Network Download Example

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#include <QtCore>
#include <QtNetwork>

#include <cstdio>

class QSslError;

using namespace std;

class DownloadManager: public QObject
{
    Q_OBJECT
    QNetworkAccessManager manager;
    QVector<QNetworkReply *> currentDownloads;

public:
    DownloadManager();
    void doDownload(const QUrl &url);
    static QString saveFileName(const QUrl &url);
    bool saveToDisk(const QString &filename, QIODevice *data);
    static bool isHttpRedirect(QNetworkReply *reply);

public slots:
    void execute();
    void downloadFinished(QNetworkReply *reply);
    void sslErrors(const QList<QSslError> &errors);
};

DownloadManager::DownloadManager()
{
    connect(&manager, SIGNAL(finished(QNetworkReply*)),
            SLOT(downloadFinished(QNetworkReply*)));
}

void DownloadManager::doDownload(const QUrl &url)
{
    QNetworkRequest request(url);
    QNetworkReply *reply = manager.get(request);

#if QT_CONFIG(ssl)
    connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
            SLOT(sslErrors(QList<QSslError>)));
#endif

    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;
}

bool DownloadManager::saveToDisk(const QString &filename, QIODevice *data)
{
    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(data->readAll());
    file.close();

    return true;
}

bool DownloadManager::isHttpRedirect(QNetworkReply *reply)
{
    int statusCode = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
    return statusCode == 301 || statusCode == 302 || statusCode == 303
           || statusCode == 305 || statusCode == 307 || statusCode == 308;
}

void DownloadManager::execute()
{
    QStringList args = QCoreApplication::instance()->arguments();
    args.takeFirst();           // skip the first argument, which is the program's name
    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;
    }

    for (const QString &arg : qAsConst(args)) {
        QUrl url = QUrl::fromEncoded(arg.toLocal8Bit());
        doDownload(url);
    }
}

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

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 {
        if (isHttpRedirect(reply)) {
            fputs("Request was redirected.\n", stderr);
        } 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();
    }
}

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

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

    app.exec();
}

#include "main.moc"