Can you create QML from a string/byte array?

[VRHans]

I am trying to create run-time dynamically created QML content for use in a QQuickView or QQmlApplicationEngine.

I am doing this because I am writing a host application that users will be able to extend with their own QML extension plugins. I will not know the names of these components so there's no way for me to create a *.qml file that references their components.

I have been able to dynamically discover, load, and register their types currently (and am able to supply a base URI for these plugins when registering) - but I have to be able to instantiate their components and add them to my base 'host' QML (or replace that QML entirely with theirs.)

I am at a loss as how to do this.

I was hoping I could query the plugin for the names it registers and then programmatically generate QML that imports the correct components.

Suggestions? Thanks :)

[Wieland]

Hi! Yes, it's easy: http://doc.qt.io/qt-5/qtqml-javascript-dynamicobjectcreation.html#creating-an-object-from-a-string-of-qml

[VRHans]

Doesn't that require me to have my C++ code call into JavaScript in my host QML and then have the JavaScript call into Qt and on the C++ again - create QML components?

Surely there's a way to access the same Qt functionality that "Qt.createQmlObject" uses from C++ (and thereby avoiding the call into JavaScript and then the call back out into C++)?

[Wieland]

Oh, I see. I thought you wanted to do this from the QtQuick side. You can do this from C++ like this:

QQmlEngine engine;
QQmlComponent component(&engine);
component.setData("import QtQuick 2.4\nText { text: \"Hello world!\" }", QUrl());
QQuickItem *item = qobject_cast<QQuickItem *>(component.create());
// add item to view, etc ...

See also: QQmlEngine, QQmlComponent.

[VRHans]

Too simple... Just kidding! lol. Thanks :)

[VRHans]

@Wieland Sorry to bother you because this is probably a very simple answer but I'm beating my head against the wall over this.

I'm trying to create a very simple test of what you mentioned above, but I cannot get the text to show up in my main window. My sample code is a simple QGuiApplication.

I have a method (shown below) that loads a very simple QML file, creates the component you wrote above, and tries to add it to the Window from the QML file.

void loadUI( QQmlApplicationEngine* in_pEngine )
{
    Q_ASSERT( in_pEngine != NULL );

	// Load the main.qml which contains only a "Window" Qml component
    in_pEngine->load( QUrl(QStringLiteral("qrc:/main.qml")) );

	// Let's create a trivial component and try to add that to the Window
    QQmlComponent l_oComponent( in_pEngine );

    l_oComponent.setData("import QtQuick 2.4\nText { text: \"Hello world!\" }", QUrl());

    QObject* l_pObject = l_oComponent.create();
	Q_ASSERT( l_pObject != NULL );
	
    QQuickItem* l_pItem = qobject_cast<QQuickItem*>( l_pObject );
    Q_ASSERT( l_pItem != NULL );

	// How to add our new component to the Window?
    l_pItem->setParentItem( qobject_cast<QQuickItem*>( in_pEngine->rootObjects().first() ) );
}

The empty window shows up just fine, and the text component creates just fine as far as I can tell, but I can never see it.

How should I be adding new components to an existing QML window?

I did validate that in_pEngine->rootObjects().first() has the object name of the Window I have in my QML file...

Thanks!

[Wieland]

Your code is ok, the problem lies in a dark secret of QtQuick.Window. Have a look at the following snippet:

Window {
    id: myWindow
    width: 800
    height: 600
    visible: true

    Rectangle {
        id: myRect
        color: "orange"
        anchors.fill: parent // This doesn't work with Window !
    }
}

Here myWindow is the parent of myRect but it is not its parentItem (aka visual parent). This is a speciality of Window. For stuff like Item, Rectangle, etc, parent and parentItem are always the same. For Window the parentItem is contentItem. So the following code is correct:

Window {
    id: myWindow
    width: 800
    height: 600
    visible: true

    Rectangle {
        id: myRect
        color: "orange"
        anchors.fill: contentItem // This works !
    }
}

With this in mind it should be easy to understand why the following does what you want:

main.qml

import QtQuick 2.5
import QtQuick.Window 2.2

Window {
    width: 800
    height: 600
    visible: true

    Rectangle {
        id: myContent
        objectName: "myContentObject" // Needed for findChild() in C++
        anchors.fill: contentItem
        color: "orange"
    }
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QQuickItem>
#include <QQmlContext>
#include <QDebug>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    QQmlComponent component(&engine);
    component.setData("import QtQuick 2.4\nRectangle{color:\"black\";width:100;height:100}", QUrl());
    QQuickItem *item = qobject_cast<QQuickItem *>(component.create());

    QObject * rootObject = engine.rootObjects().at(0);
    QObject * targetItem = rootObject->findChild<QObject*>("myContentObject");

    item->setParentItem( qobject_cast<QQuickItem *>(targetItem) );

    return app.exec();
}

Hope it helps!

[VRHans]

Bingo! Thanks! I'll note this carefully so I don't ever run into this again ;).

Cheers.