[英]Why does this Qt signal never get emitted?
我正在嘗試從網頁中獲取數據,但是信號finished()
從未發出!!! 我知道我一定在做錯事,但無法弄清楚那是什么。
# webservice.h
class WebService:public QObject
{
Q_OBJECT
public:
explicit WebService(QObject *parent=0);
void getRequest(const QString &urlString);
signals:
void networkError(QNetworkReply::NetworkError ne);
void finished(QNetworkReply*);
public slots:
void parseNetworkResponse(QNetworkReply* finished);
private:
QNetworkAccessManager *netMgr;
public:
QByteArray data;
};
#webservice.cpp
WebService::WebService(QObject *parent):QObject(parent)
{
netMgr = new QNetworkAccessManager;
connect(netMgr, SIGNAL(finished(QNetworkReply*)),
this, SLOT(parseNetworkResponse(QNetworkReply*)));
}
void WebService::getRequest(const QString &urlString)
{
QUrl url(urlString);
QNetworkRequest req;
emit finished(netMgr->get(req));
}
void WebService::parseNetworkResponse(QNetworkReply *finished)
{
if (finished->error() != QNetworkReply::NoError)
{
emit networkError(finished->error());
return;
}
data = finished->readAll();
qDebug() << data;
}
data
永遠不會得到預期的值。
您沒有將URL傳遞給您創建的QNetworkRequest
。 嘗試:
QNetworkRequest req(url);
在WebService::getRequest()
內部。
根據要求,對源進行了修改,使其可以在QtCreator中作為控制台應用程序項目中的單個main.cpp
文件進行編譯和工作:
#include <QCoreApplication>
#include <QNetworkReply>
#include <QNetworkRequest>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QByteArray>
#include <QString>
#include <QDebug>
//# webservice.h
class WebService:public QObject
{
Q_OBJECT
public:
explicit WebService(QObject *parent=0);
void getRequest(const QString &urlString);
signals:
void networkError(QNetworkReply::NetworkError ne);
void finished(QNetworkReply*);
public slots:
void parseNetworkResponse(QNetworkReply* finished);
private:
QNetworkAccessManager *netMgr;
public:
QByteArray data;
};
//#webservice.cpp
WebService::WebService(QObject *parent):QObject(parent)
{
netMgr = new QNetworkAccessManager;
connect(netMgr, SIGNAL(finished(QNetworkReply*)),
this, SLOT(parseNetworkResponse(QNetworkReply*)));
}
void WebService::getRequest(const QString &urlString)
{
QUrl url(urlString);
QNetworkRequest req(url);
emit finished(netMgr->get(req));
}
void WebService::parseNetworkResponse(QNetworkReply *finished)
{
if (finished->error() != QNetworkReply::NoError)
{
qDebug() << "QNetworkReply error: " << finished->error();
emit networkError(finished->error());
return;
}
data = finished->readAll();
qDebug() << data;
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
WebService web;
web.getRequest("http://www.google.com");
return a.exec();
}
#include "main.moc"
進行的較小修改:
main()
,它調用了WebService::getRequest()
main.cpp
文件的末尾添加了#include "main.moc"
,因此qmake可以將它作為單個獨立的.cpp文件正確地“ moc-ify” qDebug()
輸出 最后要做的一件事是在項目的.pro文件中添加QT += network
,以便將Qt網絡模塊添加到鏈接步驟和標頭搜索路徑中。
2013年10月15日更新
從您的評論看來,您希望QNetworkAccessManager::get()
調用是同步的。 我添加了示例程序的另一個版本,它將在WebService::getRequest()
阻塞,直到收到請求的finished
信號為止。 請注意,此示例在錯誤處理方面的執行不多,並且如果netwrok請求未能及時完成,可能會執行得很差。 除了示例代碼或學習代碼以外,必須適當地處理錯誤和超時。
此示例中的基本思想是,異步Qt網絡模型中發出的信號由框架的事件循環驅動。 因此,當發出請求時,將創建一個新的“嵌套”事件循環,然后WebService::getRequest()
函數執行該循環(並保持在那里),直到finished
信號的處理程序告訴事件循環退出為止。
#include <QCoreApplication>
#include <QNetworkReply>
#include <QNetworkRequest>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QEventLoop>
#include <QByteArray>
#include <QString>
#include <QDebug>
//# webservice.h
class WebService:public QObject
{
Q_OBJECT
public:
explicit WebService(QObject *parent=0);
void getRequest(const QString &urlString);
signals:
void networkError(QNetworkReply::NetworkError ne);
//void finished(QNetworkReply*);
public slots:
void parseNetworkResponse(QNetworkReply* finished);
private:
QNetworkAccessManager *netMgr;
QEventLoop request_event_loop;
public:
QByteArray data;
};
//#webservice.cpp
WebService::WebService(QObject *parent):QObject(parent)
{
netMgr = new QNetworkAccessManager;
connect(netMgr, SIGNAL(finished(QNetworkReply*)),
this, SLOT(parseNetworkResponse(QNetworkReply*)));
}
void WebService::getRequest(const QString &urlString)
{
QUrl url(urlString);
QNetworkRequest req(url);
netMgr->get(req);
request_event_loop.exec(); // wait here until the WebService::parseNetworkResponse() slot runs
// emit finished(netMgr->get(req));
}
void WebService::parseNetworkResponse(QNetworkReply *finished)
{
qDebug() << "enter parseNetworkResponse()";
if (finished->error() != QNetworkReply::NoError)
{
qDebug() << "QNetworkReply error: " << finished->error();
emit networkError(finished->error());
}
else {
data = finished->readAll();
qDebug() << data;
}
qDebug() << "request_event_loop.exit()";
request_event_loop.exit();
qDebug() << "exit parseNetworkResponse()";
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
WebService web;
qDebug() << "main() getRequest()";
web.getRequest("http://www.stackoverflow.com");
qDebug() << "main() getRequest() completed";
return a.exec();
}
#include "main.moc"
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