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Signal and slot from other class not working

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  • Cobra91151C Cobra91151

    @Ucn_

    Hello!

    I have created a simple example to illustrate a clickable label, button and connect using lambdas, so you can simplify your code.

    clickablelabel.h

    #ifndef CLICKABLELABEL_H
    #define CLICKABLELABEL_H
    
    #include <QWidget>
    #include <QLabel>
    #include <QEvent>
    
    class ClickableLabel : public QLabel
    {
        Q_OBJECT
    public:
        using QLabel::QLabel;
    
    signals:
        void clicked();
    
    protected:
        void mousePressEvent(QMouseEvent *event) override;
    };
    
    #endif // CLICKABLELABEL_H
    

    clickablelabel.cpp

    #include "clickablelabel.h"
    
    void ClickableLabel::mousePressEvent(QMouseEvent *event)
    {
        emit clicked();
        QLabel::mousePressEvent(event);
    }
    

    test.h

    #ifndef TEST_H
    #define TEST_H
    
    #include <QDialog>
    #include <QVBoxLayout>
    #include "clickablelabel.h"
    
    namespace Ui {
    class Test;
    }
    
    class Test : public QDialog
    {
        Q_OBJECT
    
    public:
        explicit Test(QWidget *parent = nullptr);
        ~Test();
    
    private:
        Ui::Test *ui;
    };
    
    #endif // TEST_H
    

    test.cpp

    #include "test.h"
    #include "ui_test.h"
    
    Test::Test(QWidget *parent) :
        QDialog(parent),
        ui(new Ui::Test)
    {
        ui->setupUi(this);
        setFixedSize(600, 500);
        setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowCloseButtonHint);
        ClickableLabel *testLabel = new ClickableLabel("This is a test label...", this);
        connect(testLabel, &ClickableLabel::clicked, [testLabel]() {
           testLabel->setText("Some text from mouse click");
        });
        ui->pushButton->setText("Test button");
        connect(ui->pushButton, &QPushButton::clicked, [testLabel]() {
           testLabel->setText("Some text from button click");
        });
    
        QVBoxLayout *testLayout = new QVBoxLayout();
        testLayout->setAlignment(Qt::AlignCenter);
        testLayout->addWidget(testLabel);
        testLayout->addWidget(ui->pushButton);
        setLayout(testLayout);
    }
    
    Test::~Test()
    {
        delete ui;
    }
    

    Screenshot:

    ClickableLabel.gif

    Happy coding!

    U Offline
    U Offline
    Ucn_
    wrote on last edited by Ucn_
    #9

    @Cobra91151 Thanks, I will give it a try.

    1 Reply Last reply
    0
    • Cobra91151C Cobra91151

      @Ucn_

      Hello!

      I have created a simple example to illustrate a clickable label, button and connect using lambdas, so you can simplify your code.

      clickablelabel.h

      #ifndef CLICKABLELABEL_H
      #define CLICKABLELABEL_H
      
      #include <QWidget>
      #include <QLabel>
      #include <QEvent>
      
      class ClickableLabel : public QLabel
      {
          Q_OBJECT
      public:
          using QLabel::QLabel;
      
      signals:
          void clicked();
      
      protected:
          void mousePressEvent(QMouseEvent *event) override;
      };
      
      #endif // CLICKABLELABEL_H
      

      clickablelabel.cpp

      #include "clickablelabel.h"
      
      void ClickableLabel::mousePressEvent(QMouseEvent *event)
      {
          emit clicked();
          QLabel::mousePressEvent(event);
      }
      

      test.h

      #ifndef TEST_H
      #define TEST_H
      
      #include <QDialog>
      #include <QVBoxLayout>
      #include "clickablelabel.h"
      
      namespace Ui {
      class Test;
      }
      
      class Test : public QDialog
      {
          Q_OBJECT
      
      public:
          explicit Test(QWidget *parent = nullptr);
          ~Test();
      
      private:
          Ui::Test *ui;
      };
      
      #endif // TEST_H
      

      test.cpp

      #include "test.h"
      #include "ui_test.h"
      
      Test::Test(QWidget *parent) :
          QDialog(parent),
          ui(new Ui::Test)
      {
          ui->setupUi(this);
          setFixedSize(600, 500);
          setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowCloseButtonHint);
          ClickableLabel *testLabel = new ClickableLabel("This is a test label...", this);
          connect(testLabel, &ClickableLabel::clicked, [testLabel]() {
             testLabel->setText("Some text from mouse click");
          });
          ui->pushButton->setText("Test button");
          connect(ui->pushButton, &QPushButton::clicked, [testLabel]() {
             testLabel->setText("Some text from button click");
          });
      
          QVBoxLayout *testLayout = new QVBoxLayout();
          testLayout->setAlignment(Qt::AlignCenter);
          testLayout->addWidget(testLabel);
          testLayout->addWidget(ui->pushButton);
          setLayout(testLayout);
      }
      
      Test::~Test()
      {
          delete ui;
      }
      

      Screenshot:

      ClickableLabel.gif

      Happy coding!

      U Offline
      U Offline
      Ucn_
      wrote on last edited by Ucn_
      #10

      @Cobra91151 said in Signal and slot from other class not working:

      connect(ui->pushButton, &QPushButton::clicked, testLabel {
      testLabel->setText("Some text from button click");
      });

      The example works, thank. But, is there a way I can pass functions or widgets created in designer to the lambda capture list? Let's say I want to run a function after QLabel click or change a specific label.

      mrjjM 1 Reply Last reply
      0
      • U Ucn_

        @Cobra91151 said in Signal and slot from other class not working:

        connect(ui->pushButton, &QPushButton::clicked, testLabel {
        testLabel->setText("Some text from button click");
        });

        The example works, thank. But, is there a way I can pass functions or widgets created in designer to the lambda capture list? Let's say I want to run a function after QLabel click or change a specific label.

        mrjjM Offline
        mrjjM Offline
        mrjj
        Lifetime Qt Champion
        wrote on last edited by
        #11

        @Ucn_

        Hi
        You can capture "this" to allow to use both
        UI->xxx and member functions

         connect(ui->pushButton, &QPushButton::clicked, [this]() {
            ui->xxxxx;
            });
        
        U 1 Reply Last reply
        4
        • mrjjM mrjj

          @Ucn_

          Hi
          You can capture "this" to allow to use both
          UI->xxx and member functions

           connect(ui->pushButton, &QPushButton::clicked, [this]() {
              ui->xxxxx;
              });
          
          U Offline
          U Offline
          Ucn_
          wrote on last edited by
          #12

          @mrjj Thanks, It works. however, I don't understand why it works in one example and in the other says

          'QMetaObject::Connection QObject::connect(const QObject *,const char *,const char *,Qt::ConnectionType) const': cannot convert argument 1 from 'QLabel *' to 'const ClickableLabels *'
          
          1 Reply Last reply
          0
          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #13

            Because you are using a pointer to a QLabel as first parameter and a signal from your ClickableLabel as second parameter.

            Interested in AI ? www.idiap.ch
            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

            U 1 Reply Last reply
            2
            • SGaistS SGaist

              Because you are using a pointer to a QLabel as first parameter and a signal from your ClickableLabel as second parameter.

              U Offline
              U Offline
              Ucn_
              wrote on last edited by
              #14

              @SGaist what is the viable solution. Cast?

              1 Reply Last reply
              0
              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #15

                Ensure your original parameter is a ClickableLabel instance.

                Interested in AI ? www.idiap.ch
                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                U 1 Reply Last reply
                1
                • SGaistS SGaist

                  Ensure your original parameter is a ClickableLabel instance.

                  U Offline
                  U Offline
                  Ucn_
                  wrote on last edited by
                  #16

                  @SGaist Thanks, I promoted QLabel widget in designer to ClickableLabel and it's working. Is it the correct way?

                  1 Reply Last reply
                  0
                  • SGaistS Offline
                    SGaistS Offline
                    SGaist
                    Lifetime Qt Champion
                    wrote on last edited by
                    #17

                    Since you are using Designer, yes.

                    Interested in AI ? www.idiap.ch
                    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                    U 1 Reply Last reply
                    1
                    • SGaistS SGaist

                      Since you are using Designer, yes.

                      U Offline
                      U Offline
                      Ucn_
                      wrote on last edited by
                      #18

                      @SGaist Thanks.

                      1 Reply Last reply
                      0
                      • U Offline
                        U Offline
                        Ucn_
                        wrote on last edited by Ucn_
                        #19

                        I know I marked this as solved, and thanks to everyone who helped me understand what I was missing. I'm able to make it work when is widget to widget or promoted with with custom events. However when there is nothing triggered or clicked like a button I'm not able to connect. For example. Function to function, if one function from another class is executed, connect and execute another function in another class. Where the signal is the first executed function and slot is the function executed after the first was triggered. Thanks

                        Pl45m4P 1 Reply Last reply
                        0
                        • U Ucn_

                          I know I marked this as solved, and thanks to everyone who helped me understand what I was missing. I'm able to make it work when is widget to widget or promoted with with custom events. However when there is nothing triggered or clicked like a button I'm not able to connect. For example. Function to function, if one function from another class is executed, connect and execute another function in another class. Where the signal is the first executed function and slot is the function executed after the first was triggered. Thanks

                          Pl45m4P Offline
                          Pl45m4P Offline
                          Pl45m4
                          wrote on last edited by
                          #20

                          @Ucn_

                          Set up the connect in your constructor and emit the signal, you connect to, in your function A to execute function B in your second class.


                          If debugging is the process of removing software bugs, then programming must be the process of putting them in.

                          ~E. W. Dijkstra

                          U 1 Reply Last reply
                          0
                          • Pl45m4P Pl45m4

                            @Ucn_

                            Set up the connect in your constructor and emit the signal, you connect to, in your function A to execute function B in your second class.

                            U Offline
                            U Offline
                            Ucn_
                            wrote on last edited by
                            #21

                            @Pl45m4

                            This only works when I setup in main.cpp and when it fires in won't update anything in the function except print. When I set up in the constructor and place the emit in the function it does not fire,

                            QObject::connect(&timer, SIGNAL(printPerSecond()), &w, SLOT(anotherFuncion()));
                            
                            emit printPerSecond()
                            
                            jsulmJ 1 Reply Last reply
                            0
                            • U Ucn_

                              @Pl45m4

                              This only works when I setup in main.cpp and when it fires in won't update anything in the function except print. When I set up in the constructor and place the emit in the function it does not fire,

                              QObject::connect(&timer, SIGNAL(printPerSecond()), &w, SLOT(anotherFuncion()));
                              
                              emit printPerSecond()
                              
                              jsulmJ Offline
                              jsulmJ Offline
                              jsulm
                              Lifetime Qt Champion
                              wrote on last edited by
                              #22

                              @Ucn_ What is timer in your connect?
                              Is there a warning at runtime that connect failed?
                              You should use the new connect syntax to get compiler error instead of runtime warning if your connect call is wrong.

                              https://forum.qt.io/topic/113070/qt-code-of-conduct

                              U 1 Reply Last reply
                              0
                              • jsulmJ jsulm

                                @Ucn_ What is timer in your connect?
                                Is there a warning at runtime that connect failed?
                                You should use the new connect syntax to get compiler error instead of runtime warning if your connect call is wrong.

                                U Offline
                                U Offline
                                Ucn_
                                wrote on last edited by
                                #23

                                @jsulm It does not give any warning. I tried different connects, it just won't execute the function.

                                jsulmJ 1 Reply Last reply
                                0
                                • U Offline
                                  U Offline
                                  Ucn_
                                  wrote on last edited by
                                  #24
                                  This post is deleted!
                                  1 Reply Last reply
                                  0
                                  • U Ucn_

                                    @jsulm It does not give any warning. I tried different connects, it just won't execute the function.

                                    jsulmJ Offline
                                    jsulmJ Offline
                                    jsulm
                                    Lifetime Qt Champion
                                    wrote on last edited by
                                    #25

                                    @Ucn_ Without more code I can't tell you what the problem is. One possibility: "timer" is not the same instance from where you emit the signal.

                                    https://forum.qt.io/topic/113070/qt-code-of-conduct

                                    U 1 Reply Last reply
                                    1
                                    • jsulmJ jsulm

                                      @Ucn_ Without more code I can't tell you what the problem is. One possibility: "timer" is not the same instance from where you emit the signal.

                                      U Offline
                                      U Offline
                                      Ucn_
                                      wrote on last edited by
                                      #26

                                      @jsulm I have as follow in my MainWindow constructor:

                                          Someclass send;
                                          QObject::connect(&send, SIGNAL(valueChanged()), SLOT(someFunction()));
                                      

                                      I have this signal in Someclass

                                      signals:
                                        void valueChanged();
                                      

                                      And this Slot in my MainWindow

                                      private slots:
                                          void someFunction();
                                      

                                      then I emit from a function

                                      void Someclass::anotherfunction(){
                                      
                                          emit valueChanged();
                                      }
                                      
                                      jsulmJ 1 Reply Last reply
                                      0
                                      • U Ucn_

                                        @jsulm I have as follow in my MainWindow constructor:

                                            Someclass send;
                                            QObject::connect(&send, SIGNAL(valueChanged()), SLOT(someFunction()));
                                        

                                        I have this signal in Someclass

                                        signals:
                                          void valueChanged();
                                        

                                        And this Slot in my MainWindow

                                        private slots:
                                            void someFunction();
                                        

                                        then I emit from a function

                                        void Someclass::anotherfunction(){
                                        
                                            emit valueChanged();
                                        }
                                        
                                        jsulmJ Offline
                                        jsulmJ Offline
                                        jsulm
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #27

                                        @Ucn_ said in Signal and slot from other class not working:

                                        Someclass send;
                                        QObject::connect(&send, SIGNAL(valueChanged()), SLOT(someFunction()));

                                        Here send is a local variable and is destroyed as soon as it goes out of scope!

                                        https://forum.qt.io/topic/113070/qt-code-of-conduct

                                        U 1 Reply Last reply
                                        1
                                        • jsulmJ jsulm

                                          @Ucn_ said in Signal and slot from other class not working:

                                          Someclass send;
                                          QObject::connect(&send, SIGNAL(valueChanged()), SLOT(someFunction()));

                                          Here send is a local variable and is destroyed as soon as it goes out of scope!

                                          U Offline
                                          U Offline
                                          Ucn_
                                          wrote on last edited by Ucn_
                                          #28

                                          @jsulm If I declare Someclass *send = new Someclass(); it gives error:

                                          mainwindow.cpp:18:14: error: no matching member function for call to 'connect'
                                          qobject.h:467:41: note: candidate function not viable: no known conversion from 'Someclass **' to 'const QObject *' for 1st argument; remove &
                                          qobject.h:264:13: note: candidate template ignored: requirement 'int(QtPrivate::FunctionPointer<const char *>::ArgumentCount) >= 0' was not satisfied [with Func1 = const char *, Func2 = const char *]
                                          qobject.h:304:13: note: candidate template ignored: substitution failure [with Func1 = const char *, Func2 = const char *]: no type named 'Object' in 'QtPrivate::FunctionPointer<const char *>'
                                          qobject.h:232:43: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
                                          qobject.h:273:13: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
                                          qobject.h:312:13: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
                                          qobject.h:212:36: note: candidate function not viable: requires at least 4 arguments, but 3 were provided
                                          qobject.h:215:36: note: candidate function not viable: requires at least 4 arguments, but 3 were provided
                                          

                                          I handle keypress event in Someclass, but is just an example. I want to emit valueChanged(); when a specific key is pressed or when a function is run, even though I did remove & to:

                                          Someclass *send = new Someclass()
                                          QObject::connect(send, SIGNAL(valueChanged()), SLOT(someFunction()));
                                          

                                          Still doesn't fire.

                                          jsulmJ 1 Reply Last reply
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