C++ - QT Framework / Cannot evoke method - c++

Question

everyone. First I want to apologize for my bad English. Here is the code of one of my personal project and I need help. The current code at execution should download this video - http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4, but does not work. The video is downloaded only when the following code be placed in
// main.cpp
int main (int argc, char * argv [])
{
QApplication a (argc, argv);
MainWindow w;
w.show ();
QtDownload dl;
dl.setTarget ( "http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4");
dl.download ();
// Quit when the download is done.
// QObject :: connect (& dl, SIGNAL (done ()), & app, SLOT (quit ()));
return a.exec ();
}
and I want to run in the method MainWindow - example:
// mainwindow.cpp
MainWindow :: MainWindow (QWidget * parent):
QMainWindow (parent),
ui (new Ui :: MainWindow)
{
QtDownload dl;
dl.setTarget ( "http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4");
dl.download ();
// Quit when the download is done.
// QObject :: connect (& dl, SIGNAL (done ()), & app, SLOT (quit ()));
........
}
Here is the current code of the program.
// downloader.pro
#-------------------------------------------------
#
# Project created by QtCreator 2017-01-12T15:22:17
#
#-------------------------------------------------
QT += core gui
QT += network
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = downloader
TEMPLATE = app
SOURCES += main.cpp\
mainwindow.cpp
HEADERS += mainwindow.h
FORMS += mainwindow.ui
// mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QObject>
#include <QString>
#include <QtNetwork/QNetworkAccessManager>
#include <QtNetwork/QNetworkReply>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
};
class QtDownload : public QObject {
Q_OBJECT
public:
explicit QtDownload();
QNetworkAccessManager manager;
QString target;
void setTarget(const QString& t);
private:
signals:
void done();
public slots:
void download();
void downloadFinished(QNetworkReply* data);
void downloadProgress(qint64 recieved, qint64 total);
};
#endif // MAINWINDOW_H
//main.cpp
#include "mainwindow.h"
#include <QApplication>
#include <QtCore>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
// mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QUrl>
#include <QtNetwork/QNetworkRequest>
#include <QFile>
#include <QDebug>
#include <QtNetwork/QNetworkReply>
#include <QtNetwork/QNetworkReply>
#include <QByteArray>
#include <QObject>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
// QCoreApplication app(argc, argv);
QtDownload dl;
dl.setTarget("http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4");
dl.download();
//quit when the download is done.
// QObject::connect(&dl, SIGNAL(done()), &app, SLOT(quit()));
}
MainWindow::~MainWindow()
{
delete ui;
}
QtDownload::QtDownload() : QObject(0) {
QObject::connect(&manager, SIGNAL(finished(QNetworkReply*)),this, SLOT(downloadFinished(QNetworkReply*)));
}
void QtDownload::setTarget(const QString &t) {
this->target = t;
}
void QtDownload::downloadFinished(QNetworkReply *data) {
QFile localFile("C:/downloadedfile.mp4");
if (!localFile.open(QIODevice::WriteOnly))
return;
const QByteArray sdata = data->readAll();
localFile.write(sdata);
qDebug() << sdata;
localFile.close();
emit done();
}
void QtDownload::download() {
QUrl url = QUrl::fromEncoded(this->target.toLocal8Bit());
QNetworkRequest request(url);
QObject::connect(manager.get(request), SIGNAL(downloadProgress(qint64,qint64)), this, SLOT(downloadProgress(qint64,qint64)));
}
void QtDownload::downloadProgress(qint64 recieved, qint64 total) {
qDebug() << recieved << total;
}

Answer 1

I have tried your code, I had the same problem that you report, only change the creation of the object to a pointer and magically arranged, I still do not understand the reason for error, I will continue to search for reason.
You must be:
...
ui->setupUi(this);
QtDownload *dl = new QtDownload();
dl->setTarget( "http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4");
dl->download();
...

Answer 2

So.. if you want to run the download at QMainWindowthe object QtDownload dl should be a pointer property of MainWindow or it will not be destroyed (unless dl, as a QObject, hasMainWindow` as parent).
Run it into main() function does not work in this case because the application will run when exec() was called, so the download
So it will be something like this:
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
// Delete the `dl` object
~MainWindow() { delete dl; }
private:
Ui::MainWindow *ui;
QtDownload *dl; // Declare it here
};
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow),
dl(nullptr)
{
ui->setupUi(this);
// QCoreApplication app(argc, argv);
dl = new QtDownload();
dl->setTarget("http://media09.vbox7.com/s/21/21bbc2dca3r3634e3389.mp4");
dl->download();
// connect the signal `done()` to save the file or anything like that
QObject::connect(dl, SIGNAL(done()), this, SLOT(downloadFinished()));
}