Updating c++ data in Qml

dziko147

@SGaist yes two instance of Backend class.
the first in UploadCSV.cpp to emit signal when button clicked

Backend *back = new Backend(); // I think that this line is the problem ?
    connect(ui->parambtn_2, SIGNAL(clicked()),back , SLOT(status()),Qt::QueuedConnection);

the second in mainwindow.cpp to create a contextproperty :

back = new Backend();
    m_ui->quickWidget->rootContext()->setContextProperty("cppObject",back);```

SGaist

And you expect these two unrelated instances to communicate between different widgets from your UI ?

dziko147

@SGaist said in Updating c++ data in Qml:

expect these two unrelated instances to communicate between different widgets from your UI ?

no I mentioned that these two instances cause a problem.

So can you suggest a way to do this !

BTW UploadCSV is a QDialog , Backend is QObject .

when I create instance in UploadCSV.h it cause problem !

SGaist

Review your application architecture. There's an issue with what is created where and how they interact with each other.

You seem to have an issue with the proper composition of your various classes and GUI elements.

dziko147

@SGaist there no way to solve it ?

SGaist

Yes there is and was already suggested in your other threads. You need to properly create your objects hierarchy.

You were creating a MainWidget object in your dialog trying to get data from it while it was in fact created by a different instance.

You seem to reproduce a similar scheme here.

dziko147

@SGaist In my situation , when I would like to create an Backend instance in UploadCSV to emit signal .
And create Backend instance in MainWindow to set a contextProperty .
what should I do .
Please :)

KroMignon

@dziko147 said in Updating c++ data in Qml:

In my situation , when I would like to create an Backend instance in UploadCSV to emit signal .
And create Backend instance in MainWindow to set a contextProperty .
what should I do .
Please :)

You should first explain why you think you have to create a Backend instance in UploadCSV?
It is impossible to understand (for me) what is the purpose of this.
And emitting a signal which is connected to nothing, results to nothing.

dziko147

@KroMignon in uploadcsv ui , i have a button . I want to emit clicked signal connected to a slot in backend which change data (see my code please) .

KroMignon

@dziko147 said in Updating c++ data in Qml:

in uploadcsv ui , i have a button . I want to emit clicked signal connected to a slot in backend which change data (see my code please) .

Your code is incomprehensible!
Please learn C++ basics, like what are classes, class members, class methods and class instances.

I don't understand what is your problem, on click event the method status() instance back which is created in UploadCSV.cpp will be called and you should see a debug message on console.

I don't understand why you expect anything on QML side as this instance is not exposed to QML.

dziko147

there is no one who did something like this ?

KroMignon

@dziko147 said in Updating c++ data in Qml:

there is no one who did something like this ?

Again, explain what you want to do, not what you have done.
Because what you have done don't made sense.

If you want to update the Backend instance back create in MainWindow on button click in UploadCSV.cpp, then you have to connect the click event with this instance, not create a new instance of Backend in UploadCSV.cpp.

dziko147

@KroMignon if I understand you . I should create an MainWindow Instance in UploadCSV.cpp then I call the back defined in MainWindow ?
Is that ?

KroMignon

@dziko147 said in Updating c++ data in Qml:

if I understand you . I should create an MainWindow Instance in UploadCSV.cpp then I call the back defined in MainWindow ?
Is that ?

No, you don't understand anything, and that is very bad :(
You have to connect the click signal from the UploadCSV instance to the back variable (which should be a class member ==> https://www.learncpp.com/cpp-tutorial/classes-and-class-members/) of the MainWindow instance.

And NOT create a new instance.

jsulm

@dziko147 said in Updating c++ data in Qml:

Is that ?

No.
Why should UploadCSV create main window?!
Please read more carefully.

You should really learn what instances are! Two different instance are exactly that: two different instances/objects, they are NOT same.
If you create two instances of a class (lets name them A and B), then connect a slot from A to a signal, then do you expect that the slot in B will also be called if the signal is emitted? Yes or No?

dziko147

@jsulm Sure No .

@KroMignon If I don't create MainWindow instance, how can i access to mainwindow variable from UploadCSV.

MainWindow *mymain=new MainWindow();
    connect(ui->parambtn_2, SIGNAL(clicked()),mymain->back, SLOT(status()),Qt::QueuedConnection);

this is what I think .
Correct me please :)

jsulm

@dziko147 said in Updating c++ data in Qml:

If I don't create MainWindow instance, how can i access to mainwindow variable from UploadCSV.

Don't create a SECOND MainWindow instance! Use the one you already have (you probably create it in main.cpp, right?)...

dziko147

@jsulm Yes sure . so I use it ?

jsulm

@dziko147 said in Updating c++ data in Qml:

so I use it ?

Yes, the one you already have

dziko147

@KroMignon Is this code correct ??

I create a class member :

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    Backend *back;

then in uploadCSV.cpp

  connect(ui->parambtn_2, SIGNAL(clicked()),MainWindow().back, SLOT(statuspican()),Qt::QueuedConnection);

But still doesn't work !