Qt：使用QEventLoop等待信号，如果信号发得太早怎么办？

Question

I am working on an small client-server application. The client sends a query and has to wait for an answer. I think the most convenient way is to use a QEventLoop:

connect(&parser, SIGNAL(answer_received()), this, SLOT(react_to_answer()));
...
...
QEventLoop loop;
connect(&parser, SIGNAL(answer_received()), &loop, SLOT(quit()));
this.sendQuery();
loop.exec();

This worked for me at the moment, but what happens, if the signal answer_received() is emitted very fast, even before loop.exec() was called? Will my application stuck in the QEventLoop forever?

Thank you!

Answer 1

Given your code, the only way you will encounter an issue is if the answer_received() signal is emitted before loop.exec() is called. Which is equivalent to say that answer_received() is emitted during the call to this.sendQuery(); .

In your case it is not likely to happen because you rely on server/client interactions and you likely use a QNetworkAccessManager or a QSocket of some sort. If this is the case the QNetworkAccessManager/QSocket will not emit readyRead() or finished() signals until you enter the event loop.

However, in a more general case if answer_received() can be emitted from this.sendQuery() you have to change your code:

1. You could make the connection between to be queued. This way even if answer_received() is emitted during this.sendQuery() the slot will not be called until you enter the event loop.

    connect(&parser, SIGNAL(answer_received()), &loop, SLOT(quit()), Qt::QueuedConnection); 

2. You could ensure that answer_received() is never emitted during this.sendQuery() . On way to do so is to use 0ms QTimer, which will trigger as soon as all events in the event queue are processed, ie during loop.exec()

   Replace:

    emit answer_received(); 

   By:

    QTimer::singleShot(0, this, *receiver, &MyClass::answer_received);