QEventLoop *loop = new QEventLoop;
connect(m_netWorkManager, SIGNAL(finished(QNetworkReply*)), loop, SLOT(quit()));
m_reply = m_netWorkManager->get(request);
loop->exec();

通过以上get访问到需要跳转的url时，速度明显快于不需要跳转的。具体原因还不清楚。

四、界面效果

粗略的绘制了一下界面，比较丑，主要以实现效果为主。

五、主要代码

启动下载：

void Http::startDownLoad(const QString &url)
{
    m_url = url;
    if(m_url.isEmpty()) return;

    if(!m_IsDownloading)
    {
        //获取请求头
        QNetworkRequest request;
        QUrl url = QUrl(m_url);
        request.setUrl(url);
        m_fileName = url.fileName();
        qDebug()<<"fileName = "<<m_fileName;
        m_reply = m_netWorkManager->head(request);
        m_state = requestHead;
        getCurrentFileSize();
        connect(m_reply,SIGNAL(finished()),this,SLOT(onfinishedRequest()));
    }
}

通过setUrl来装载我们请求的地址，随后获取请求头，请求结束后，进入槽函数做处理。

暂停下载：

void Http::stopDownLoad()
{
    if(m_reply == nullptr) return;

    disconnect(m_reply,SIGNAL(readyRead()),this,SLOT(onReadyRead()));
    disconnect(m_reply,SIGNAL(error(QNetworkReply::NetworkError)),this,SLOT(onError(QNetworkReply::NetworkError)));
    disconnect(m_reply,SIGNAL(finished()),this,SLOT(onfinishedRequest()));

    m_reply->abort();
    m_reply->deleteLater();

    m_file.close();
    getCurrentFileSize();

    m_IsDownloading = false;
}

暂停下载主要是断开信号槽的连接，然后释放资源和关闭文件操作符。

文件大小回读：

void Http::getCurrentFileSize()
{
    QFileInfo fileInfo(m_fileName);
    if(fileInfo.exists())
    {
        m_currentLoadedBytes = fileInfo.size();
    }
    else
    {
        m_currentLoadedBytes = 0;
    }
}

通过QFileInfo类提供的size()接口，获取文件当前的字节大小。

槽函数onfinishedRequest():

void Http::onfinishedRequest()
{
    if(m_reply==nullptr) return;

    if(m_state == requestHead)
    {       
        m_fileSize = m_reply->rawHeader("Content-Length").toInt();
        qDebug()<<"m_fileSize = "<<m_fileSize;
        if(m_currentLoadedBytes == m_fileSize)
        {
            qDebug()<<"文件已经存在!";
            return;
        }

        QEventLoop *loop = new QEventLoop;
        connect(m_netWorkManager, SIGNAL(finished(QNetworkReply*)), loop, SLOT(quit()));
        QNetworkRequest request;
        request.setUrl(m_url);
        m_reply = m_netWorkManager->get(request);
        loop->exec();

        //获取状态码
        m_statusCode = m_reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
        qDebug()<<"statusCode="<<m_statusCode;
        m_state = requestBody;
    }

    QNetworkRequest request;
    if(m_statusCode==200)
    {
        request.setUrl(m_url);
    }
    else if(m_statusCode == 302)    //存在转调url
    {
        //获取实际下载地址
        QUrl realUrl = m_reply->attribute(QNetworkRequest::RedirectionTargetAttribute).toUrl();
        request.setUrl(realUrl);
    }
    else
    {
        return;
    }

    QString downLoadSize = QString::number(m_fileSize);
    QString selectSize = QString("bytes=%1-%2").arg(m_currentLoadedBytes).arg(downLoadSize);
    request.setRawHeader("Range",selectSize.toLatin1());
    m_reply = m_netWorkManager->get(request);

    connect(m_reply,SIGNAL(finished()),this,SLOT(onfinishedRequest()));
    connect(m_reply,SIGNAL(readyRead()),this,SLOT(onReadyRead()));
    connect(m_reply,SIGNAL(error(QNetworkReply::NetworkError)),this,SLOT(onError(QNetworkReply::NetworkError)));
}

这里需要注意的是做了一个转调功能，因为有些资源的实际下载地址并不是当前所给的Url。当返回302的时候，说明需要获取背后正确的下载地址。

槽函数onReadyRead()：

void Http::onReadyRead()
{
    if(m_reply==nullptr) return;

    if(!m_file.isOpen())
    {
        m_file.setFileName(m_fileName);
        m_file.open(QIODevice::WriteOnly|QIODevice::Append);
    }

    m_file.write(m_reply->readAll());

    m_downLoadedBytes =m_file.size();

    emit fileDownloadProgress(m_downLoadedBytes, m_fileSize);
    if(m_file.size() == m_fileSize)
    {
        qDebug()<<"download finished!";
        stopDownLoad();
    }
}

将接收到的数据，写入文件中，并且将当前下载字节数通过信号槽发送出去，提供给进度条对话框使用。

当下载的字节总大小等于请求时服务器返回过来的文件大小时，就认为下载结束。

当然，更严谨的做法是再计算文件的MD5码，与服务器回传的MD5码进行比较，以确保下载的是正确的文件。

收录于