QQmlPropertyMap, model

[sBoff]

Hi wysota,

Thanks for your reply. Do you mean derive my own model object from say QAbstractTableModel or similar?

I originally started with QQmlPropertyMap as I need a way to easily map key/value pairs and to use this data to bind to QML objects - for that purpose QQmlPropertyMap seemed like the ideal choice. Maybe now that I am needing to display the data in a tableview that approach is no longer the best way?

Regards

[wysota]

Thanks for your reply. Do you mean derive my own model object from say QAbstractTableModel or similar?

More likely QAbstractListModel. But in general, yes.

I originally started with QQmlPropertyMap as I need a way to easily map key/value pairs and to use this data to bind to QML objects - for that purpose QQmlPropertyMap seemed like the ideal choice. Maybe now that I am needing to display the data in a tableview that approach is no longer the best way?

Maybe

[sBoff]

More likely QAbstractListModel. But in general, yes.

Thanks for your input - Ill try this out and post up if I have any issues with it. I had thought about doing this but I was sure there must have been an 'easy' way (it seems like a common use case to me?), but thankyou again; its nice to have another brain out there provide some clarity!

Regards

[sBoff]

It looks like I've come full circle.

I originally started this project a long time ago but then with my daughters birth had put things on hold. I'm now managing to scrape some time here and there so thought I would start it up again.

wysota, I've now created a QAbstractListModel derived object which stores my data in an internal QVector. This is working beautifully in the QML side and I have a TableView that shows all entries and updates correctly when the internal data updates.

class DataEngine : public QAbstractListModel
{
private:
  QVector<Data> m_Data;
};

class DataEngine : public QAbstractListModel
{
private:
  QVector<Data> m_Data;
};

To copy to clipboard, switch view to plain text mode 

This leads me onto the second issue which was what I originally faced so long ago - Data is added to my 'data engine' dynamically at runtime, I dont know prior to runtime what data will be available. I need to be able to bind a QML object's value to a data entry and have the QML object also update when the data changes.

MyQmlObject
{
  value: runtime.abc //  <-- abc is a dynamic, we dont know until runtime that there is a property called abc
}

MyQmlObject
{
  value: runtime.abc //  <-- abc is a dynamic, we dont know until runtime that there is a property called abc
}

To copy to clipboard, switch view to plain text mode 

As far as I'm aware there is no mechanism exposed in Qt to allow for dynamic properties (other than using QQmlPropertyMap, QQmlListProperty etc)? If this is correct do I go about binding a UI element's value to a data entry? I guess I could always use a Q_INVOKABLE member to return the data however, how would I then get the QML object to upadte when the value has changed?

I'm currently thinking I'll have to use a QQmlListProperty as my internal data storage for my QAbstractListModel derived object. The problem is this leads back to the original issue - I now have to either:

- Make sure a data entry can only ever be modified through the QAbstractListModel derived object (to keep the update notifications of QAbstractListModel and the QQmlPropertyList in sync)
OR
- Find a way to detect a change in a data entry and inform the QAbstractListModel that it also needs to update.

I hope that makes sense? Any thoughts/suggestions?

[wysota]

how would I then get the QML object to upadte when the value has changed?

Bind the object's property to a function call that will return the data or invoke some imperative code when contents of the model change.

I'm currently thinking I'll have to use a QQmlListProperty as my internal data storage for my QAbstractListModel derived object.

No, that's not a good idea.

- Make sure a data entry can only ever be modified through the QAbstractListModel derived object

That's always the proper approach.

[sBoff]

Bind the object's property to a function call that will return the data or invoke some imperative code when contents of the model change.

This is fine, I have the QML object's property bound to the a function (I can see it hitting the breakpoint where I return a value) however, this only gets called once while the QML object is loading. From that point on, it never updates despite the data changing (this the issue QQmlPropertyMap and the likes resolved). I guess the question then boils down to, how can I update the QML object when I know data that it references has been updated?

Edit: I've had a bit of a play with this and the only solution I can come up with is binding the QML object's value a C++ object's property and provide a NOTIFY signal

C++:
class Runtime : public QObject
{
    Q_OBJECT
    Q_PROPERTY(RuntimeData* data READ GetData NOTIFY modelChanged)
public:

C++:
class Runtime : public QObject
{
    Q_OBJECT
    Q_PROPERTY(RuntimeData* data READ GetData NOTIFY modelChanged)
public:

To copy to clipboard, switch view to plain text mode 

The RuntimeData object is my QAbstractListModel derived object. In this object I've then created a Q_INVOKABLE member that retuns the data value base on an index (for now).

class RuntimeData : public QAbstractListModel
{
    Q_OBJECT
public:
    Q_INVOKABLE int GetData(int index)
    {
        if (index >= 0 && index < m_Data.count())
            return m_Data[index].Value().toInt();
        return 0;
    }

class RuntimeData : public QAbstractListModel
{
    Q_OBJECT
public:
    Q_INVOKABLE int GetData(int index)
    {
        if (index >= 0 && index < m_Data.count())
            return m_Data[index].Value().toInt();
        return 0;
    }

To copy to clipboard, switch view to plain text mode 

Finally, its all working but there is a remaining issue. With this implementaion, anytime I have to update any data, I also have to emit the modelChanged signal to inform the QML to update. Hypothetically if I have 1000 data entries and 1000 QML objects bound to these, if one data item changes all QML objects will update.

Again, I feel like I must be missing something? Why is this so complicated?