Question

我怀疑应该如何使用QEventLoop。我有两段代码，它们都适用于我（获取Web资源下载）。

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，在信号被激活后，事件循环是否在第二个中退出？或者我必须像第一个一样打电话给quit()吗？我在某个地方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。

Answer 1

我同意@ Mher-Didaryan - 事件循环由第二个代码片段中的以下代码行loop.exec();启动 - 永远不会退出。这是因为SIGNAL和SLOT之间的 connect（）正在完成与通过EventLoop loop;指示的事件循环不同的事件循环。

在第一个代码片段的情况下，逻辑取决于与{1}相关联的finished(QNetworkReply*)信号。将相同的GET请求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()));

在manager->get( request ) ;发出finished(QNetworkReply*)信号后，

可能会执行。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，第一个代码段中loop.exec();开始的事件循环也不会退出。我想这就是@Mher-Didaryan也在回答他的问题。

也许你可以使用下面的QEventLoop逻辑来处理以下负面执行场景

提示退出GET请求（例如由于网络连接问题）

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

在你的第二个例子中，事件循环永远不会退出，另一方面，在你的第一个例子中，循环将在finished(QNetworkReply*)发出时退出。但是，如果在连接循环之前manager->get( request );导致finished(QNetworkReply*)信号被激活，该怎么办呢？

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();

而且你还需要以某种方式处理经理根本不会发出SIGNAL(finished(QNetworkReply*))的情况。

QEventLoop正确用法

2 个答案: