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Display text for 4 seconds



  • How can I display a string to a label for 4 seconds? Then after 4 seconds, the string can be changed to whatever string value.

    This is what I have tried so far:

    void Dialog::onDisplayMessage(string sMessage, bool bWithTimeout)
    {
        if(bWIthTimeout)
        {
            pMessageTimer_ = new QTimer(this);
    
            connect(pMessageTimer_, SIGNAL(timeout()), this, SLOT(stopTimer()));
            pMessageTimer_->start(4000);
    
            ui->LabelMessage->setText(QString::fromStdString(sMessage));
            ui->LabelMessage->update();
            QCoreApplication::processEvents();
    
        }
        else
        {
            ui->LabelMessage->setText(QString::fromStdString(sMessage));
            ui->LabelMessage->update();
            QCoreApplication::processEvents();
        }
    }
    
    void Dialog::stopTimer()
    {
        pMessageTimer_->stop();
    }
    

    With the above code, the text is not changing after 4 seconds.

    onDisplayMessage() is a SLOT and can be called anytime. I put a Boolean flag (bWithTimeout) to know which message will be displayed for four seconds. After four seconds and the onDisplayMessage is called again and the flag is false, the timer should not start and the text will be changed.```


  • Lifetime Qt Champion

    QCoreApplication::processEvents();

    This is useless and wrong

    void Dialog::stopTimer()
    {
        pMessageTimer_->stop();
    }
    

    Shouldn't you clear your label within there?

    pMessageTimer_ = new QTimer(this);

    You create this every time onDisplayMessage() for no good reason.



  • Hi, look at QTimer::singleShot.


  • Lifetime Qt Champion

    @ollarch said in Display text for 4 seconds:

    Hi, look at QTimer::singleShot.

    This does not fix his problem though...



  • You can have a QString member variable to store the message to show and when the timer triggers you can show the variable text.



  • @User123456
    2 lines will do this, if you know how to use C++ lambdas (only a couple of lines longer if you don't and write an explicit slot):

    ui->LabelMessage->setText("Hello");
    QTimer::singleShot(4000, this, [this] () { ui->LabelMessage->setText(""); });
    

    No need for your update(), processEvents().



  • @JonB said in Display text for 4 seconds:

    QTimer:singleShot(4000, this, [this] () { ui->LabelMessage->setText(""); });

    Thank you @JonB but when I try this, it gives me this error:
    no matching function for call to ‘QTimer::singleShot(int, Dialog*, Dialog::onDisplayMessage(std::__cxx11::string, bool)::<lambda()>)’
    QTimer::singleShot(4000, this, [this] () { ui->LabelMessage->setText(""); });

    I am not familiar with c++ lambdas right now.
    ^



  • @User123456
    I don't know how I mis-pasted since I tested it :( But anyway where I had a single colon QTimer: I meant a double colon (and have corrected above):

    QTimer::singleShot(4000, this, [this] () { ui->LabelMessage->setText(""); });
    

    Then again, you seem to have that in your error message, so maybe you had already done that? It works for me, are you at least Qt 5.8 (i.e. not old)?



  • Hi @User123456

    If i remember i had similar issue with Qt 5.6 or so i guess, but with preset Qt update it works fine (I am using Qt 5.12)

    Can you please try including CONFIG += c++11 in your .pro file if not included.
    AFAIK the Lambdas are included since C++11 .

    then the code should work fine as given by @JonB

        QTimer::singleShot(4000, this, [this] () { 
            ui->LabelMessage->setText(""); 
        });
    


  • @JonB, I am using an old version, 4.5. I cannot just upgrade it since it is part of the requirements. Could you please give an example which will work on that version?



  • @User123456

    Did you try including CONFIG += c++11 in your .pro file if not included.
    Lambda expressions (since C++11).

    Or for normal Signal Slot simple code below would be fine.
    d0d47cbe-9f08-455e-94e6-da297ae5be97-image.png



  • @Pradeep-P-N said in Display text for 4 seconds:

    CONFIG += c++11

    yes, it's already included



  • @Pradeep-P-N said in Display text for 4 seconds:

    Did you try including CONFIG += c++11 in your .pro file if not included.

    That doesn't matter. Qt4 doesn't support using lambda as slots at all.
    Qt4.5 was released even before c++11 was published


  • Lifetime Qt Champion

    @User123456 said in Display text for 4 seconds:

    Could you please give an example which will work on that version?

    Simply use a normal slot instead of a lambda.
    There is even an example in the documentation...



  • @User123456 said in Display text for 4 seconds:

    @JonB, I am using an old version, 4.5. I cannot just upgrade it since it is part of the requirements. Could you please give an example which will work on that version?

    Given that you appear to be a beginner, I am surprised you are using such an old version of Qt. That must mean you have an existing, pretty ancient Qt code base to work with. I understand it's not automatic/trivial to upgrade, but it's shame. If you post any future questions in this forum you should state each time that you are using Qt4, as it may be relevant to answers. Bear in mind when looking through documentation/examples you may need to check for Qt 4 ones/compatibility.

    Anyway, with an explicit method and not a lambda:

    // header file:
    private slots:
        void onTimeout();
    
    // cpp file:
    void Dialog::onTimeout()
    {
        ui->LabelMessage->setText("");
    }
    
    // caller:
    // next line connects using *new style* syntax, which is nicer...
    QTimer::singleShot(4000, this, &Dialog::onTimeout);
    // ... but if that doesn't work in Qt4 you can use *old style* syntax
    QTimer::singleShot(4000, this, SLOT(onTimeout()));
    

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