利用QNetworkAccessManager和QHttp完成文件下载有关问题

// 头文件
#ifndef SYCHRONIZATION_TASK_H
#define SYCHRONIZATION_TASK_H

#include <QHttp>
#include <QFile>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QNetworkReply>

class SynchronizationTask : public QObject
{
    Q_OBJECT

public:
    SynchronizationTask();

    void run();
    void startRequest(QUrl url);

signals:
    void synchronizationFinish();
    void synchronizationFail(const QString& msg);

private slots:
    void httpFinished();
    void httpReadyRead();

private:
    QHttp *m_http;
    QString m_state;
    QUrl m_url;
    QNetworkAccessManager m_qnam;
    QNetworkReply *m_reply;
    QFile *m_file;
    int m_httpGetId;
};

#endif

// 源文件
#include "sychronizationtask.h"
#include <QMessageBox>
#include <QFileInfo>
#include <QNetworkRequest>

SynchronizationTask::SynchronizationTask(HostWindow* host, const QString& state) : 
    m_host(host), 
    m_state(state)
{

}

void SynchronizationTask::run()
{
    try
    {
        m_url = tr("http://www.baidu.com");
        QString fileName = QFileInfo(m_url.path()).fileName();
        if (fileName.isEmpty())
            fileName = "index.html";

        if (QFile::exists(fileName)) {
            QFile::remove(fileName);
        }

        m_file = new QFile(fileName);
        if (!m_file->open(QIODevice::WriteOnly)) {
            delete m_file;
            m_file = 0;
            emit synchronizationFail(tr("Unable to save the file %1: %2.")
                .arg(fileName).arg(m_file->errorString()));
        }

        // schedule the request
        startRequest(m_url);
    }
    catch(const std::exception &e)
    {
        emit synchronizationFail(e.what());
    }
    catch(...)
    {
        emit synchronizationFail(tr("Unknown exception"));
    }
}

void SynchronizationTask::startRequest(QUrl url)
{
    try
    {
        QNetworkRequest request;
        request.setUrl(url);
        m_reply = m_qnam.get(request);
        connect(m_reply, SIGNAL(finished()),    this, SLOT(httpFinished()));
        connect(m_reply, SIGNAL(readyRead()),this, SLOT(httpReadyRead()));
    }
    catch(const std::exception &e)
    {
        emit synchronizationFail(e.what());
    }
    catch(...)
    {
        emit synchronizationFail(tr("Unknown exception"));
    }
}

void SynchronizationTask::httpFinished()
{
    try
    {
        m_file->flush();
        m_file->close();


        QVariant redirectionTarget = m_reply->attribute(QNetworkRequest::RedirectionTargetAttribute);
        if (m_reply->error()) {
            m_file->remove();
        } else if (!redirectionTarget.isNull()) {
            QUrl newUrl = m_url.resolved(redirectionTarget.toUrl());
            if (QMessageBox::question(m_host, tr("HTTP"),
                tr("Redirect to %1 ?").arg(newUrl.toString()),
                QMessageBox::Yes | QMessageBox::No) == QMessageBox::Yes) {
                    m_url = newUrl;
                    m_reply->deleteLater();
                    m_file->open(QIODevice::WriteOnly);
                    m_file->resize(0);
                    startRequest(m_url);
                    return;
            }
        } 

        m_reply->deleteLater();
        m_reply = 0;
        delete m_file;
        m_file = 0;
        emit synchronizationFinish();
    }
    catch(const std::exception &e)
    {
        emit synchronizationFail(e.what());
    }
    catch(...)
    {
        emit synchronizationFail(tr("Unknown exception"));
    }
}

void SynchronizationTask::httpReadyRead()
{
    if (m_file)
        m_file->write(m_reply->readAll());
}