Updating c++ data in Qml

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 !

jsulm

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

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

You're creating a new MainWindow instance in the connect.
I give up...

dziko147

@jsulm Sorry :( :( :(
why you don't/want give an example ?
I can understand you with example :)

jsulm

@dziko147 You should really learn C++ basics first before you continue with Qt.
This is now second thread from you where people are trying to explain to you what you are doing wrong. Your problem is not Qt but missing C++ knowledge.

For example: why did you write "MainWindow()" bellow? Do you know what MainWindow() does? And how does this relate to what I wrote (use existing MainWindow instance)? You again create a new MainWindow instance, even if you was told many times to not to do so!

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

dziko147

@jsulm ok told me how to do this . I am a student in first year . So i am already learning :/ .
Yes I understand that i create a second instance but i haven't another choice . the only solution for me is to ask you . :(

jsulm

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

Yes I understand that i create a second instance

You need to understand that it is quite disappointing for others to repeat the same thing again and again and again and then see that you are still doing EXACTLY the same mistake even if you know that it is wrong! So, why do you again post code which is wrong if you know that it is wrong?

I can't tell you how exactly you need to do it because I don't have your code and so don't know how it is structured...
From architectural point of view it is strange why uploadCSV should connect a signal to a slot in a member variable in another class? uploadCSV should not even know that MainWindow has a member called "back". This was already pointed out by @SGaist

Where is the uploadCSV instance created? Does MainWindow create it?

dziko147

@jsulm this is my situation.

My project compouned of 3 classes :
1/Mainwindow
2/backend
/UploadCSV
In Mainwindow :
1/ I have a quickwidget which integrate a qml file .
2/ I create a backend instance to get data from it and fill the QMl File

In Backend
1/I have the defenition of my data (getter, setter, change slots).

In UploadCSV
1/I have a button when it's clicked a slot in Backend will change some data .
2/After change data I would like to display it on QML .

I hope it's clear

jsulm

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

I hope it's clear

No, you did not answer my question, which is actually quite clear...

dziko147

I give uuuuuuuuuuuuuup

J.Hilk

alright @jsulm
alt text

@dziko147 this does actually help

My project compouned of 3 classes :
1/Mainwindow
2/backend
/UploadCSV
In Mainwindow :
1/ I have a quickwidget which integrate a qml file .
2/ I create a backend instance to get data from it and fill the QMl File

In Backend
1/I have the defenition of my data (getter, setter, change slots).

In UploadCSV
1/I have a button when it's clicked a slot in Backend will change some data .
2/After change data I would like to display it on QML .

I hope it's clear

some more required information:

how do you expose backend to the qml context, and where (in your code)
name of your backend and uploadcsv instances, and where they are defined
does UploadCSV have its own UI components? do you mean that with "button"

I have the high suspicion that everything is centered in and around mainwindow already and its only a matter of connecting it. But truthfully code will help, it doesn't look like you have much yet, so post as much as you can

dziko147

@J-Hilk
1/ I expose backend to qml like this :

back= new Backend();
    m_ui->quickWidget->setVisible(true);
    m_ui->quickWidget->rootContext()->setContextProperty("cppObject",back);
    m_ui->quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
    m_ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/qml/automotive.qml")));

2/ And UploadCSV in MainWindow just for show . I dont use UploadCSV in MainWindow

//BTW CAN i define the connect signal of UploadCSV button here ?
void MainWindow::on_actionUpload_CSV_File_triggered()
{
    uploadcsv = new UploadCSV();
    uploadcsv->show();
}

3/ Yes Sure UploadCSV has a UI form .

J.Hilk

@dziko147 alright,

we are getting closer,

I assume this:

back= new Backend();
    m_ui->quickWidget->setVisible(true);
    m_ui->quickWidget->rootContext()->setContextProperty("cppObject",back);
    m_ui->quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
    m_ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/qml/automotive.qml")));

is inside the MainWindow constructor ?

//BTW CAN i define the connect signal of UploadCSV button here ?
void MainWindow::on_actionUpload_CSV_File_triggered()
{
    uploadcsv = new UploadCSV();
    uploadcsv->show();
}

The way you have it currently set up, you have to make the connect call here.

but that won't work quite yet.

I assume the button you want to press, to update your backend is part of the UploadCSV ui ? Therefore ui->parambtn_2 is the button you want to press and is part of the UploadCSV ui?
have you yet defined a (custom)signal inside the UploadCSV class ?

J.Hilk

If my assumptions are correct, I'm writing this before a response,

you can/should make the following changes:

inside UploadCSV .h add a custom signal, for example purpose we'll name it myCustomButtonClicked(),

public:
    explicit UploadCSV(QObject *parent = nullptr);
.....

signals:
      void myCustomButtonClicked();

inside UploadCSV.cpp constructor definition, after ui->setupUi(this), connect the button to the recently created signal:

{
    ui->setupUi(this);
    connect(ui->parambtn_2, &QAbstractButton::clicked, this, &UploadCSV::myCustomButtonClicked); // this connects/forwards the button clicked signal to the  class signal 'myCustomButtonClicked'
}

in mainwindow constructor, instantiate the UploadCSV in the constructor and connect it to the backend:

{
    ui->setupUi(this);
back= new Backend();
    m_ui->quickWidget->setVisible(true);
    m_ui->quickWidget->rootContext()->setContextProperty("cppObject",back);
    m_ui->quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
    m_ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/qml/automotive.qml")));

uploadcsv = new UploadCSV();
connect(uploadcsv, & UploadCSV:: myCustomButtonClicked, backend, &Backend::status); //status has to be public!

}

your on_actionUpload_CSV_File_triggered now only contains this:

void MainWindow::on_actionUpload_CSV_File_triggered()
{
    uploadcsv->show();
}

let's see if that worked.

code my contain typos, so be aware :D