Does a loop continue running even if Signals and slot is not complete?



  • 0_1530603344305_34ea8264-5de8-423b-98ae-e94895f8340c-image.png

    Hi everyone, this is the code i've written.
    Basically, my code has errors that cause it to terminate which i can't debug.

    But, the main question i'm trying to ask, is whether while loops will continue its next iteration even before the signal and slot is done.

    So, in the code, i'm looping through a map to determine the key and value inside. For each set of key and value pair, i'm going to use a connect to allow the webpage to load finished and then bring it to parse2(bool) to parse information. So far, it has not been giving me the right outcome and sometimes it says "QWaitCondition: Destroyed while threads are still waiting".
    But, i was wondering is it because the next iteration of the while loop causes this error to come out because it takes time for a webpage to load and to parse the information out?

    I have no idea how to test this out or debug.


  • Moderators

    A QObject::connect() does not call in to any other code of yours; in particular, it won't execute slots. The same is true for all the other calls yo made, so you should be safe there.

    Your problem must be elsewhere.



  • @kkoehne just to be sure, the connect() function will cause my parse2(bool) to be complete before executing the next loop?


  • Moderators

    @GCDX Again: connect does NOT call the slot, it just connects the signal to the slot. The slot (parse2() in your case) will be called as soon as the signal is emitted. Please read http://doc.qt.io/qt-5.9/signalsandslots.html



  • This post is deleted!


  • @kkoehne @jsulm
    Okay i get it! but basically, it links a signal and a slot, when the signal is emitted, the corresponding slot will be called.

    My question is in pseudo code:

    connect(object,SIGNAL(function1()),SLOT(function2());
    int x=0;
    while(true when has next value){
    x++;
    print(x);
    function1();
    }
    function2(){print("executed last");}

    The problem i'm facing now is, the for loop will execute finish before function2.

    output:
    1
    2
    executed last

    Why is this so? shouldnt it be, the while will execute function1 first and then 2 and then run the next iteration?



  • @GCDX
    Unrelated, but you'll (usually) do yourself a favor using the new syntax https://wiki.qt.io/New_Signal_Slot_Syntax: :you get type checks at compile time.
    You can also hit F2 on your sig/slot methods and navigate direct instead of hunt.

    I don't have experience using / abusing / this style, I'm pretty sure it's wrong for the old sytax, pretty sure you need to specify a receiver object instance.

    connect(object,SIGNAL(function1()),SLOT(function2());// nearly certain this is invalid as you have no receiver object set but use SLOT
    

    can become:

    connect(object, &ObjectT::function1, whateverObjectThisWasMeantToBe, &OtherOrSameObjectT::function2); 
    //or
    connect(object, &ObjectT::function1, function2); 
    
    //or even a lambda expression!
    

    You should also see this: http://doc.qt.io/qt-5/threads-qobject.html#signals-and-slots-across-threads


  • Moderators

    @GCDX Did you really try this pseudo code, I mean as real code?
    If you connect like this:

    connect(object,SIGNAL(function1()),SLOT(function2());
    

    then it will be direct connection. That means: as soon as the signal is emitted all connected slots will be executed. This is more or less same as calling all connected slots directly.
    So here

    connect(object,SIGNAL(function1()),SLOT(function2());
    int x=0;
    while(true when has next value){
        x++;
        print(x);
        function1();
    }
    function2(){print("executed last");}
    

    I would expect
    1
    executed last
    2
    executed last
    as output.


  • Moderators

    @GCDX I just tried this code:

    connect(this, SIGNAL(somethingHappend()), this, SLOT(doSomething()));
    for (int i = 0; i < 2; ++i) {
        qDebug() << i;
        emit somethingHappend();
    }
    
    void MainWindow::doSomething()
    {
        qDebug() << "do something";
    }
    

    Output was as expected:
    0
    do something
    1
    do something



  • @jsulm yes :-) specifying the receiving object ( this ptr /any obj/instance really) is how I'd expect old sytnax to work too. I didn't see it in the other code, looked like a hybrid old and new syntax. Didn't feel right.



  • @6thC
    take a look at this connect overload:
    http://doc.qt.io/qt-5/qobject.html#connect-2

    you can omit the receiver object, connect falls than back to this as the receiver. Not consistent and OCD-triggering, but its a valid connect statement.



  • @jsulm hmmm yup thanks i found my error but i don't know how to fix it.
    void MainWindow::on_pushButton_clicked()
    {
    QString modules=stringpath;
    QUrl url2(modules);
    innerPage->setUrl(url2);//innerpage is a QWebview
    connect(innerPage, SIGNAL(loadFinished(bool)), SLOT(parse(bool)));
    }

    void MainWindow::parse(bool)
    {
    QWebFrame *frameInner = innerPage->page()->mainFrame();
    QWebElement doc = frameInner->documentElement();
    QString json=doc.toPlainText();
    //parsing code
    print("parsing executed");
    }

    i realised i put on_pushButton_clicked in a loop as such:
    int i=0;
    While(i!=2){
    i++;
    print(i)
    on_pushbutton_clicked();
    }
    outcome:
    1
    2
    parsing executed.

    i want parsing so be executed in every loop however i cant seem to be able to solve it?



  • @GCDX said in Does a loop continue running even if Signals and slot is not complete?:
    ok first of all remove that line from the on_pushButton_clicked and place it in your constructor of MainWindow

    connect(innerPage, SIGNAL(loadFinished(bool)), SLOT(parse(bool)));

    otherwise you create additional connections each time you call on_pushButton_clicked.
    -> 2nd time on_pushButton_clicked is called -> parse is called twice
    -> 3ed timeon_pushButton_clicked is called -> parse is called 3 times
    etc.

    2nd, make a container, QList or QVector for example, that contains all of your targeted urls, create a member variable cnt to help you navigate through the container.

    in on pushbutton clicked you set cnt to 0 and start loading the first url.

    on load finished, you increase cnt by 1, check if its still smaler that your url list. If it is, load next one, if not, your're done.


  • Moderators

    @GCDX This is easy: MainWindow::parse(bool) will be called as soon as innerPage emits the signal loadFinished, it does not care about your loop. In fact your while() loop is blocking the event loop, so MainWindow::parse(bool) will NOT be called as long as your loop is running.



  • @jsulm how do a rewrite it so that it will work???

    @J-Hilk my method is what you describe which doesn't work! something is preventing the parse from running until the while loop is completed


  • Moderators

    @GCDX said in Does a loop continue running even if Signals and slot is not complete?:

    how do a rewrite it so that it will work???

    I don't understand what you actually want to achieve.
    I don't understand why you call on_pushbutton_clicked() inside the loop.
    If you want to call something immediately inside the loop then simply call the method instead of emitting a signal.



  • @GCDX said in Does a loop continue running even if Signals and slot is not complete?:

    something is preventing the parse from running

    Yes, your while loop! What I describted eleminates the loop and manages the next url just, when the first parse is done



  • @J-Hilk but don't you still need a for loop to execute what you said?



  • @GCDX
    untested of course.

    MainWindow::MainWindow(...){
        ....
        connect(innerPage, SIGNAL(loadFinished(bool)), SLOT(parse(bool)));
        urlList = QList<QUrl> ({QUrl("Url1"), QUrl("Url"), ...});
    }
    
    void MainWindow::continue(){
         if(cnt < urlList.size()){
             innerPage->setUrl(urlList.at(cnt));      
             print(cnt); 
         } 
    }
    
    void MainWindow::on_pushButton_clicked()
    {
       cnt = 0;
       continue();
    }
    
    void MainWindow::parse(bool)
    {
    QWebFrame *frameInner = innerPage->page()->mainFrame();
    QWebElement doc = frameInner->documentElement();
    QString json=doc.toPlainText();
    //parsing code
    print("parsing executed");
       continue();
    }
    }
    


  • @J-Hilk wow it works, thanks a lot, its a very smart solution that i would never have thought of!!! Thanks so much :)



  • @J.Hilk said in Does a loop continue running even if Signals and slot is not complete?:

    connect(innerPage, SIGNAL(loadFinished(bool)), SLOT(parse(bool)));

    well there you go - I mean, I'd have expected that to be just fine in the new syntax but there it is specifying SIGNAL and SLOT too =) - that's new to me.


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