QEventLoop正确用法

Question

我想知道我应该如何使用QEventLoop .我有 2 段代码，它们都对我有用(下载网络资源(。

第一个：

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();

第二个：

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();

我

想知道的是我应该使用哪一个。我的意思是，在发出信号后，事件循环是否会在第二个事件循环中退出？还是我必须像第一个那样打电话给quit()？我在某处找到了第二个解决方案，但对我来说似乎不合适，所以我将其修改为第一段代码。

Answer 1

我同意 @Mher-Didaryan 的观点——事件循环由第二个代码片段中的代码行loop.exec();启动——永远不会退出。这是因为 SIGNAL 和 SLOT 之间的 connect(( 是为与通过 EventLoop loop; 指示的事件循环不同的事件循环完成的

。

在第一个代码片段的情况下，逻辑取决于与发送到两个不同事件循环的一个和相同的GET请求相关联的finished(QNetworkReply*)信号。但很有可能

    connect(manager, SIGNAL(finished(QNetworkReply*)),&loop, SLOT(quit()));

很可能在manager->get( request ) ;发出finished(QNetworkReply*)信号后执行。对于涉及非常小的文件或响应的GET类型HTTP操作，可能会发生这种情况。在这种情况下，由第一个代码段中的loop.exec();启动的事件循环也不会退出。我想这也是@Mher-迪达良在他的回答中询问的。

也许您可以使用下面的QEventLoop逻辑来处理以下负面执行场景

1. GET请求超时(例如由于网络连接问题(

2. 来自网络服务器端的错误类型响应

   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;
   

Answer 2

在第二个示例中，事件循环永远不会退出，另一方面，在第一个示例中，当finished(QNetworkReply*)发出时，循环将退出。但是manager->get( request );如果在连接环路退出之前导致信号finished(QNetworkReply*)发出怎么办？

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();

而且您还需要以某种方式处理管理器根本不发出SIGNAL(finished(QNetworkReply*))的情况。