使用QNetworkAccessManager实现Qt的FTP下载服务

从Qt5开始，官方推荐使用QNetworkAccessManager进行Ftp和http的上传和下载操作；Qt4中使用的QtFtp模块即作为独立模块，需要自己从github上进行下载编译后使用（官方地址：https://github.com/qt/qtftp）。

官方的QtFtp最后一次更新为2014年，根据搜索的资料，其尚存在若干bug。不过有人对此代码在Github上进行维护和更新，如果需要使用的话，可以搜索一下。

QNetworkAccessManager的相关API比较丰富，但是相应也比较低级。如果需要对Ftp进行较为复杂的操作，在缺少资料的基础上就会很麻烦，需要较好的功底。

因为个人对Ftp的操作仅限于下载或者上传，因此使用`QNetworkAccessManager`即可满足要求。此处仅对下载进行示范，上传基本一致。

 #ifndef FTPGETWINDOW_H
 #define FTPGETWINDOW_H

 #include <QWidget>
 #include <QUrl>
 #include <QDir>
 #include <QNetworkReply>

 class QFile;
 class QLabel;
 class QLineEdit;
 class QTextEdit;
 class QPushButton;
 class QProgressBar;
 class QGridLayout;

 class QTimer;
 class QNetworkAccessManager;

 class FtpgetWindow : public QWidget
 {
     Q_OBJECT

 public:
     FtpgetWindow(QWidget *parent = );
     ~FtpgetWindow();

 private slots:
     void timeOut();
     void updateSelectSaveDir();
     void updateTaskRunningState();
     void slotReadyRead();
     void readReplyError(QNetworkReply::NetworkError error);
     void downloadFinishReply(QNetworkReply* reply);
     void downloadProgress(qint64 bytesReceived, qint64 bytesTotal);

 private:
     bool checkUrl();
     bool checkSaveDir();
     bool createDownloadFile();
     void startDownloadFile();

 private:
     qint64 fileDownloadSize;
     qint64 lastDownloadSize;
     QUrl url;
     QDir saveDir;
     QFile *file;
     QTimer *timer;
     QNetworkReply *downloadReply;
     QNetworkAccessManager *downloadManager;

     QLabel *urlLabel;
     QLabel *dirLoactionLabel;
     QLabel *downlaodInfoLabel;
     QLabel *runningTipLabel;
     QLineEdit *urlTextEdit;
     QLineEdit *dirTextEdit;
     QTextEdit *downloadInfoTextEdit;
     QPushButton *runningTaskButton;
     QPushButton *dirLocationButton;
     QProgressBar *progressBar;
     QGridLayout *mainLayout;
 };

 #endif // FTPGETWINDOW_H

头文件无需赘述。

 #include "ftpgetwindow.h"

 #include <QLabel>
 #include <QLineEdit>
 #include <QTextEdit>
 #include <QPushButton>
 #include <QProgressBar>
 #include <QGridLayout>
 #include <QFileDialog>

 #include <QUrl>
 #include <QDir>
 #include <QFile>
 #include <QTimer>
 #include <QFileInfo>
 #include <QMetaEnum>
 #include <QNetworkAccessManager>

 FtpgetWindow::FtpgetWindow(QWidget *parent)
     : QWidget(parent),
       fileDownloadSize(),
       lastDownloadSize(),
       file(Q_NULLPTR)
 {
     downloadManager = new QNetworkAccessManager(this);
     connect(downloadManager, SIGNAL(finished(QNetworkReply*)),SLOT(downloadFinishReply(QNetworkReply*)));

     //初始化超时检查定时器，30秒查询一次
     timer = new QTimer;
     connect(timer, SIGNAL(timeout()), SLOT(timeOut()));

     urlLabel = new QLabel;
     urlLabel->setText(tr("Url:"));

     urlTextEdit = new QLineEdit;
     urlLabel->setBuddy(urlTextEdit);

     runningTaskButton = new QPushButton;
     runningTaskButton->setText("Run");
     connect(runningTaskButton, SIGNAL(clicked(bool)), SLOT(updateTaskRunningState()));

     dirLoactionLabel = new QLabel;
     dirLoactionLabel->setText(tr("Save Dir:"));

     dirTextEdit = new QLineEdit;
     dirTextEdit->setReadOnly(true);
     dirLoactionLabel->setBuddy(dirTextEdit);

     dirLocationButton = new QPushButton;
     dirLocationButton->setText("Select Save Dir");
     connect(dirLocationButton, SIGNAL(clicked(bool)), SLOT(updateSelectSaveDir()));

     runningTipLabel = new QLabel;
     runningTipLabel->setText(tr("Runing task:"));

     progressBar = new QProgressBar;
     runningTipLabel->setBuddy(progressBar);

     downlaodInfoLabel = new QLabel;
     downlaodInfoLabel->setText(tr("Download Info:"));

     downloadInfoTextEdit = new QTextEdit;
     downloadInfoTextEdit->setReadOnly(true);
     downlaodInfoLabel->setBuddy(downloadInfoTextEdit);

     mainLayout = new QGridLayout;
     mainLayout->setColumnStretch(, );
     mainLayout->setColumnStretch(, );
     mainLayout->setColumnStretch(, );
     mainLayout->setMargin();
     mainLayout->setColumnMinimumWidth(, );

     mainLayout->addWidget(urlLabel, , );
     mainLayout->addWidget(urlTextEdit, , );
     mainLayout->addWidget(runningTaskButton, , );
     mainLayout->addWidget(dirLoactionLabel, , );
     mainLayout->addWidget(dirTextEdit, , );
     mainLayout->addWidget(dirLocationButton, , );
     mainLayout->addWidget(runningTipLabel, , , , );
     mainLayout->addWidget(progressBar, , , , );
     mainLayout->addWidget(downlaodInfoLabel, , , , );
     mainLayout->addWidget(downloadInfoTextEdit, , , , );
     setLayout(mainLayout);

     setFixedWidth();
     setWindowTitle(tr("FpGet Window"));
 }

 FtpgetWindow::~FtpgetWindow()
 {
     if(file != Q_NULLPTR)
     {
         file->deleteLater();
         file = Q_NULLPTR;
     }
     //downloadManager的父对象是窗体，会自动进行析构
 }

 /**
  * @brief 进行下载超时判断，错误则发送超时信号
  */
 void FtpgetWindow::timeOut()
 {
     if(lastDownloadSize != fileDownloadSize)
         lastDownloadSize = fileDownloadSize;
     else
         emit downloadReply->error(QNetworkReply::TimeoutError); //下载超时，发送超时错误信号
 }

 /**
  * @brief 检查Url地址合法性
  * @return
  */
 bool FtpgetWindow::checkUrl()
 {
     url = QUrl(urlTextEdit->text());
     if(!url.isValid())
     {
         downloadInfoTextEdit->append("Error: Invalid URL");
         return false;
     }

     if(url.scheme() != "ftp")
     {
         downloadInfoTextEdit->append("Error: URL must start with 'ftp:'");
         return false;
     }

     if (url.path().isEmpty()) {
         downloadInfoTextEdit->append("Error: URL has no path");
         return false;
     }
     return true;
 }

 /**
  * @brief 检查文件下载地址
  * @return
  */
 bool FtpgetWindow::checkSaveDir()
 {
     QString dir = dirTextEdit->text();
     if(dir.isEmpty())
         dir = QDir::currentPath() + "/Download/";
     saveDir = QDir(dir);

     if(!saveDir.exists())
     {
         auto ok = saveDir.mkdir(dir);
         if(!ok) return false;
     }
     return true;
 }

 bool FtpgetWindow::createDownloadFile()
 {
     auto localFileName = QFileInfo(url.path()).fileName();
     if (localFileName.isEmpty())
         localFileName = "ftpget.out";

     file = new QFile;
     file->setFileName(saveDir.absoluteFilePath(localFileName));
     if(!file->open(QIODevice::WriteOnly))
     {
         auto info = "Error: Cannot write file " + file->fileName()
                 + ": " + file->errorString();
         downloadInfoTextEdit->append(info);
         return false;
     }
     return true;
 }

 /**
  * @brief 开始下载文件操作
  */
 void FtpgetWindow::startDownloadFile()
 {
     if(!createDownloadFile()) return;

     if(timer->isActive())
         timer->stop();
     fileDownloadSize = lastDownloadSize = ; //重新设置定时器以及相关变量

     downloadInfoTextEdit->append("Download file: " + url.fileName());

     downloadReply = downloadManager->get(QNetworkRequest(url));

     //分块获取文件信息，并写入文件中
     connect(downloadReply, SIGNAL(readyRead()), SLOT(slotReadyRead()));

     //获取下载进度信息
     connect(downloadReply, SIGNAL(downloadProgress(qint64,qint64)),
             SLOT(downloadProgress(qint64,qint64)));

     //下载过程出错，进行报错处理（超时处理也是丢出超时信号，交由此槽函数进行处理）
     connect(downloadReply, SIGNAL(error(QNetworkReply::NetworkError)),
             SLOT(readReplyError(QNetworkReply::NetworkError)));

     timer->start( * ); //启动超时检查定时器，每30秒查询下载情况
 }

 void FtpgetWindow::updateSelectSaveDir()
 {
     dirTextEdit->setText("");
     QString dir = QFileDialog::getExistingDirectory(this, tr("Open Directory"),
                                                     "C://",
                                                     QFileDialog::ShowDirsOnly
                                                     | QFileDialog::DontResolveSymlinks);
     if(!dir.isEmpty())
         dirTextEdit->setText(dir);
 }

 void FtpgetWindow::updateTaskRunningState()
 {
     if(!checkUrl() || !checkSaveDir())
         return;

     downloadInfoTextEdit->clear(); //清空信息栏

     runningTaskButton->setEnabled(false);
     dirLocationButton->setEnabled(false);
     startDownloadFile();
 }

 /**
  * @brief 文件下载完成的清尾操作
  * @param reply
  */
 void FtpgetWindow::downloadFinishReply(QNetworkReply *reply)
 {
     file->waitForBytesWritten( * ); //等待文件写入结束
     if( == file->size())
         //此处下载失败，不再进行重新下载操作
         downloadInfoTextEdit->append("Nothing be download.");
     else
         downloadInfoTextEdit->append("Download file success.");

     if(timer->isActive())
         timer->stop(); //停止超时计时器

     file->deleteLater();
     file = Q_NULLPTR;

     reply->deleteLater();
     reply = Q_NULLPTR;

     runningTaskButton->setEnabled(true);
     dirLocationButton->setEnabled(true);
 }

 void FtpgetWindow::slotReadyRead()
 {
     file->write(downloadReply->readAll());
     fileDownloadSize = file->size(); //更新下载字节数
 }

 /**
  * @brief 下载异常，重新进行下载
  * @param error
  */
 void FtpgetWindow::readReplyError(QNetworkReply::NetworkError error)
 {
     auto metaEnum = QMetaEnum::fromType<QNetworkReply::NetworkError>();
     //PS:字符串转换为枚举值
     //Qt::Alignment alignment = (Qt::Alignment)metaEnum.keyToValue("Qt::AlignLeft");
     //alignment = (Qt::Alignment)metaEnum.keysToValue("Qt::AlignLeft | Qt::AlignVCenter");
     //枚举值转换为字符串
     auto errStr = metaEnum.valueToKey(error);
     downloadInfoTextEdit->append("Download file occur error: " + QString(errStr));

     file->deleteLater();
     file = Q_NULLPTR;

     downloadReply->deleteLater();
     downloadReply = Q_NULLPTR;

     startDownloadFile(); //重新尝试下载文件
 }

 void FtpgetWindow::downloadProgress(qint64 bytesReceived, qint64 bytesTotal)
 {
     if( != bytesTotal)
     {
         progressBar->setMaximum(bytesTotal);
         progressBar->setValue(bytesReceived);
     }
 }

（1）超时操作：

在下载过程中，经常出现假死操作，因为不清楚如何进行续传操作，现有做法是取消当前下载任务并重新开始。

在启动下载操所时，启动定时器，每隔30秒记录当前下载数值和上一次记录的下载数值比较，如果相同，则可以认为在30秒内无操作，发送超时信号，断开连接重新开始下载任务。

（2）大文件下载：

现有仅测试了上百M的文件，可以在下载结束的时候，一次读取所有字节并写入文件，但是这样的压力比较大。

因此，当QNetworkReply发送信号告知有分段数据可供读取的时候，即读取并写入文件中。

（3）大文件上传：

调用put函数时，主要有两种方式，将文件信息读取出保存至QByteArray中，或者上传文件的操作指针。使用后者即可实现大型文件的上传操作。

（4）下载进度信息：

下载过程中，QNetworkReply会发送下载进度信息，用户可以根据此刷新QProgressBar控件，或者在命令行刷新进度条。

以下代码为在命令行实现进度条刷新操作，关键在于每次输出进度信息的时候，不要添加换行符，并且在输出信息头部添加"\r"即可。

 /**
  * @brief 实现命令行下进度条，提示下载进度
  * @param bytesReceived
  * @param bytesTotal
  */
 void FtpGet::downloadProgress(qint64 bytesReceived, qint64 bytesTotal)
 {
     int barLength = ;
     int percent = int(qreal(bytesReceived) / qreal(bytesTotal) * barLength);
     QString out = "\rPercent: " + QString(percent, '#') + QString(barLength - percent, ' ');
     out += " " + QString::number(bytesReceived) + " / " + QString::number(bytesTotal);
     std::cout << qPrintable(out) << std::flush;
 }

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