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Proper way to update widget from other class

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  • H Offline
    H Offline
    hills
    wrote on last edited by hills
    #1

    Hello everyone!
    I'm stuck with the problem, so: i have a form that have input widget and i want to update that widget from another class:

    Here is the form:
    H-file

    #pragma once
    #include <QtWidgets/QMainWindow>
    #include "ui_QtWidgetsApplication1.h"
    #include "test_class.h"
    
    class QtWidgetsApplication1 : public QMainWindow {
        Q_OBJECT
        Ui::QtWidgetsApplication1Class ui; 
    
    public:
        QtWidgetsApplication1(QWidget* parent = Q_NULLPTR);
    
    public slots:
        void updateText(const QString& text);
    };
    

    CPP-file

    #include "QtWidgetsApplication1.h"
    #include "test_class.h"
    
    QtWidgetsApplication1::QtWidgetsApplication1(QWidget *parent) : QMainWindow(parent) {
        ui.setupUi(this);
    
    }
    
    void QtWidgetsApplication1::updateText(const QString& text) {
        ui.plainTextEdit->appendPlainText(text);
    }
    

    And class from within i need to update a widget:
    H-file

    #pragma once
    #include <QObject>
    #include "QtWidgetsApplication1.h"
    
    class Counter : public QObject {
    
    public:
        Counter() {
            connect();
        }
    
        void connect();
    
    signals:
        void sendText(const QString& newText);
     
    private:
        Q_OBJECT;
    };
    

    and CPP-file

    #include "test_class.h"
    #include <QObject>
    #include "QtWidgetsApplication1.h"
    
    void Counter::connect() {
        emit sendText("WOW");
    }
    

    Main CPP

    #include "QtWidgetsApplication1.h"
    #include <QtWidgets/QApplication>
    #include "test_class.h"
    #include <QObject>
    int main(int argc, char *argv[])
    {
        QApplication a(argc, argv);
        QtWidgetsApplication1 w;
    
        w.show();
        return a.exec();
    }
    

    I'll try to connect it in form class like

    Counter c;
    connect(&c, SIGNAL(sendText()), this, SLOT(updateText()));
    // or
    connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);
    

    It doesn't work

    But if i instantiate connect in main, like this:

    ...
    QtWidgetsApplication1 w;
    Counter c;
    QObject::connect(&c, &Counter::sendText, &w, &QtWidgetsApplication1::updateText);
    // and then assign
    c.sendText("No-no-no");
    

    It obviously work.
    So the question is how to show up a message "WOW" from Counter::connect() method when program launches.
    That's just an example what i want to do.
    I use a Qt6.0.1 with VS 2019 win 10 if that matter.

    1 Reply Last reply
    0
    • Christian EhrlicherC Online
      Christian EhrlicherC Online
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on last edited by
      #2

      Since you can't use signals and slots you can for example set a callback function in Counter which gets executed every time you want to send a signal from Counter to Qt. This callback function can then emit a Qt signal.

      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
      Visit the Qt Academy at https://academy.qt.io/catalog

      H 1 Reply Last reply
      3
      • Christian EhrlicherC Christian Ehrlicher

        Since you can't use signals and slots you can for example set a callback function in Counter which gets executed every time you want to send a signal from Counter to Qt. This callback function can then emit a Qt signal.

        H Offline
        H Offline
        hills
        wrote on last edited by
        #3

        @Christian-Ehrlicher said in Update widget from nonQT class (signal & slot problem):

        Since you can't use signals and slots you can for example set a callback function in Counter which gets executed every time you want to send a signal from Counter to Qt. This callback function can then emit a Qt signal.

        Can you give me an example? I'm not clearly understand. I thought it's a common usage: i have isolated class with form and UI stuff and some other classes that connected with that form thru some method's. And i thought slots and signals implemented that routine.
        That what i'm trying to do is weird? And if it's does can you have a better workaround?

        1 Reply Last reply
        0
        • Christian EhrlicherC Online
          Christian EhrlicherC Online
          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on last edited by
          #4

          @hills said in Update widget from nonQT class (signal & slot problem):

          I thought it's a common usage:

          Emitting a signal from a n object which is not derived from QObject is not common usage. So either derive from QObject or use a callback function.

          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
          Visit the Qt Academy at https://academy.qt.io/catalog

          H 1 Reply Last reply
          1
          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #5

            Hi and welcome to devnet,

            If I may, adding a "connect" method to a QObject subclass is not really a good idea. You might want to consider a more accurate name.

            That said, you do not call said method in your code, hence "WOW" will never be shown.

            On a side note, signals should only be called from within the class that defines them.

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

            1 Reply Last reply
            0
            • Christian EhrlicherC Christian Ehrlicher

              @hills said in Update widget from nonQT class (signal & slot problem):

              I thought it's a common usage:

              Emitting a signal from a n object which is not derived from QObject is not common usage. So either derive from QObject or use a callback function.

              H Offline
              H Offline
              hills
              wrote on last edited by hills
              #6

              @SGaist said in Update widget from nonQT class (signal & slot problem):

              If I may, adding a "connect" method to a QObject subclass is not really a good idea. You might want to consider a more accurate name.

              Thanks!
              It just an example, for real: i have CLI-based class that has function's with many std::cout log messages inside and i want to translate them into input widgets .

              @Christian-Ehrlicher said in Update widget from nonQT class (signal & slot problem):

              @hills said in Update widget from nonQT class (signal & slot problem):

              I thought it's a common usage:

              Emitting a signal from a n object which is not derived from QObject is not common usage. So either derive from QObject or use a callback function.

              So, i trying to realize callback function, but it fails on QtWidgetsApplication1::UpdateText (error C2228: left of '.textEdit' must have class/struct/union).
              How to make this works?

              FORM H

              class QtWidgetsApplication1 : public QMainWindow {
                  Q_OBJECT
                  Ui::QtWidgetsApplication1Class ui; 
                  static void updateText(const QString& text);
              public:
                  explicit QtWidgetsApplication1(QWidget* parent = Q_NULLPTR);
              };
              

              FORM CPP

              QtWidgetsApplication1::QtWidgetsApplication1(QWidget *parent) : QMainWindow(parent) {
                  ui.setupUi(this);
                  Counter* c = new Counter();
                  c->setCallbackFunc(updateText);
              }
              
              void QtWidgetsApplication1::updateText(const QString& text) {
                  ui.textEdit->setText(text);                    // error C2228: 
                  ui.plainTextEdit->appendPlainText(text);       //  left of '.textEdit' must have class/struct/union
              }
              

              Counter H

              class Counter : public QMainWindow {
              public:
                  explicit Counter(QObject* parent = 0) {
                      connect();
                  }
                  void setCallbackFunc(void (*func) (const QString& text));
                  void connect();
              private:
                  void (*callbackFunc)(const QString& text);
                  Q_OBJECT;
              };
              

              Counter CPP

              void Counter::setCallbackFunc(void(*func)(const QString& text)) {
                  callbackFunc = func;
              }
              
              void Counter::connect() {
                  //QString text = "Maybe-maybe";
                  //callbackFunc(text);
                  callbackFunc("WOW");
              }
              
              1 Reply Last reply
              0
              • mrjjM Offline
                mrjjM Offline
                mrjj
                Lifetime Qt Champion
                wrote on last edited by
                #7

                Hi
                When you say nonQT class, we think of class that does not inherit QObject
                but in your code shown, it does ! `?

                so if the real issues were

                Counter c;
                connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                only works in main.cpp,
                i was wondering if your actual issue was just the use of a local variable that will
                get deleted as soon as the function ends. ( the variable c has a short life span if in a function)

                So maybe the real fix would have been

                Counter * c = new Counter (this);
                connect(c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                H 1 Reply Last reply
                2
                • mrjjM mrjj

                  Hi
                  When you say nonQT class, we think of class that does not inherit QObject
                  but in your code shown, it does ! `?

                  so if the real issues were

                  Counter c;
                  connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                  only works in main.cpp,
                  i was wondering if your actual issue was just the use of a local variable that will
                  get deleted as soon as the function ends. ( the variable c has a short life span if in a function)

                  So maybe the real fix would have been

                  Counter * c = new Counter (this);
                  connect(c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                  H Offline
                  H Offline
                  hills
                  wrote on last edited by
                  #8

                  @mrjj said in Update widget from nonQT class (signal & slot problem):

                  Hi
                  When you say nonQT class, we think of class that does not inherit QObject
                  but in your code shown, it does ! `?

                  so if the real issues were

                  Counter c;
                  connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                  only works in main.cpp,
                  i was wondering if your actual issue was just the use of a local variable that will
                  get deleted as soon as the function ends. ( the variable c has a short life span if in a function)

                  So maybe the real fix would have been

                  Counter * c = new Counter (this);
                  connect(c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                  Hello!
                  Unfortunately it didn't work. Fail with error:
                  error C3867: 'Counter::sendText': non-standard syntax; use '&' to create a pointer to member
                  error C3867: 'QtWidgetsApplication1::updateText': non-standard syntax; use '&' to create a pointer to member
                  And if i add &

                  connect(c, &Counter::sendText, this, &QtWidgetsApplication1::updateText);
                  

                  It compiles without errors and warnings but nothing happened at text widget

                  Christian EhrlicherC mrjjM 2 Replies Last reply
                  0
                  • H hills

                    @mrjj said in Update widget from nonQT class (signal & slot problem):

                    Hi
                    When you say nonQT class, we think of class that does not inherit QObject
                    but in your code shown, it does ! `?

                    so if the real issues were

                    Counter c;
                    connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                    only works in main.cpp,
                    i was wondering if your actual issue was just the use of a local variable that will
                    get deleted as soon as the function ends. ( the variable c has a short life span if in a function)

                    So maybe the real fix would have been

                    Counter * c = new Counter (this);
                    connect(c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                    Hello!
                    Unfortunately it didn't work. Fail with error:
                    error C3867: 'Counter::sendText': non-standard syntax; use '&' to create a pointer to member
                    error C3867: 'QtWidgetsApplication1::updateText': non-standard syntax; use '&' to create a pointer to member
                    And if i add &

                    connect(c, &Counter::sendText, this, &QtWidgetsApplication1::updateText);
                    

                    It compiles without errors and warnings but nothing happened at text widget

                    Christian EhrlicherC Online
                    Christian EhrlicherC Online
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by
                    #9

                    @hills said in Update widget from nonQT class (signal & slot problem):

                    It compiles without errors and warnings but nothing happened at text widget

                    Are you sure you call sendText() and updateText() is actually doing something? Please add some debug output and make sure the correct instance of Counter is connected.

                    And please change the title of your question - it's irritating.

                    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                    Visit the Qt Academy at https://academy.qt.io/catalog

                    H 1 Reply Last reply
                    1
                    • H hills

                      @mrjj said in Update widget from nonQT class (signal & slot problem):

                      Hi
                      When you say nonQT class, we think of class that does not inherit QObject
                      but in your code shown, it does ! `?

                      so if the real issues were

                      Counter c;
                      connect(&c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                      only works in main.cpp,
                      i was wondering if your actual issue was just the use of a local variable that will
                      get deleted as soon as the function ends. ( the variable c has a short life span if in a function)

                      So maybe the real fix would have been

                      Counter * c = new Counter (this);
                      connect(c, Counter::sendText, this, QtWidgetsApplication1::updateText);

                      Hello!
                      Unfortunately it didn't work. Fail with error:
                      error C3867: 'Counter::sendText': non-standard syntax; use '&' to create a pointer to member
                      error C3867: 'QtWidgetsApplication1::updateText': non-standard syntax; use '&' to create a pointer to member
                      And if i add &

                      connect(c, &Counter::sendText, this, &QtWidgetsApplication1::updateText);
                      

                      It compiles without errors and warnings but nothing happened at text widget

                      mrjjM Offline
                      mrjjM Offline
                      mrjj
                      Lifetime Qt Champion
                      wrote on last edited by
                      #10

                      @hills

                      Hi. well, it has to have &, as we take the address of a member function.

                      Also, make sure you mean to actually mean create a new Counter variable.
                      The one you connect with - is the only one connected so if you have other Counters, you created, they
                      might not been connected.
                      That can explain why nothing happens if you have code in the slot that should set some text on the screen.

                      1 Reply Last reply
                      1
                      • Christian EhrlicherC Christian Ehrlicher

                        @hills said in Update widget from nonQT class (signal & slot problem):

                        It compiles without errors and warnings but nothing happened at text widget

                        Are you sure you call sendText() and updateText() is actually doing something? Please add some debug output and make sure the correct instance of Counter is connected.

                        And please change the title of your question - it's irritating.

                        H Offline
                        H Offline
                        hills
                        wrote on last edited by hills
                        #11

                        @Christian-Ehrlicher said in Update widget from nonQT class (signal & slot problem):

                        @hills said in Update widget from nonQT class (signal & slot problem):

                        It compiles without errors and warnings but nothing happened at text widget

                        Are you sure you call sendText() and updateText() is actually doing something? Please add some debug output and make sure the correct instance of Counter is connected.

                        Yeah, i tested it in main.cpp - from main he works fine

                        And please change the title of your question - it's irritating.

                        No problem, that's better?

                        @mrjj hi! there is only one variable called Counter an it's within QtWidgetsApplication1.cpp

                        Here the full project, maybe this will clarify something
                        https://pastebin.com/h0NFXJrP - main.cpp
                        https://pastebin.com/YHnYZkwp - QtWidgetsApplication1.h
                        https://pastebin.com/Z45b5uWx - QtWidgetsApplication1.cpp
                        https://pastebin.com/c502CUjy - QtWidgetsApplication1.ui
                        https://pastebin.com/BfKCdUdr - test_class.h
                        https://pastebin.com/WU3ThMu3 - test_class.cpp

                        1 Reply Last reply
                        0
                        • Christian EhrlicherC Online
                          Christian EhrlicherC Online
                          Christian Ehrlicher
                          Lifetime Qt Champion
                          wrote on last edited by
                          #12

                          Where do you call Counter::connect(). And no, calling it in the ctor will not work since the connection is done afterwards.

                          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                          Visit the Qt Academy at https://academy.qt.io/catalog

                          H 1 Reply Last reply
                          2
                          • Christian EhrlicherC Christian Ehrlicher

                            Where do you call Counter::connect(). And no, calling it in the ctor will not work since the connection is done afterwards.

                            H Offline
                            H Offline
                            hills
                            wrote on last edited by
                            #13

                            @Christian-Ehrlicher said in Proper way to update widget from other class:

                            Where do you call Counter::connect(). And no, calling it in the ctor will not work since the connection is done afterwards.

                            Oh, sweet lord, i finally got something... I call the constructor and after that make connection between form and Counter class. That leads to the fact that all messages that were before the connection disappeared.
                            So i need to re-write my real code, coz i make stuff in the default constructor that show up some messages.
                            Thanks a lot @Christian-Ehrlicher!

                            Christian EhrlicherC 1 Reply Last reply
                            0
                            • H hills

                              @Christian-Ehrlicher said in Proper way to update widget from other class:

                              Where do you call Counter::connect(). And no, calling it in the ctor will not work since the connection is done afterwards.

                              Oh, sweet lord, i finally got something... I call the constructor and after that make connection between form and Counter class. That leads to the fact that all messages that were before the connection disappeared.
                              So i need to re-write my real code, coz i make stuff in the default constructor that show up some messages.
                              Thanks a lot @Christian-Ehrlicher!

                              Christian EhrlicherC Online
                              Christian EhrlicherC Online
                              Christian Ehrlicher
                              Lifetime Qt Champion
                              wrote on last edited by
                              #14

                              @hills Glad you found the problem. Please mark the topic as solved then.

                              Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                              Visit the Qt Academy at https://academy.qt.io/catalog

                              1 Reply Last reply
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