如何使用 QEventLoop 同步等待多个 QNetwork 回复？答案

【问题标题】：How to wait for multiple QNetwork replies synchronously using QEventLoop?如何使用 QEventLoop 同步等待多个 QNetwork 回复？
【发布时间】：2020-03-29 21:26:20
【问题描述】：

我的函数使用 QNetworkAccessManager 同时下载多个文件。然后它应该等待所有 QNetworkReply 回复完成，然后再继续执行其余代码。

但是我不能让 QEventLoop 处理多个连接，它要么在所有回复完成之前返回，要么就陷入我认为是无限循环的状态。

这是我的代码：

void DownloadFiles(QStringList urls)
{
    QNetworkAccessManager *manager = new QNetworkAccessManager(this);
    QEventLoop loop;
    QNetworkReply *replies[6];

    for (int i = 0, i < urls.lenght(); i++){
        replies[i] = manager->get(QNetworkRequest(QUrl(urls[i]));
        connect(replies[i], SIGNAL(finished()), &loop, SLOT(quit()));
    }
    loop.exec();  // Wait here
    // Rest of code
}

另一个尝试，也许更荒谬：

void DownloadFiles(QStringList urls)
{
    QNetworkAccessManager *manager = new QNetworkAccessManager(this);
    QEventLoop loops[6];
    QNetworkReply *replies[6];

    for (int i = 0, i < urls.lenght(); i++){
        replies[i] = manager->get(QNetworkRequest(QUrl(urls[i]));
        connect(replies[i], SIGNAL(finished()), &loops[i], SLOT(quit()));
    }
    for (int i = 0; i < 6; i++)
       loops[i]->exec();  // Wait here
    // Rest of code
}

同样，我的目标是同时下载多个文件，并等待所有文件都完成，然后再继续执行其余代码。

【问题讨论】：

标签： c++ qt qtnetwork

【解决方案1】：

您可以使用计数器来检查信号被触发的次数。

#include <QtNetwork>

static void DownloadFiles(const QList<QUrl> & urls){
    QNetworkAccessManager manager;
    QList<QNetworkReply *> replies;
    QEventLoop loop;
    int total = urls.length();
    for(const QUrl & url: urls){
        QNetworkReply *reply = manager.get(QNetworkRequest(url));
        QObject::connect(reply, &QNetworkReply::finished, [&total, &loop](){
            total--;
            if(total == 0){
                loop.quit();
            }
        });
        replies << reply;
    }
    loop.exec();
    for(QNetworkReply *reply: replies){
        qDebug() << "===============================";
        qDebug() << "url:" << reply->url() << "\nhas error?" << (reply->error() != QNetworkReply::NoError);
        qDebug() << "===============================";
    }
    qDebug() << "Completed";
    qDeleteAll(replies.begin(), replies.end());
    replies.clear();
}

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

    QList<QUrl> urls { QUrl("https://www.qt.io/"),
                       QUrl("https://github.com/"),
                       QUrl("https://stackoverflow.com/") };
    DownloadFiles(urls);

    return 0;
}

输出：

===============================
url: QUrl("https://www.qt.io/") 
has error? false
===============================
===============================
url: QUrl("https://github.com/") 
has error? false
===============================
===============================
url: QUrl("https://stackoverflow.com/") 
has error? false
===============================
Completed

【讨论】：