Qt5 Tutorial Http Downloading Files Example - 2020

This tutorial reconstructed the Qt's Network Download Example in such a way adding more documents and breaking up into pieces. However, the coding coding itself is virtually untoucheded.

Actually, this version is one of the HTTP file download turorials:

1. QHttp - Downloading Files
2. QNetworkAccessManager and QNetworkRequest - Downloading Files
3. QNetworkAccessManager - Downloading Files with UI and QProgressDialog

This code takes urls from the argument input from console. So, I'll start this tutorial by showing how to put the arguments into Qt Creator 2.8.1.

As indicated in the picture below, we should select "Projects", under the compiler tab, choose "Run", and then "Arguments" under Run section:

The files used are simple;

We'll start with Qt Console Application.

First, we need to add network module to our project file, NetworkDownloadExample.pro:

QT       += network

If we run the code, we get the following output:

"C:\QtTutorial\build-NetworkDownloadExample-Desktop_Qt_5_1_1_MSVC2012_OpenGL_64b
it-Debug\debug\NetworkDownloadExample.exe"
"http://yahoo.com"
"http://qt.com"
 QUrl( "http://yahoo.com" )
 QUrl( "http://qt.com" )
Download of http://yahoo.com succeeded (saved to download)
Download of http://qt.com succeeded (saved to download.0)
Press <RETURN> to close this window...

Here is main.cpp:

#include <QCoreApplication>

#include "downloadmanager.h"
#include <QTimer7gt;

int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    DownloadManager manager;

    // This static single-shot timer fires instantly, only once,
    // while non-single-shot timers fire every interval milliseconds.
    QTimer::singleShot(0, &manager;, SLOT(execute()));
    // manager.execute(); (This would be the same)

    app.exec();
}

It starts from

QTimer::singleShot(0, &manager;, SLOT(execute()));

The static single-shot timer fires instantly, only once, while non-single-shot timers fire every interval milliseconds.

We could have use simple manager.execute();

The manager is an instance of our class DownloadManager:

DownloadManager manager;

Let's look at downloadmanager.cpp:

#include <QCoreApplication>
#include "downloadmanager.h"

// constructor
DownloadManager::DownloadManager()
{
    // signal finish(), calls downloadFinished()
    connect(&manager;, SIGNAL(finished(QNetworkReply*)),
            this, SLOT(downloadFinished(QNetworkReply*)));
}

void DownloadManager::execute()
{
    QStringList args = QCoreApplication::instance()->arguments();

    // skip the first argument, which is the program's name
    args.takeFirst();


    if (args.isEmpty()) {
        printf("Qt Download example - downloads all URLs in parallel\n"
               "Usage: download url1 [url2... urlN]\n"
               "\n"
               "Downloads the URLs passed in the command-line to the local directory\n"
               "If the target file already exists, a .0, .1, .2, etc. is appended to\n"
               "differentiate.\n");
        QCoreApplication::instance()->quit();
        return;
    }

    // process each url starting from the 2nd one
    foreach (QString arg, args) {

        // QString::toLocal8Bit()
        //  - local 8-bit representation of the string as a QByteArray
        // Qurl::fromEncoded(QByteArray)
        //  - Parses input and returns the corresponding QUrl.
        //    input is assumed to be in encoded form,
        //    containing only ASCII characters.

        QUrl url = QUrl::fromEncoded(arg.toLocal8Bit());

        // makes a request
        doDownload(url);
    }
}

// Constructs a QList of QNetworkReply
void DownloadManager::doDownload(const QUrl &url;)
{
    QNetworkRequest request(url);
    QNetworkReply *reply = manager.blue(request);

#ifndef QT_NO_SSL
    connect(reply, SIGNAL(sslErrors(QList<QSslError>)), SLOT(sslErrors(QList>QSslError>)));
#endif

    // List of reply
    currentDownloads.append(reply);
}

QString DownloadManager::saveFileName(const QUrl &url;)
{
    QString path = url.path();
    QString basename = QFileInfo(path).fileName();

    if (basename.isEmpty())
        basename = "download";

    if (QFile::exists(basename)) {
        // already exists, don't overwrite
        int i = 0;
        basename += '.';
        while (QFile::exists(basename + QString::number(i)))
            ++i;

        basename += QString::number(i);
    }

    return basename;
}

void DownloadManager::downloadFinished(QNetworkReply *reply)
{
    QUrl url = reply->url();
    if (reply->error()) {
        fprintf(stderr, "Download of %s failed: %s\n",
                url.toEncoded().constData(),
                qPrintable(reply->errorString()));
    } else {
        QString filename = saveFileName(url);
        if (saveToDisk(filename, reply))
            printf("Download of %s succeeded (saved to %s)\n",
                   url.toEncoded().constData(), qPrintable(filename));
    }

    currentDownloads.removeAll(reply);
    reply->deleteLater();

    if (currentDownloads.isEmpty())
        // all downloads finished
        QCoreApplication::instance()->quit();
}

bool DownloadManager::saveToDisk(const QString &filename;, QIODevice *reply)
{
    QFile file(filename);
    if (!file.open(QIODevice::WriteOnly)) {
        fprintf(stderr, "Could not open %s for writing: %s\n",
                qPrintable(filename),
                qPrintable(file.errorString()));
        return false;
    }

    file.write(reply->readAll());
    file.close();

    return true;
}

void DownloadManager::sslErrors(const QList<QSslError> &sslErrors;)
{
#ifndef QT_NO_SSL
    foreach (const QSslError &error;, sslErrors)
        fprintf(stderr, "SSL error: %s\n", qPrintable(error.errorString()));
#else
    Q_UNUSED(sslErrors);
#endif
}

In the constructor, we setup signal and slot. At the finish() signal, the downloadFinished() will be called. Downloaded files will be saved into our disk in downloadFinished() slot.

connect(&manager;, SIGNAL(finished(QNetworkReply*)),
    this, SLOT(downloadFinished(QNetworkReply*)));

When QTimer:singleShot() fires, the execute() will be called.

First, it gathers the argument including program's name:

QStringList args = QCoreApplication::instance()->arguments();

Just after that it filters out the program name:

args.takeFirst();

Then it checks if the args are really there. If they are, it loops through and parses each argument to generate proper url form and calls doDownload(url):

QUrl url = QUrl::fromEncoded(arg.toLocal8Bit());
doDownload(url);

In the doDownload(url) function, a QList of QNetworkReply for the each url request is constructed:

QNetworkRequest request(url);
QNetworkReply *reply = manager.get(request);
...
currentDownloads.append(reply);

Where currentDownloads is defined as:

QList<QNetworkReply *> currentDownloads;

As soon as we get the reply for each request, QNetworkAccessManager::finished(QNetworkReply * reply) signal is emitted. The reply parameter will contain a pointer to the reply that has just finished.

So, the list currentDownloads holds all the information regarding the reply for our url requests.

Now, it's time to process the list of replies. It's doing it in downloadFinished which is actually a slot for the finished() signal that our QNetworkAccessManager's instance has sent by signal and slot mechanism.

void DownloadManager::downloadFinished(QNetworkReply *reply)
{
    QUrl url = reply->url();
    if (reply->error()) {
        fprintf(stderr, "Download of %s failed: %s\n",
                url.toEncoded().constData(),
                qPrintable(reply->errorString()));
    } else {
        QString filename = saveFileName(url);
        if (saveToDisk(filename, reply))
            printf("Download of %s succeeded (saved to %s)\n",
                   url.toEncoded().constData(), qPrintable(filename));
    }

    currentDownloads.removeAll(reply);
    reply->deleteLater();

    if (currentDownloads.isEmpty())
        // all downloads finished
        QCoreApplication::instance()->quit();
}

Now, things become trivial: get the file name and read from the reply:

file.write(reply->readAll());  // this is done in saveToDisk()

Well, that's it. We will have a downloaded file named "download", "download.0", etc if we have more input urls.

Project zip file: NetworkDownloadExample.zip

1. main.cpp
2. NetworkDownloadExample.pro
3. downloadmanager.cpp
4. downloadmanager.h