QEventLoop正确使用

我有疑问应该如何使用QEventLoop。我有两段代码，它们都适合我（下载网络资源）。

第一：

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，信号发出后，事件循环是否会在第二个事件循环中退出？或者我必须打电话quit()就像第一个一样？我在某处找到了第二个解决方案，但它对我来说似乎不合适，所以我将其修改为第一段代码。

我同意@Mher-Didaryan - 事件循环是通过以下代码行开始的loop.exec();在第二个代码片段中 - 永远不会退出。这是因为 SIGNAL 和 SLOT 之间的 connect() 是针对不同的事件循环完成的，而不是通过EventLoop loop; .

对于第一个代码片段，逻辑取决于finished(QNetworkReply*)与一个且相同的 GET 请求关联的信号被发送到两个不同的事件循环。但很有可能

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

可能会在之后执行manager->get( request ) ;已发出finished(QNetworkReply*)信号。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，事件循环由loop.exec();第一个代码片段中的也不会退出。我想这就是@Mher-Didaryan 在他的回答中也在询问的内容。

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;