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  4. [Solved] Exporting functions and constants from C++ to QML

[Solved] Exporting functions and constants from C++ to QML

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  • I Offline
    I Offline
    Ices_Eyes
    wrote on last edited by
    #1

    Hi all.
    Probably thi will be considered a very noob question, but that's it.
    I have a QML app that make use of some constants defined on a javascript file. What I would like to do is to remove those constants from public visible javascript and put them into the c++ code. know that a possible solution would be to expose them with the rootContext()->setContextProperty(...) method, but... What if I would like to have a C++ class that use some functions in order to compute those constants? I mean:
    al the constants are strings, so I would like to have, in the QML code, something like:
    myClass.getUrl1(), myClass.getUrl2(), and so on
    but withouth the need to define a QML object
    @MyClass {
    id: myClass
    }@
    in the QML code. Is this possible? :)

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    • U Offline
      U Offline
      unai_i
      wrote on last edited by
      #2

      Hi,
      You can just register a C++ class into QML with the required slots (or invokable methods) or expose an instance of such a class through setContextProperty. All you need is inherit from QObject.
      Good luck!

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      • I Offline
        I Offline
        Ices_Eyes
        wrote on last edited by
        #3

        Wll, I didn't think I could export some classes instances as context properties, but..in effect the function accept an object as argument... :)

        so the idea is to have something like:

        @MyClass.h
        class MyClass : public QObject {
        Q_OBJECT
        public:
        explicit MyClass(QObject *parent = 0);
        QString getAnUrl();
        QString getAnotherUrl(QString parameter)
        private:
        const QString aConstant = "http://anurl";
        const QString anotherConstant = "http://anotherurl";
        }
        @
        @MyClass.cpp
        MyClass::MyClass(QObject *parent) : QObject(parent) {
        }

        QString MyClass::getAnUrl() {
        return this.aConstant;
        }

        QString MyClass::getAnotherUrl(QString parameter) {
        return this.anotherConstant + "?aparameter=" + parameter;
        }
        @

        @main.cpp
        Q_DECL_EXPORT int main(int argc, char *argv[]) {
        ....
        MyClas myClass;
        viewer.rootContext()->setContextProperty("myClass", myClass);
        ....
        }
        @

        and then to use myClass.getAnUrl() in QML and js file.

        Right? :P

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        • sierdzioS Offline
          sierdzioS Offline
          sierdzio
          Moderators
          wrote on last edited by
          #4

          Yes. Now you only need to add Q_INVOKABLE in front of your method declarations in the header file.

          (Z(:^

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          0
          • I Offline
            I Offline
            Ices_Eyes
            wrote on last edited by
            #5

            Ok! Thanks... And now let's do some programming :P

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            • sierdzioS Offline
              sierdzioS Offline
              sierdzio
              Moderators
              wrote on last edited by
              #6

              May the code be with you.

              You'll also have to correct some errors in your c++ code, but I trust the compiler to point them out for you.

              (Z(:^

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              • I Offline
                I Offline
                Ices_Eyes
                wrote on last edited by
                #7

                Well, yep, I've written down the code quickly ;)

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                • I Offline
                  I Offline
                  Ices_Eyes
                  wrote on last edited by
                  #8

                  Well... It worksss!!! Thaks guys ;)

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                  • sierdzioS Offline
                    sierdzioS Offline
                    sierdzio
                    Moderators
                    wrote on last edited by
                    #9

                    I'm happy to hear that :) And you even marked the thread as solved - perfection itself. Happy further coding.

                    (Z(:^

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                    • I Offline
                      I Offline
                      Ices_Eyes
                      wrote on last edited by
                      #10

                      Ok, I have one more question with respect to this :)
                      What I have done is something like:
                      @Q_INVOKABLE QString getUrl(QString urlParameter);@

                      But in many tutorials I have seen now, the use
                      @Q_INVOKABLE QString getNewsUrl(const QString &urlParameter);@

                      Which one is more correct?

                      Also. Having javascript "var" variables that contains integers, is there a way to pass this types to C++ instead of strings? Or should I convert them in C++? I mean... Using
                      @Q_INVOKABLE QString getNewsUrl(int urlParameter);@
                      gives me no right values in urlParameter variables...

                      :)

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                      • A Offline
                        A Offline
                        andre
                        wrote on last edited by
                        #11

                        [quote author="Ices_Eyes" date="1346753251"]Ok, I have one more question with respect to this :)
                        What I have done is something like:
                        @Q_INVOKABLE QString getUrl(QString urlParameter);@

                        But in many tutorials I have seen now, the use
                        @Q_INVOKABLE QString getNewsUrl(const QString &urlParameter);@

                        Which one is more correct?
                        [/quote]
                        Why would you assume the one is 'more' correct than the other. By what measure exactly?

                        Some coding guidelines tell you to use const references for function arguments that as used unchanged by the function, and in that context using the const QString& might be more correct. But not everybody uses this guideline. For efficieny, there is very little gain here, as QString is implicitly shared and thus very fast to copy as a value.

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                        • sierdzioS Offline
                          sierdzioS Offline
                          sierdzio
                          Moderators
                          wrote on last edited by
                          #12

                          [quote author="Ices_Eyes" date="1346753251"]
                          Also. Having javascript "var" variables that contains integers, is there a way to pass this types to C++ instead of strings? Or should I convert them in C++? I mean... Using
                          @Q_INVOKABLE QString getNewsUrl(int urlParameter);@
                          gives me no right values in urlParameter variables...
                          :)[/quote]

                          As Andre already covered the top part, I'll take on the bottom one. QML automatically translates JS values to C++ ones. So, for int you get c++ int, for real you get Qt's qreal, for string it's QString, for arrays is complicated (as you can store anything there. But basically QObjectList tends to work in most cases, or QVariantList etc.). There is no need for conversion of those basic types.

                          (Z(:^

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                          • I Offline
                            I Offline
                            Ices_Eyes
                            wrote on last edited by
                            #13

                            So, I need to understand why
                            @
                            var parameter = 1;
                            ...
                            myClass.myFunc(parameter)
                            @
                            works if myFunct is defined with QString and does not work if defined with int... ;)

                            Thanks guys!!! :)

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                            • I Offline
                              I Offline
                              Ices_Eyes
                              wrote on last edited by
                              #14

                              Damn!!! My bad... It was a conversion problem in C++ of integer to string!!! Damn, too much codin in Java and Javascript, I have completly forgotten C!!! :(

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                              • sierdzioS Offline
                                sierdzioS Offline
                                sierdzio
                                Moderators
                                wrote on last edited by
                                #15

                                This is indeed strange. Be sure to make a full app rebuild (clean, run qmake, build) just to make sure that MOC updates the definitions. It quite definitely works for me. Alternatively, you can try using QVariant in c++ declaration, and then calling ::toInt() on the value - but that should not be necessary.

                                (Z(:^

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                                • I Offline
                                  I Offline
                                  Ices_Eyes
                                  wrote on last edited by
                                  #16

                                  Solved with a QString::number(parameter)...

                                  Damn me... :P

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                                  0
                                  • sierdzioS Offline
                                    sierdzioS Offline
                                    sierdzio
                                    Moderators
                                    wrote on last edited by
                                    #17

                                    Happens to everybody :) That took me by surprise a few times, too. I tend to forget about ::number() especially when I'm tired and after prolonged QML-only phases :D

                                    (Z(:^

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