问题
I have doubts how should I use QEventLoop
. I have 2 pieces of code, both of them work for me (get web resource downloaded).
First one:
QNetworkAccessManager *manager = new QNetworkAccessManager( this );
QNetworkRequest request;
request.setUrl(QUrl(url));
request.setRawHeader("User-Agent", "Mozilla Firefox");
connect(manager, SIGNAL(finished(QNetworkReply*)),this,SLOT(replyFinished(QNetworkReply*)));
manager->get( request ) ;
QEventLoop loop;
connect(manager, SIGNAL(finished(QNetworkReply*)),&loop, SLOT(quit()));
loop.exec();
Second one:
QNetworkAccessManager *manager = new QNetworkAccessManager( this );
QNetworkRequest request;
request.setUrl(QUrl(url));
request.setRawHeader("User-Agent", "Mozilla Firefox");
manager->get( request ) ;
QEventLoop loop;
connect(manager, SIGNAL(finished(QNetworkReply*)),this, SLOT(replyFinished(QNetworkReply*)));
loop.exec();
What I want to know is which one should I use. I mean, does the event loop quit in the second one after signal is emmited? Or do I have to call quit()
like in the first one? I found the second solution somewhere but it didn't seem proper to me so I modified it into first piece of code.
回答1:
I agree with @Mher-Didaryan - that the event loop started by following line of code loop.exec();
in the 2nd code snippet - will never exit. This is because the connect() between the SIGNAL and SLOT is being done for a different event loop than the event loop indicated through EventLoop loop;
.
In the case of the 1st code snippet, the logic depends on the finished(QNetworkReply*)
signal associated with one & same GET request being emitted to two different event loops. But it is quite possible that
connect(manager, SIGNAL(finished(QNetworkReply*)),&loop, SLOT(quit()));
may well execute after the manager->get( request ) ;
has emitted the finished(QNetworkReply*)
signal. Maybe it can happen for a GET type HTTP operation involving a very small file or response. In such a scenario the event loop started out by the loop.exec();
in the 1st code snippet will also not exit. I guess this is what @Mher-Didaryan is also querying in his answer.
Maybe you can use the below QEventLoop logic that would handle the following negative execution scenarios too
- Timing out of the GET request (say due to network connectivity issues)
Error type response from server side of network
QNetworkAccessManager *manager = new QNetworkAccessManager(this); QNetworkRequest request; QEventLoop loop; QTimer getTimer; // let's use a 10 second period for timing out the GET opn request.setUrl(QUrl(url)); request.setRawHeader("User-Agent", "Mozilla Firefox"); // connect the timeout() signal of getTimer object to quit() slot of event loop QTimer::connect(&getTimer,SIGNAL(timeout()),&loop, SLOT(quit())); QObject::connect(manager, SIGNAL(finished(QNetworkReply*)),&loop, SLOT(quit())); QNetworkReply *resp = manager->get( request ); getTimer.start(10000); // 10000 milliSeconds wait period for get() method to work properly loop.exec(); if(NULL == resp) { // Error. we probably timed out i.e SIGNAL(finished()) did not happen // this handles above indicated case (1) return -1; // or return some timeout related error value } else if( QNetworkReply::NoError != resp->error() ) { // Error - SIGNAL(finished()) was raised but get() opn failed & returned with error // Refer http://doc.qt.io/qt-4.8/qnetworkreply.html#NetworkError-enum // This section of code handles above indicated case (2) } else { // get() operation was Successful !. // read the response available in the 'resp' variable as a QString & parse it. // Obtain the necessary result and etc. } delete resp; delete manager;
回答2:
In your second example event loop will never quit, on the other hand in your first example the loop will quit when finished(QNetworkReply*)
emits. But what if manager->get( request );
cause finished(QNetworkReply*)
signal to be emited before you connect loop's quit to it?
QNetworkAccessManager *manager = new QNetworkAccessManager( this );
QNetworkRequest request;
QEventLoop loop;
request.setUrl(QUrl(url));
request.setRawHeader("User-Agent", "Mozilla Firefox");
connect(manager, SIGNAL(finished(QNetworkReply*)),this,SLOT(replyFinished(QNetworkReply*)));
connect(manager, SIGNAL(finished(QNetworkReply*)),&loop, SLOT(quit()));
manager->get( request ) ;
loop.exec();
And also you need to somehow handle situation where manager does not emit SIGNAL(finished(QNetworkReply*))
at all.
来源:https://stackoverflow.com/questions/29449561/qeventloop-proper-usage