当在错误槽中启动事件循环时发生ContentNotFoundError时,QNetworkReply会发出两次错误信号

Question

我使用QtSDK 4.7.3

我在(void test())中这样做:

mgr = new QNetworkAccessManager();
reply = mgr->get(QNetworkRequest(QUrl("http://developer.qt.nokia.com/fileNotExisting.txt")));

connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
    SLOT(onError(QNetworkReply::NetworkError)), Qt::ConnectionType::UniqueConnection);

当然,调用插槽onError:

if (networkError == QNetworkReply::NetworkError::ContentNotFoundError)
{
// Messagebox starts an event loop which
// causes this slot to be called again
QMessageBox m;
m.exec();
}

如果我在onError插槽中没有messagebox/eventloop,则没有崩溃,一切正常.但是当它存在时,调用m.exec()时会再次调用onError槽.当两个消息框都关闭并且我将函数保留为onError时,应用程序崩溃.发生这种情况时,应用程序会尝试删除/释放内存.错误"访问冲突读取位置"没有任何帮助,调用堆栈深入到Qt dll.

我检查了什么:
信号没有连接两次.
尝试在QApplication调用它的exec函数之前和之后调用test().(没关系).
像HostNotFound这样的另一个错误不会两次调用onError插槽.
我的所有代码都在主线程中执行.
尝试断开onError插槽,因此只调用一次,但它仍然崩溃.
试图在onError()中对请求进行中止调用.
在Qt论坛(帖子)上发表了同样的问题.

任何人都可以帮我弄清楚这里发生了什么？

这是我用于测试的代码:main.cpp

#include "contentnotfound.h"
#include <QtGui/QApplication>
#include <QTimer>

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

ContentNotFound cnf;

// false: start test after application's event loop have started
if (true) { cnf.test(); }
else { QTimer::singleShot(2000, &cnf, SLOT(test())); }

return a.exec();
}

contentnotfound.h

#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QMessageBox>

class ContentNotFound : public QObject
{
Q_OBJECT

public slots:
void test()
{
    mgr = new QNetworkAccessManager();
    reply = mgr->get(QNetworkRequest(QUrl("http://developer.qt.nokia.com/fileNotExisting.txt")));

    connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        SLOT(onError(QNetworkReply::NetworkError)), Qt::ConnectionType::UniqueConnection);
}

private slots:
void onError(QNetworkReply::NetworkError networkError)
{
    //reply->disconnect(); // Disconnect all signals

    if (networkError == QNetworkReply::NetworkError::ContentNotFoundError)
    {
        // Messagebox starts an event loop which
        // causes this slot to be called again
        QMessageBox m;
        m.exec();
    }
}

private:
QNetworkAccessManager* mgr;
QNetworkReply* reply;

};

Answer 1

Qt < 4.8.0 中有一个错误： https: //bugreports.qt.io/browse/QTBUG-16333

修改与排队连接的连接可以解决该问题：

内容未找到.h：

#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QMessageBox>

class ContentNotFound : public QObject
{
Q_OBJECT

public slots:
void test()
{
    qRegisterMetaType<QNetworkReply::NetworkError>("QNetworkReply::NetworkError");
    mgr = new QNetworkAccessManager(this);
    reply = mgr->get(QNetworkRequest(QUrl("http://developer.qt.nokia.com/fileNotExisting.txt")));

    connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        SLOT(onError(QNetworkReply::NetworkError)), Qt::QueuedConnection);
}

private slots:
void onError(QNetworkReply::NetworkError networkError)
{
    //reply->disconnect(); // Disconnect all signals

    if (networkError == QNetworkReply::ContentNotFoundError)
    {
        // Messagebox starts an event loop which
        // causes this slot to be called again
        QMessageBox m;
        m.exec();
    }
}

private:
QNetworkAccessManager* mgr;
QNetworkReply* reply;

};