Creating a C++ defined object in QML

mzimmers

Hi all -

This is similar to, but different from, an earlier thread.

I have a QML file that accepts an argument. The argument is a C++ defined object, and may be null, depending on the use case. If it's not null, I want to make a local copy (which seems to be working). If it is null, I want to create a new instance of this object (which isn't working).

Here's the class:

class Schedule : public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    Q_INVOKABLE explicit Schedule(QObject *parent = nullptr);
    Q_INVOKABLE static Schedule *cloneSchedule(Schedule *source);

And the QML file:

ColumnLayout {
    property Schedule schedule // sometimes null depending on caller
    property Schedule scheduleCopy
    property Schedule newSchedule: Schedule

    Component.onCompleted: {
        if (schedule !== null) {
            console.log("ScheduleCreateEdit.qml: creating clone of Schedule in list.")
            scheduleCopy = schedule.cloneSchedule(schedule)
        } else {
            console.log("ScheduleCreateEdit.qml: creating new Schedule.")
            // nothing I've tried here works; it's always null.
            scheduleCopy = new Schedule//newSchedule//schedule.cloneSchedule(newSchedule)
        }
    }

How do I correctly code the creation of a new instance of this class?

Thanks...

Bob64

@mzimmers I think it's the parent as the first argument and then an optional JS object argument of any properties you want to initialise it with.

Something like:

    scheduleComponent.createObject(scheduleCreate, {"prop1": prop1Val, "prop2": prop2Val, ...})

I don't know if that's the correct parent for your case and of course you might not want to set any initial properties, in which case leave out the second argument. (Edit: looking more closely at your example, you don't seem to set any properties, but I guess you might have simplified it compared with the real thing.)

GrecKo

@mzimmers said in Creating a C++ defined object in QML:

If it is null, I want to create a new instance of this object (which isn't working).

What will be the use of this new instance?

mzimmers

@GrecKo said in Creating a C++ defined object in QML:

What will be the use of this new instance?

To serve as a template for the user to populate, and then send a command to add the new instance to the model list. It's perfectly fine if this is a temporary copy; it only exists for the user to edit.

EDIT:

It seems like all I need to do is invoke the class c'tor, but I can't figure out the correct syntax to do this in QML.

Bob64

@mzimmers in the simplest case you would construct a C++ object exposed to QML simply by using it as a QML object:

ColumnLayout {
    MyCppObject { ...}
}

You want to create it dynamically though so can you do something like wrapping it in a Component and then using createObject to instantiate it?

Component {
    id: myCppObjectComponent
    MyCppObject {}
}

...

    var myCppObj =  myCppObjectComponent.createObject(...)

mzimmers

@Bob64 I tried this, and it seems to work:

ColumnLayout {
    id: scheduleCreate
    property Schedule schedule
    property Schedule scheduleCopy
    Schedule {
        id: newSchedule
    }

    Component.onCompleted: {
        if (schedule !== null) {
            scheduleCopy = schedule.cloneSchedule(schedule)
        } else {
            scheduleCopy = newSchedule.cloneSchedule(newSchedule)
        }

If I understand your suggestion, it would be more like this:

Component {
    id: scheduleComponent
    Schedule {
        id: newSchedule
    }
}
property Schedule scheduleCopy

Component.onCompleted: {
    if (schedule !== null) {
        scheduleCopy = schedule.cloneSchedule(schedule)
    } else {
        scheduleCopy = scheduleComponent.createObject(*** what goes in here? ***)

Right? So, what are the arguments to createObject() in this case?

Thanks...

Bob64

@mzimmers I think it's the parent as the first argument and then an optional JS object argument of any properties you want to initialise it with.

Something like:

    scheduleComponent.createObject(scheduleCreate, {"prop1": prop1Val, "prop2": prop2Val, ...})

I don't know if that's the correct parent for your case and of course you might not want to set any initial properties, in which case leave out the second argument. (Edit: looking more closely at your example, you don't seem to set any properties, but I guess you might have simplified it compared with the real thing.)

mzimmers

@Bob64 yep, that works. I chose not to parent the object -- I believe this will cause it to be destroyed when this file goes out of scope.

property Schedule schedule
property Schedule scheduleCopy
Component {
    id: scheduleComponent
    Schedule {
        id: newSchedule
    }
}

Component.onCompleted: {
    if (schedule !== null) {
        scheduleCopy = schedule.cloneSchedule(schedule)
    } else {
        scheduleCopy = scheduleComponent.createObject()
    }
}

Thanks for the help.