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QT：多线程下载文件

第七根弦 2015 年 7 月 14 日 QT 发表评论 (0)

这里的线程是指下载的通道（和操作系统中的线程不一样），一个线程就是一个文件的下载通道，多线程也就是同时开起好几个下载通道.当服务器提供下载服务时，使用下载者是共享带宽的，在优先级相同的情况下，总服务器会对总下载线程进行平均分配。不难理解，如果你线程多的话，那下载的越快。现流行的下载软件都支持多线程。

思路：
1：用阻塞的方式获取目标地址的HTTP头部，得到目标文件的大小。
2：算出每段文件的开始点，结尾点，并分别向目标地址发出请求。
3：每次目标地址有数据返回，都将得到的数据写入文件。

4：等待各段文件下载结果。

运行截图：
0_1328014168VgK5

源代码：
main.cpp

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#include <QCoreApplication>
#include "downloadcontrol.h"
 
//多线程下载的线程数
const int PointCount = 5;
//目标文件的地址（千千静听的下载地址，我用来做实验的）
 
 
int main(int argc, char **argv)
{
    QCoreApplication a(argc, argv);
    //用阻塞的方式下载文件，完成后退出
    DownloadControl *control = new DownloadControl;
    QEventLoop loop;
    QObject::connect(control, SIGNAL(FileDownloadFinished()),
                     &loop, SLOT(quit()));
    control->StartFileDownload(strUrl, PointCount);
    loop.exec();
    control->deleteLater();
    return 0;
}

downloadcontrol.h

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#ifndef DOWNLOADCONTROL_H
#define DOWNLOADCONTROL_H
 
#include <QObject>
#include <QFile>
#include <QUrl>
 
const QString strUrl = "http://ttplayer.qianqian.com/otherdown/alladin/ttpsetup_5713.exe";
 
//用于管理文件的下载
class DownloadControl : public QObject
{
    Q_OBJECT
private:
    int m_DownloadCount;
    int m_FinishedNum;
    int m_FileSize;
    QUrl m_Url;
    QFile *m_File;
public:
    DownloadControl(QObject *parent = 0);
    void StartFileDownload(const QString &url, int count);
    qint64 GetFileSize(QUrl url);
signals:
    void FileDownloadFinished();
private slots:
    void SubPartFinished();
};
 
#endif // DOWNLOADCONTROL_H

downloadcontrol.cpp

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#include "downloadcontrol.h"
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QEventLoop>
#include <QFileInfo>
#include "download.h"
 
//用阻塞的方式获取下载文件的长度
qint64 DownloadControl::GetFileSize(QUrl url)
{
    QNetworkAccessManager manager;
    qDebug() << "Getting the file size...";
    QEventLoop loop;
    //发出请求，获取目标地址的头部信息
    QNetworkReply *reply = manager.head(QNetworkRequest(url));
    QObject::connect(reply, SIGNAL(finished()), &loop, SLOT(quit()), Qt::DirectConnection);
    loop.exec();
    QVariant var = reply->header(QNetworkRequest::ContentLengthHeader);
    reply->deleteLater();
    qint64 size = var.toLongLong();
    qDebug() << "The file size is: " << size;
    return size;
}
 
DownloadControl::DownloadControl(QObject *parent)
    : QObject(parent)
{
    m_DownloadCount = 0;
    m_FinishedNum = 0;
    m_FileSize = 0;
    m_File = new QFile;
}
 
void DownloadControl::StartFileDownload(const QString &url, int count)
{
    m_DownloadCount = count;
    m_FinishedNum = 0;
    m_Url = QUrl(url);
    m_FileSize = GetFileSize(m_Url);
    //先获得文件的名字
    QFileInfo fileInfo(m_Url.path());
    QString fileName = fileInfo.fileName();
    if (fileName.isEmpty())
        fileName = "index.html";
 
    m_File->setFileName(fileName);
    //打开文件
    m_File->open(QIODevice::WriteOnly);
    Download *tempDownload;
 
    //将文件分成PointCount段，用异步的方式下载
    qDebug() << "Start download file from " << strUrl;
    for(int i=0; i<m_DownloadCount; i++)
    {
        //先算出每段的开头和结尾（HTTP协议所需要的信息）
        int start = m_FileSize * i / m_DownloadCount;
        int end = m_FileSize * (i+1) / m_DownloadCount;
        if( i != 0 )
            start--;
 
        //分段下载该文件
        tempDownload = new Download(i+1, this);
        connect(tempDownload, SIGNAL(DownloadFinished()),
                this, SLOT(SubPartFinished()));
        connect(tempDownload, SIGNAL(DownloadFinished()),
                tempDownload, SLOT(deleteLater()));
        tempDownload->StartDownload(m_Url, m_File, start, end);
    }
}
 
void DownloadControl::SubPartFinished()
{
    m_FinishedNum++;
    //如果完成数等于文件段数，则说明文件下载完毕，关闭文件，发生信号
    if( m_FinishedNum == m_DownloadCount )
    {
        m_File->close();
        emit FileDownloadFinished();
        qDebug() << "Download finished";
    }
}

download.h

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#ifndef DOWNLOAD_H
#define DOWNLOAD_H
 
#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QFile>
 
 
//用于下载文件（或文件的一部分）
class Download : public QObject
{
    Q_OBJECT
private:
    QNetworkAccessManager m_Qnam;
    QNetworkReply *m_Reply;
    QFile *m_File;
 
    const int m_Index;
    qint64 m_HaveDoneBytes;
    qint64 m_StartPoint;
    qint64 m_EndPoint;
 
public:
    Download(int index, QObject *parent = 0);
    void StartDownload(const QUrl &url, QFile *file,
                       qint64 startPoint=0, qint64 endPoint=-1);
signals:
    void DownloadFinished();
 
public slots:
    void FinishedSlot();
    void HttpReadyRead();
};
 
#endif // DOWNLOAD_H

download.cpp

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#include "download.h"
#include <QNetworkRequest>
 
Download::Download(int index, QObject *parent)
    : QObject(parent), m_Index(index)
{
    m_HaveDoneBytes = 0;
    m_StartPoint = 0;
    m_EndPoint = 0;
    m_File = NULL;
}
 
void Download::StartDownload(const QUrl &url,
                             QFile *file,
                             qint64 startPoint/* =0 */,
                             qint64 endPoint/* =-1 */)
{
    if( NULL == file )
        return;
 
    m_HaveDoneBytes = 0;
    m_StartPoint = startPoint;
    m_EndPoint = endPoint;
    m_File = file;
 
    //根据HTTP协议，写入RANGE头部，说明请求文件的范围
    QNetworkRequest qheader;
    qheader.setUrl(url);
    QString range;
    range.sprintf("Bytes=%lld-%lld", m_StartPoint, m_EndPoint);
    qheader.setRawHeader("Range", range.toLatin1());
 
    //开始下载
    qDebug() << "Part " << m_Index << " start download";
    m_Reply = m_Qnam.get(QNetworkRequest(qheader));
    connect(m_Reply, SIGNAL(finished()),
            this, SLOT(FinishedSlot()));
    connect(m_Reply, SIGNAL(readyRead()),
            this, SLOT(HttpReadyRead()));
}
 
//下载结束
void Download::FinishedSlot()
{
    m_File->flush();
    m_Reply->deleteLater();
    m_Reply = 0;
    m_File = 0;
    qDebug() << "Part " << m_Index << " download finished";
    emit DownloadFinished();
}
 
void Download::HttpReadyRead()
{
    if ( !m_File )
        return;
 
    //将读到的信息写入文件
    QByteArray buffer = m_Reply->readAll();
    m_File->seek( m_StartPoint + m_HaveDoneBytes );
    m_File->write(buffer);
    m_HaveDoneBytes += buffer.size();
}

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