Qt从基于Qt示例的QThread下载文件不起作用

Posted 2023-02-17

【中文标题】Qt从基于Qt示例的QThread下载文件不起作用

【英文标题】：Qt Download Files from QThread Based On Qt Example Doesn't work

【发布时间】：2011-04-13 05:52:55

【问题描述】：

您好，我构建了一个简单的示例，删除了所有不相关的代码，只剩下有问题的代码 一般来说，我有执行线程的应用程序，并且在这个线程工作者内部，我放置了应该下载超过 30 个文件的下载代码，这基于 Qt 给出的下载示例，问题是 QNetworkAccessManager 插槽从不调用。 你能告诉我我在这里做错了什么吗？

class MainWindowContainer : public QMainWindow 

    Q_OBJECT

public:
    MainWindowContainer(QWidget *parent = 0);


 public slots:
    void InvokeDownloadThread();

 private:
    QPushButton *pushButtonInvokeThread;
    PhotosDownloadWorker* pm_hotosDownloadWorker;
  ;

#include <QtGui>
#include "MainWindowContainer.h"

MainWindowContainer::MainWindowContainer(QWidget* parent) : 
                                    QMainWindow(parent) 

    pushButtonInvokeThread  = new QPushButton(this); 
    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(pushButtonInvokeThread);
    setLayout(layout);
    QObject::connect(pushButtonInvokeThread,SIGNAL(clicked()),this, SLOT(InvokeDownloadThread()));


void MainWindowContainer::InvokeDownloadThread()

     pm_hotosDownloadWorker = new PhotosDownloadWorker(this);
     pm_hotosDownloadWorker->Execute();


class PhotosDownloadWorker : public QThread 

    Q_OBJECT

 public :
    PhotosDownloadWorker(QObject *parent);
    ~PhotosDownloadWorker();
    void  Execute();
    void append(const QStringList &urlList);
    bool saveToDisk(const QString &filename, QIODevice *data);
    QString saveFileName(const QUrl &url);

 protected:
    void run();

private:
    bool m_abort;
     QList<QNetworkReply *> currentDownloads;
     QFile output;
     QNetworkAccessManager* networkMgr ;  

  public slots:
     void downloadFinished(QNetworkReply *reply);
     void startNextDownload(const QUrl &url);



;

#include "PhotosDownloadWorker.h"
PhotosDownloadWorker::PhotosDownloadWorker(QObject *parent)
    : QThread(parent)

    m_abort = false;
    networkMgr = new QNetworkAccessManager(this);
    connect(networkMgr, SIGNAL(finished(QNetworkReply*)), this, SLOT(downloadFinished(QNetworkReply*)));
 

 PhotosDownloadWorker::~PhotosDownloadWorker()

   m_abort = true; 
    wait();


void PhotosDownloadWorker::Execute()

    m_abort = false;
    start();

void PhotosDownloadWorker::run()

    QStringList m_urlList;

    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");
    m_urlList.append("http://....xxxx......jpg");

    // x 30 
    append(m_urlList);



void PhotosDownloadWorker::append(const QStringList &urlList)

    foreach (QString url, urlList)
    
         startNextDownload(QUrl::fromEncoded(url.toLocal8Bit()));
    
 

void PhotosDownloadWorker::startNextDownload(const QUrl &url)

    QNetworkRequest request(url);
    QNetworkReply *reply = networkMgr->get(request);
    currentDownloads.append(reply);

 bool PhotosDownloadWorker::saveToDisk(const QString &filename, QIODevice *data)

    QFile file(filename);
    if (!file.open(QIODevice::WriteOnly)) 
        fprintf(stderr, "Could not open %s for writing: %s\n",
                qPrintable(filename),
                qPrintable(file.errorString()));
        return false;
    

    file.write(data->readAll());
    file.close();

    return true;


void PhotosDownloadWorker::downloadFinished(QNetworkReply *reply)

    QUrl url = reply->url();
    if (reply->error()) 
        fprintf(stderr, "Download of %s failed: %s\n",
                url.toEncoded().constData(),
                qPrintable(reply->errorString()));
     else 
        QString filename = saveFileName(url);
        if (saveToDisk(filename, reply))
            printf("Download of %s succeeded (saved to %s)\n",
                   url.toEncoded().constData(), qPrintable(filename));
    

     reply->deleteLater();
     currentDownloads.removeAll(reply);
     if (currentDownloads.isEmpty())
     

         this->exit();
     


 QString PhotosDownloadWorker::saveFileName(const QUrl &url)

    QString path = url.path();
    QString basename = QFileInfo(path).fileName();

    if (basename.isEmpty())
        basename = "download";

    if (QFile::exists(basename)) 
        // already exists, don't overwrite
        int i = 0;
        basename += '.';
        while (QFile::exists(basename + QString::number(i)))
            ++i;

        basename += QString::number(i);
    

    return basename;


//main

#include "MainWindowContainer.h"

 int main(int argc, char *argv[])
 

     QApplication app(argc, argv);
     MainWindowContainer mainWindowContainer;
     mainWindowContainer.show();
     return app.exec();
 

【问题讨论】：

你用了什么例子？您能给我们指出一个在线链接吗？

【参考方案1】：

尽管您在代码中调用了“networkMgr->get(request)”，但您的线程可能会在实际下载开始之前退出。尝试将“exec()”调用放在“QThread::run()”方法的最底部，以进入 QNetworkAccessManager 需要能够工作的线程中的事件循环。这也将防止线程过早完成其执行。您的代码如下所示：

void PhotosDownloadWorker::run()

  QStringList m_urlList;

  m_urlList.append("http://....xxxx......jpg");
  m_urlList.append("http://....xxxx......jpg");    
  // x 30 
  append(m_urlList);

  exec();

【讨论】：

是的答案，谢谢最后一个问题.. exec() 调用后如何终止线程最好的方法是什么？

您可以使用 QThread::quit() 或 QThread::exit()。使用最后一个，您可以传递结果代码以从 QThread::exec() 调用返回。通常我使用 QThread::quit()。因此，您甚至可以将最终下载完成（）信号连接到 QThread::quit() 插槽。

你能举个我没看懂的例子吗

当我把这个->退出 currentDownloads.isEmpty() 它永远不会到达那里。

不确定你的意思。你能解释一下吗？

【参考方案2】：

我认为如果参数列表不匹配，它甚至不会将信号连接到插槽。请参阅调试器输出以获取有关运行时的警告（应用程序输出选项卡）。

所以，而不是

connect(networkMgr, SIGNAL(readyRead()), this, SLOT(downloadFinished(QNetworkReply*)));

做

connect(networkMgr, SIGNAL(finished(QNetworkReply*)), this, SLOT(downloadFinished(QNetworkReply*)));

通过 QNetworkReply 的 error signal 处理错误。在你get 之后，你就可以连接到信号了。