从Cpp到QML的结构内部写入数组

Question

I need to write an array (from external C code) from Cpp to QML. As it was mentioned that this may be an XY-problem, now I first describe my basic intention. I use an external c-program that is given as it is and that I can't change. This c-program provides the following struct and the function mx_get to get the data from a certain position:

typedef struct Tmatrix
{
    size_t rows;
    size_t cols;
    double *data;
} Tmatrix;

double mx_get(const Tmatrix *matrix, const size_t i, const size_t j)
{
    return *(matrix->data + i * matrix->cols + j);
}

I need to use this c-program in my QML app, that is 99% QML and 1% Cpp. The cpp-part is a class to connect the QML to the c-program. I use qmlRegisterType<MyTerminal> to use it inside QML.

MyTerminal.h:

#include <QObject>

extern "C"
{
    #include "external-c-program.h"
}
Q_DECLARE_METATYPE(Tmatrix)

class MyTerminal: public QObject
{
    Q_OBJECT

public:
    MyTerminal();
    Q_INVOKABLE void getAllPartitionQualities();
    /* and other stuff not relevant here... */

private:
    TPanel* Panel; /* this contains the Tmatrix at Panel->Q */
};

In the QML part, I want to display the content of Tmatrix->data. Therefore I currently use Text inside a Repeater :

Repeater {
    model: nRows*nColumns
    Text {
        text: myTerminal.quality[indexY][indexX].toFixed(2)
        property int indexX: index - parseInt(index/nColumns)*nColumns
        property int indexY: parseInt(index/nRows)
    }
}

I did it element by element, but this now takes about 20 secs as the array size increased to 19*19. (In the following code samples, Panel->Q is of type Tmatrix .)

MyTerminal.cpp:

for (int y=0; y<PARTITION_ROWS; y++)
{
    for (int x=0; x<PARTITION_COLUMNS; x++)
    {
        QMetaObject::invokeMethod(this, "setQuality",
                                  Q_ARG(QVariant, x),
                                  Q_ARG(QVariant, y),
                                  Q_ARG(QVariant, mx_get(Panel->Q, x, y)));
    }
}

QML:

MyTerminal {
    id: myTerminal
    property int nRows: 19
    property int nColumns: 19
    property var quality: [[]]

    Component.onCompleted: {
        // Initialize quality as 2d array with size nRows*nColumns
        var i, j;
        var temp = new Array(nRows);
        for (i=0; i<nRows; i++) {
            temp[i] = new Array(nColumns);
            for (j=0; j<nColumns; j++) {
                temp[i][j] = 0.0;
            }
        }
        quality = temp;
    }

    function setQuality(x, y, q) {
        quality[y][x] = q;
        // Assign to itself to trigger an update of the texts
        quality = quality;
    }
}

So I am now trying to set the whole array with one call in cpp:

QQmlProperty::write(this, "quality", QVariant::fromValue(Panel->Q->data));

Therefore I think I need to add this:

Q_DECLARE_METATYPE(Tmatrix)

But I still get the error "static assertion failed: Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system". Probably because the double pointer is inside the struct?

What is the best (most performant) way to send the pointer content to QML?

I also tried again with QMetaObject::invokeMethod passing Panel->Q (instead of passing Panel->Q->data to QQmlProperty::write), but I didn't find help how to do this correctly neither.

Edit: Or can I declare my array quality inside my Cpp-class which inherits QObject and still access it from QML? Maybe that's easier.

Answer 1

What is the best (most performant) way to send the pointer content to QML?

You can't send the pointer itself directly. You'll need to pack the data into a QVariantList or a QVariantMap . (Since you're using doubles, you could also store it as a QList<qreal> and pass it to QML).

You could make your TMatrix object into a QList<QList<qreal>> by

QList<QList<qreal>> tMatrix = {
    {0.0, 1.0, 2.0, 3.0},
    {4.0, 5.0, 6.0, 7.0},
    {8.0, 9.0, 10.0, 11.0}
};

This makes it easier to throw your object around from and to C++/QML.

More info: Data Type Conversion between QML and C++

Or can I declare my array quality inside my Cpp-class which inherits QObject and still access it from QML? Maybe that's easier.

Yes, that may be more appropriate, as your data is large. But note that, your data still needs to be convertible to a QML data type. (Again, I suggest the QVariantMap for your object, since that's eventually how JS handles objects.)

This may come in the form of a member variable of your C++ class, eg

private:
    QList<QList<qreal>> tMatrix = { /*...*/ };

You can then fill this matrix in the constructor with your data values. (And it doesn't necessarily have to be a QList , it could be a double or another QList<> , but that depends on what your TMatrix is for, so it's up to you. Just make sure it's a valid type .)

Then have a public slot or invokable method which will be called from QML/JS, eg

public:
    Q_INVOKABLE qreal getObj(int x, int y);

And define it like so

qreal MyClass::getObj(int x, int y)
{
    if (y <= 0 || reals.size() <= y)
        return qreal();

    if (x <= 0 || vMaps[y].size() <= x)
        return qreal();

    return tMatrix[y][x];
}

This is only the getter, if you want a setter, you could define it likewise, with void return type and qreal arguments (which allows you to then pass JS objects into it).

Remember to expose your class to QML (I believe you've done so already), then you can access tMatrix in QML/JS using...

MyClass  // depends on how you registered your type
{
    id: myClass
    Component.onCompleted
    {
        var obj = getObj(2, 1)  //  calls the function from your class
                                //  use an id if needed (e.g. myClass.getObj())

        console.debug("Retrieved Object at (2, 1):")
        console.debug(JSON.stringify(obj))            
    }
}

More info: Integrating QML and C++