如何在 Qt 中正确中止网络请求？

Question

我有一个对网络执行请求并解析数据的类。如何正确实现它的请求中止？

想象一下我有这样一门课：

class MyClass
{
public:
...
    void doRequest()
    {
        m_reply = m_manager.get(...);

        QEventLoop waitForResponse;
        connect(m_reply, &QNetworkReply::finished, &waitForResponse, &QEventLoop::quit);
        waitForResponse.exec();

        // Check if request was aborted (otherwise, application will crash)
        if (m_reply == nullptr)
            return;

        // Check for network errors, write result to m_data and delete m_reply;
        ...
    }
    void abort()
    {
        if (m_reply != nullptr)
            m_reply->abort();
    }
    QString data()
    {
        return m_data;
    }
...
private:
    QNetworkAccessManager *m_manager;
    QPiinter<QNetworkReply> m_reply;
    QString m_data;
}

这是一个按下按钮的例子：

class MainWindow : public QMainWindow
{
...
private slots:
    MainWindow::on_myButton_pressed()
    {
        m_myClass->abort();
        m_myClass->doRequest();
        ui->myTextEdit->setText(m_myClass->data());
    }

private:
    MyClass m_myClass;
}

当你按下按钮时，如果之前的请求没有完成，那么它就会被取消。这行得通。但在这种情况下，新请求将数据写入QTextEdit 并退出函数，然后旧请求从它自己的循环返回并再次将相同的m_data 写入QTextEdit。

正确吗？也许有更正确的方法来实现这一点？

Answer 1

嵌套事件循环是the root of all evil。编写像doRequest 这样的函数要简单得多，而无需向用户假装它是一个同步函数。似乎您已经跟踪了调用abort() 时发生的复杂控制流，并且您了解了对doRequest() 的后续调用如何由于重新进入事件循环而最终成为嵌套调用。如果您多次重新启动请求，您的堆栈将看起来像（堆栈向下增长）：

1.  main function
2.  main event loop
3.  [...] (Qt functions)
4.  MainWindow::on_myButton_pressed()
5.  MyClass::doRequest()
6.  QEventLoop::exec()
7.  [...] (Qt functions)
8.  MainWindow::on_myButton_pressed()
9.  MyClass::doRequest()
10. QEventLoop::exec()
11. [...] (Qt functions)
12. MainWindow::on_myButton_pressed()   and so on...

对MainWindow::on_myButton_pressed() 的每个调用都需要在其嵌套事件循环退出时调用ui->myTextEdit->setText()。

如果您需要在特定操作完成时执行某些操作，则另一种方法是使您的函数完全异步并依赖信号/插槽。这是您要实现的目标的最小实现：

#include <QtNetwork>
#include <QtWidgets>

/// A class responsible for communication with the web backend
class Backend : public QObject {
  Q_OBJECT
public:
  explicit Backend(QObject *parent = nullptr)
      : QObject(parent), m_reply(nullptr) {}
public slots:
  void doRequest() {
    // abort and delete ongoing request (if any)
    if (m_reply) {
      m_reply->abort();
      delete m_reply;
      m_reply = nullptr;
    }
    emit textUpdated(QString());
    // send request
    QUrl url("http://wtfismyip.com/text");
    QNetworkRequest request(url);
    m_reply = m_manager.get(request);
    // when the request is finished,
    QObject::connect(m_reply, &QNetworkReply::finished, [this] {
      // if the request ended successfully, read the received ip into m_lastData
      if (m_reply->error() == QNetworkReply::NoError)
        m_lastData = QString::fromUtf8(m_reply->readAll());
      // otherwise, emit errorOccured() signal (if the request has not been
      // actively canceled)
      else if (m_reply->error() != QNetworkReply::OperationCanceledError)
        emit errorOccured(m_reply->errorString());
      // in all cases, emit updateText and do cleanup
      emit textUpdated(m_lastData);
      m_reply->deleteLater();
      m_reply = nullptr;
    });
  }

  void abort() {
    if (m_reply != nullptr)
      m_reply->abort();
  }

signals:
  void textUpdated(const QString &);
  void errorOccured(const QString &);

private:
  QNetworkAccessManager m_manager;
  QNetworkReply *m_reply;
  QString m_lastData;
};

/// A minimal widget that contains a QPushButton and a QLabel
class Widget : public QWidget {
  Q_OBJECT
public:
  explicit Widget(QWidget *parent = nullptr) : QWidget(parent) {
    m_layout.addWidget(&m_pushButton);
    m_layout.addWidget(&m_label);

    connect(&m_pushButton, &QPushButton::clicked, this, &Widget::buttonClicked);
  }

signals:
  void buttonClicked();

public slots:
  void updateText(const QString &text) { m_label.setText(text); }
  void showError(const QString &error) {
    QMessageBox::warning(this, tr("Error"), error);
  }

private:
  QVBoxLayout m_layout{this};
  QPushButton m_pushButton{"Retrieve Name"};
  QLabel m_label;
};

int main(int argc, char *argv[]) {
  QApplication a(argc, argv);

  Backend backend;
  Widget widget;

  // connect components
  QObject::connect(&backend, &Backend::textUpdated, &widget,
                   &Widget::updateText);
  QObject::connect(&backend, &Backend::errorOccured, &widget,
                   &Widget::showError);
  QObject::connect(&widget, &Widget::buttonClicked, &backend,
                   &Backend::doRequest);
  widget.show();

  return a.exec();
}

#include "main.moc"