QT 文件下载问题

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。

举了说明：

#include <QtCore>
#include <QtNetwork>

//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）
const QString strUrl = "http://ttplayer.qianqian.com/otherdown/alladin/ttpsetup_5713.exe";

//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();
};

//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();
};

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.toAscii());

	//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();
}

//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);
	delete reply;
	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";
	}
}

#include "main.moc"

int main(int argc, char **argv)
{
	QCoreApplication app(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);//strUrl指的是想要下载的文件的url地址，PointCount指的是多线程下载的线程数

loop.exec(); return 0; }