and knowledge. I am still trying to nail it down. This is what I have now:
[code]
#include "Downloader.h"Downloader::Downloader(QObject *parent) :
QObject(parent)
{
}void Downloader::doDownload()
{
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)),this, SLOT(replyFinished(QNetworkReply*)));
manager->get(QNetworkRequest(QUrl("ftp://ftp.com/ftp.TXT")));}
void Downloader::replyFinished (QNetworkReply *reply)
{
if(reply->error())
{
qDebug() << "ERROR!";
qDebug() << reply->errorString();
}
else
{
dataByte = reply->readAll();
emit downloadFinished(this);
reply->deleteLater();
}
}void Downloader::downloadFinished(Downloader *dfinished)
{
QString dataStr(dataByte);
dataFinal = dataStr;
dfinished->deleteLater();
}QString Downloader::getData()
{
return dataFinal;
}[/code]
MainWindow.cpp
[code]
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "Downloader.h"MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}MainWindow::~MainWindow()
{
delete ui;
}void MainWindow::on_pushButton_clicked()
{
d = new Downloader(this); // declared in header
connect(d, SIGNAL(downloadFinished(Downloader *)), this, SLOT(someSlot(Downloader *)));
d->doDownload();ui->plainTextEdit->setPlainText(metarData);}
void MainWindow::someSlot(Downloader* down)
{
metarData = d->getData();
down->deleteLater();
}
[/code]Now, I get a weird error saying that
[code]
multiple definition of Downloader::downloadFInished(Downloader*)
first define here
[/code]Weird, because I only have it defined in Downloader.h
[code]
#ifndef DOWNLOADER_H
#define DOWNLOADER_H#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QUrl>
#include <QDateTime>
#include <QFile>
#include <QDebug>
#include <QEventLoop>class Downloader : public QObject
{
Q_OBJECT
public:
explicit Downloader(QObject *parent = 0);
void doDownload();
QString getData();signals:
void downloadFinished(Downloader *dfinished);public slots:
void replyFinished (QNetworkReply *reply);private:
QNetworkAccessManager *manager;
QByteArray dataByte;
QString dataFinal;
};#endif // DOWNLOADER_H
[/code]I love Qt, but it is so discouraging to spend many days on something that took me a day to figure out how to do it in libcurl and Java.
sierdzio. Thank you again!
-
If you want easy, go with QHttp or QFtp ;) QNAM is powerful, but also more complicated. The asynchronous paradigm is much better, but also more complicated and harder to get.
In your case, you've hit a small detail that some people new to Qt do stumble upon: a signal is not a method, only the slots need implementation. So, you should delete all this:
@
// Delete!
void Downloader::downloadFinished(Downloader *dfinished)
{
QString dataStr(dataByte);
dataFinal = dataStr;
dfinished->deleteLater();
}
@And only leave this line in the header file:
@
signals:
void downloadFinished(Downloader *dfinished);
@ -
sierdzio. You are the MAN! Thank you. Now the file is getting downloaded. I see it in qDebug. One problem still remains though due the asynchronous process, I believe. The thing is that when in the following code:
[code]
void MainWindow::on_pushButton_clicked()
{
d = new Downloader(this);
d->doDownload();
connect(d, SIGNAL(downloadFinished(Downloader *)), this, SLOT(someSlot(Downloader *)));qDebug() << "MetarData: " << metarData; ui->plainTextEdit->setPlainText(metarData);}
void MainWindow::someSlot(Downloader* down)
{
metarData = d->getData();
down->deleteLater();
}
[/code]when I do this:
[code]
ui->plainTextEdit->setPlainText(metarData);
[/code]it does not do anything. Apparently, it does not wait for the file to be first downloaded and goes straight to setPlainText because
[code]
qDebug() << "MetarData: " << metarData
[/code]returns an empty string. In my main application, where metarData is further passed to other functions, I even get a CTD.
I thought that connect should take care of this and the code would wait for the full data to be downloaded first.Thanks!
-
You assign the variable at the wrong time. Do this instead:
@
void MainWindow::someSlot(Downloader* down)
{
metarData = d->getData();
ui->plainTextEdit->setPlainText(metarData);
down->deleteLater();
}
@
