在QCoreApplication中下载文件

Question

我在一个用c++开发QT应用程序的团队中工作。除此之外，该应用程序还需要从互联网上下载文件。下面是我用来下载文件的代码：

int main(int argc, char *argv[]){
        QCoreApplication app(argc, argv);
        QNetworkAccessManager man;
        std::string urlc = "https://upload.wikimedia.org/wikipedia/commons/7/73/Flat_tick_icon.svg"; //Ramdom svg file
        QUrl url(QString::fromStdString(urlc));
        QNetworkRequest req(url);
        QNetworkReply* reply = man.get(req);
        QObject::connect(reply, &QNetworkReply::finished, [department, region, &reply](){
            QByteArray read = reply->readAll();
            QString savefile = QString::fromStdString("file.png");
            QFile out(savefile);
            out.open(QIODevice::WriteOnly);
            out.write(read);
            out.close();
            reply->close();
            reply->deleteLater();
            app.quit();
        });
        return app.exec();
  }

上面的代码运行良好，并以指定的url下载文件。但是，如果我试图将下载的文件解压到一个单独的函数中(参见下面的代码)，程序就不能工作。事实上，它甚至从来没有开始下载文件。

std::string download_file(){
    QNetworkAccessManager man;
    std::string urlc = "https://upload.wikimedia.org/wikipedia/commons/7/73/Flat_tick_icon.svg"; //Ramdom svg file
    QUrl url(QString::fromStdString(urlc));
    QNetworkRequest req(url);
    QNetworkReply* reply = man.get(req);
    QObject::connect(reply, &QNetworkReply::finished, [department, region, &reply](){
        QByteArray read = reply->readAll();
        QString savefile = QString::fromStdString("file.png");
        QFile out(savefile);
        out.open(QIODevice::WriteOnly);
        out.write(read);
        out.close();
        reply->close();
        reply->deleteLater();
    });
    return "file.png";
}

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

    std::string path = download_file();
    std::cout << path << std::endl;

    return app.exec();
}

这种行为的原因可能是什么？

Answer 1

不同之处在于，QNetworkAccessManager在第一种情况下有更大的作用域，而第二种情况只是一个局部变量，因此它将在尝试时被销毁。一种解决方法是创建一个处理所有逻辑的类：

#include <QCoreApplication>
#include <QFile>
#include <QNetworkAccessManager>
#include <QNetworkReply>

class Downloader: public QObject{
    Q_OBJECT
public:
    Downloader(QObject *parent=nullptr):QObject(parent){
        connect(&m_manager, &QNetworkAccessManager::finished, this, &Downloader::handle_finished);
    }
    void download(const QUrl & url, const QString & filename){
        QNetworkRequest request;
        request.setUrl(url);
        request.setAttribute(QNetworkRequest::User, filename);
        m_manager.get(request);
    }
    Q_SIGNAL void finished(bool);
private:
    void handle_finished(QNetworkReply *reply){
        bool ok = false;
        if(reply->error() == QNetworkReply::NoError){
            QByteArray read = reply->readAll();
            QString filename = reply->request().attribute(QNetworkRequest::User).toString();
            QFile out(filename);
            if(out.open(QIODevice::WriteOnly)){
                out.write(read);
                out.close();
                ok = true;
            }
        }
        else{
            qDebug() << reply->error() << reply->errorString();
        }
        reply->deleteLater();
        Q_EMIT finished(ok);
    }
    QNetworkAccessManager m_manager;
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Downloader downloader;

    QObject::connect(&downloader, &Downloader::finished, [](bool sucess){
        qDebug() << "download" << sucess;
    });

    downloader.download(QUrl("https://upload.wikimedia.org/wikipedia/commons/7/73/Flat_tick_icon.svg"), "file.svg");

    return a.exec();
}