当再次请求相同的url时，Qt QNetworkReply readAll返回空

Qt QNetworkReply readAll returns empty when request the same url again

本文关键字：Qt QNetworkReply readAll 返回 url 请求      更新时间：2023-10-16

这纯粹是为了测试和我的好奇心。我使用DataDownloader类从服务器获取数据列表。它适用于不同的url，比如http://tiny-file-url，http://big-file-url.但如果我传入两个相同的url，http://tiny-file-url，http://tiny-file-url，以及来自http://tiny-file-url非常小，则来自QNetworkAccessManager的第二个回复为空。然而两个http://big-file-url似乎有效。我想知道QNetworkAccessManager在从同一个url请求小数据时是否有魔力？我似乎在Qt文档网站上找不到任何相关信息。

这是我的DataDownloader类：

class DataDownloader : public QObject
{
    Q_OBJECT
public:
    explicit DataDownloader(QObject* parent = 0);
    virtual ~DataDownloader();
    QByteArray data() const;
    void cancel();
    void download(QUrl url);
    signals:
    void isComplete();
    private slots:
    void complete(QNetworkReply* reply);
    private:
    QNetworkAccessManager m_NetworkManager;
    QByteArray m_Data;
    QNetworkReply* m_pReply;
};
DataDownloader::DataDownloader(QUrl url, QObject* parent) :QObject(parent)
{
    connect(&m_NetworkManager, SIGNAL(finished(QNetworkReply*)),
            SLOT(complete(QNetworkReply*)));
}
DataDownloader::~DataDownloader()
{
}
void DataDownloader::complete(QNetworkReply* reply)
{
    // in the first download it returns data
    // but the second time m_Data is empty
    m_Data = reply->readAll();
    reply->deleteLater();
    if (!m_Data.isEmpty()) {
        // no signal on the second try
        emit isComplete();
    }
}
QByteArray DataDownloader::data() const
{
    return m_Data;
}
void DataDownloader::cancel()
{
    m_pReply->abort();
}
void DataDownloader::download(QUrl url)
{
    QNetworkRequest request(url);
    m_pReply = m_NetworkManager.get(request);
}

调用此DataDownloader的代码如下：

void PluginManager::downloadPlugins()
{
    m_DownloadIndex++;
    savePlugin();
    if (m_DownloadIndex < m_PluginList.size()) {
        QUrl url;
        // alway return the same url here
        QString pluginUrl = getPluginUrl(m_PluginList.at(m_DownloadIndex));
        url.setUrl(pluginUrl);
        if (m_pPluginDownloader == NULL) {
            m_pPluginDownloader = new DataDownloader();
            // call this function again when isComplete is signaled
            connect(m_pPluginDownloader, SIGNAL(isComplete()), this, SLOT(downloadPlugins()));
        }
        m_pPluginDownloader->download(url);
    }
}

我认为这里isComplete是在连接到downloadPlugins插槽之前发出的。尝试首先连接此信号，然后执行下载操作：

m_pPluginDownloader = new DataDownloader(url);
connect(m_pPluginDownloader, SIGNAL(isComplete()), this, SLOT(downloadPlugins()));
m_pPluginDownloader.download();

与其在构造函数中执行get操作，不如在名为download:的槽中执行

void DataDownloader::download()
{
    QNetworkRequest request(url);
    m_pReply = m_NetworkManager.get(request);
}

另一点是，没有必要每次都创建DataDownloader的新实例。您可以在PluginManager的构造函数中创建一个实例，并在那里连接isComplete信号一次。然后，您可以在download插槽中提供url作为参数。

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