Blocked waiting for a asynchronous Qt signal

0 votes
asked Aug 24, 2010 by cyphorious

I know, there are some similar questions to the following out there, but I couldn't find a concrete answer that helps me. So here's my problem:

I work on an application that does some gui-initialisations on start up. One of the things I have to do, is calling

NetworkConfigurationManager::updateConfigurations ()

This is a asynchronous call, which emits the updateCompleted() signal, when it is finished. The problem is, that all my other gui-initialisations have to wait until the updateConfigurations() is finished.

So what I could do would be something like this:

MyApp::MyApp(QWidget *parent) : ....
{
   doSomeInits();
   //Now connect the signal we have to wait for
   connect(configManager, SIGNAL(updateCompleted()), this, SLOT(networkConfigurationUpdated()));
   configManager->updateConfigurations(); //call the async function
}

void MyApp::networkConfigurationUpdated()
{
   doSomething();
   doRemainingInitsThatHadToWaitForConfigMgr();
}

To split up the initialisation doesn't seem a good way to me. I think it makes the code much harder to read - inits should remain together. The other thing is: Because updateConfiguration() is asynchronous, the user will be able to use the GUI, which doesn't give him any information yet, cause we are waiting for updateCompleted().

So is there a way to wait for the updateCompleted() signal, before the application continues?

like:

MyApp::MyApp(QWidget *parent) : ....
{
   doSomeInits();
   //Now connect the signal we have to wait for
   connect(configManager, SIGNAL(updateCompleted()), this, SLOT(doSomething()));
   ???? //wait until doSomething() is done.
   doRemainingInitsThatHadToWaitForConfigMgr();
}

In some APIs there are blocking alternatives to asynchronous functions, but not in this case.

I appreciate any help. Thanks!

3 Answers

0 votes
answered Aug 24, 2010 by chalup

The way to do this is to use nested event loops. You simply create your own QEventLoop, connect whatever signal you want to wait for to the loop's quit() slot, then exec() the loop. This way, once the signal is called, it will trigger the QEventLoop's quit() slot, therefore exiting the loop's exec().

MyApp::MyApp(QWidget *parent) : ....
{
    doSomeInits();
    {
        QEventLoop loop;
        loop.connect(configManager, SIGNAL(updateCompleted()), SLOT(quit()));
        configManager->updateConfigurations(); 
        loop.exec();
    }
    doReaminingInitsThatHadToWaitForConfigMgr();
}
0 votes
answered Aug 24, 2010 by caleb-huitt-cjhuitt

Working from chalup's answer, if you are going to be waiting for a user-noticeable time, you might want to show a progress bar instead (or a splash screen, perhaps).

MyApp::MyApp(QWidget *parent) : ....
{
    doSomeInits();
    {
        QSpashScreen splash;
        splash.connect(configManager, SIGNAL(updateCompleted()), SLOT(close()));
        configManager->updateConfigurations(); 
        splash.exec();
    }
    doReaminingInitsThatHadToWaitForConfigMgr();
}
0 votes
answered Sep 15, 2017 by piponazo

Another solution could be to use QSignalSpy::wait(). This function was introduced in Qt 5 and does exactly what you want.

The only problem I see with that class is that it is provided in the QtTest module. In my case I find it very useful when testing code, but it might be not the best solution for production code.

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