C++ to change QML object properties

p3c0

Hi,

There are multiple problems here,

"Window":http://qt-project.org/doc/qt-5/qml-qtquick-window-window.html doesn't instantiate QQuickItem but QQuickWindow. So you can't cast it to QQuickItem.
There's nothing like QWidth or Qt::560, just plainly use the number e.g 560
Window is a top level, so you can get it directly instead of finding.

To sum up, try this,
@
QQuickWindow *item = (QQuickWindow *)engine.rootObjects().first();
if (item)
item->setProperty("width", 1360);
@

Vignes

Hi,
I tried to change QML Window to fullscreen using visibility via c++
@
QQuickWindow *W = (QQuickWindow *)engine.rootObjects().first();
if (W)
W->setProperty("visibility", "Fullscreen");
@
tried
@
W->FullScreen;
@
tried
@
W->visibility() = "Fullscreen";
@

none of them are working. Help please?

Vignes

Hi,
I got it working, it the spelling of the Fullscreen should be "FullScreen"
@
QQuickWindow *W = (QQuickWindow *)engine.rootObjects().first();
if (W)
W->setProperty("visibility", "FullScreen");
@

p3c0

For FullScreen you can use directly,

@
if (W)
W->showFullScreen();
@

Vignes

Hi,
I have objects window, text and mouse area to text in qml document and I want to implement in c++: when text area is clicked: if the window is not on fullscreen, it should become fullscreen and the text in the text are should change to "Window is Fullscreen" with a animated color chage to "white". Next time it clicked it should come out of fullscreen and chanege the text and color. Also console.log equivalent output of the this event along with screen resolution should be output from c++ (not via qml).

main.qml :
@
import QtQuick 2.2
import QtQuick.Window 2.1

Window {
id: window1
visible: true
width: 360
height: 360
color: "red"
title: "MyWindow"

Text {
    objectName: "myText"
    text: qsTr("Hello World!<br><i>I'm </i><b>not Fullscreen<i> Now</i></b>")
    font.bold: true
    horizontalAlignment: Text.AlignHCenter
    font.family: "Times New Roman"
    font.pointSize: 18
    anchors.centerIn: parent
    color: "steelblue"

    MouseArea {
        anchors.fill: parent
    }
}

}
@

main.cpp
@
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuick/QQuickItem>
#include <QtQuick/QQuickWindow>

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

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

//here I need function for
//1 clicke event
//check fullscreen value
//2 text changing
//3 animation
//4 console.log output of Screen resolution and action
//5 finaly Window : showMaximized(); or showFullScreen();

return app.exec();

}
@

Could you help me with this please?
Thank you

p3c0

You can,

Create a QObject based class
Create a SLOT in it
Pass QQuickWindow pointer (which we obtained as shown above) to it
Create a signal in your QML
Do the connections
In the created class cast the QObject pointer to QQuickWindow using qobject_cast
Use this pointer to access/modify the properties of the QML window in the SLOT when the SIGNAL is fired from QML

Vignes

Hi,
I created QObject: called iobject.h
@
#ifndef IOBJECT_H
#define IOBJECT_H

#include <QObject>

class iObject : public QObject
{
Q_OBJECT
Q_PROPERTY(QString name READ name WRITE setName)
public:
explicit iObject(QObject *parent = 0);

QString name() const;

    Q_INVOKABLE void nameChang();

signals:
void nameChanged(int newValue);

public slots:
void setName(const QString &name);

private:
QString m_name;
};

#endif // IOBJECT_H
@

iobject.cpp
@
#include "iobject.h"

iObject::iObject(QObject *parent) :
QObject(parent)
{
}
QString iObject::name() const
{
return m_name;
}
iObject::setName(const QString &name){
if(name != m_name)
m_name = name;
emit nameChanged(name);
}
@

main.cpp
@
#include "iobject.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>

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

qmlRegisterType<iObject>("myObject", 1, 0, "iObject");

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

return app.exec();

}
@

main.qml
@
import QtQuick 2.2
import QtQuick.Window 2.1
import myObject 1.0

Window {
visible: true
width: 360
height: 360
title: iObject.name

MouseArea {
    anchors.fill: parent
    onClicked: {
        Qt.quit();
    }
}

Text {
    text: qsTr("Hello World!<br><i>I'm </i><b>not Fullscreen<i> Now</i></b>")
    horizontalAlignment: Text.AlignHCenter
    font.family: "Times New Roman"
    font.pointSize: 18
    anchors.centerIn: parent
    color: "steelblue"

    MouseArea {
        anchors.fill: parent
    }
}

}
@

At this point, It did not work. erros on main.cpp 'qmlRegisterType'
what am I doing wrong?

p3c0

Just create a the class and pass the QQuickWindow pointer. No need use qmlRegisterType.
Something like this,
main.cpp
@
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
QQuickWindow *W = (QQuickWindow *)engine.rootObjects().first();
iObject a(W);
QObject::connect(engine.rootObjects().first(),SIGNAL(callCpp()),&a,SLOT(onCallCpp()));
@

No need of custom properties in the class.
Remember to cast back QObject back to QQuickWindow in iObject and then you can modify QQuickWindow properties as shown earlier.

Vignes

I'm getting error "main.obj:-1: error: LNK2019: unresolved external symbol "public: __cdecl iObject::iObject(class QObject *)" (??0iObject@@QEAA@PEAVQObject@@@Z) referenced in function main"

p3c0

Have you included its corresponding header file (probably iobject.h) in main.cpp ?

Vignes

Yes
main.cpp
@
#include "iobject.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuick/QQuickWindow>

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

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

QQuickWindow *W = (QQuickWindow *)engine.rootObjects().first();
iObject a(W);
QObject::connect(engine.rootObjects().first(),SIGNAL(callCpp()),&a,SLOT(onCallCpp()));

return app.exec();

}
@

p3c0

Can you post the new iobject.cpp and .h file ?

Vignes

iobject.h
@
#ifndef IOBJECT_H
#define IOBJECT_H

#include <QObject>

class iObject : public QObject
{
Q_OBJECT
public:
explicit iObject(QObject *parent = 0);

signals:

public slots:

};

#endif // IOBJECT_H
@

iobject.cpp
@
#include "iobject.h"

iObject::iObject(QObject *parent) :
QObject(parent)
{
}
@

p3c0

Everything seems to be fine to me.
Try to clean, run qmake and build the project again.
Apart from that declare the SLOT onCallCpp in iObject class.

Vignes

Still same error, the
@
iObject a(W);
@

Creates following errors:
"main.obj:-1: error: LNK2019: unresolved external symbol "public: _cdecl iObject::iObject(class QObject *)" (??0iObject@@QEAA@PEAVQObject@@@Z) referenced in function main""
And
"debug\HEVC.exe:-1: error: LNK1120: 1 unresolved externals
"
I tried cleaning and rebuild it and created a new project and tried it, same error

All the code in new project follows:
main.qml
@
import QtQuick 2.2
import QtQuick.Window 2.1

Window {
visible: true
width: 360
height: 360

MouseArea {
    anchors.fill: parent
    onClicked: {
        Qt.quit();
    }
}

Text {
    text: qsTr("Hello World")
    anchors.centerIn: parent
}

}
@

main.cpp
@
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuick/QQuickWindow>
#include "iobject.h"

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

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
QQuickWindow *W = (QQuickWindow *)engine.rootObjects().first();
    iObject a(W);
    QObject::connect(engine.rootObjects().first(),SIGNAL(callCpp()),&a,SLOT(onCallCpp()));

return app.exec();

}
@

iobject.h
@
#ifndef IOBJECT_H
#define IOBJECT_H

#include <QObject>

class iObject : public QObject
{
Q_OBJECT
public:
explicit iObject(QObject *parent = 0);

signals:

public slots:
void onCallCpp();

};

#endif // IOBJECT_H
@

iobject.cpp
@
#include "iobject.h"

iObject::iObject(QObject *parent) :
QObject(parent)
{
}
@

I went though several examples and foud ""Animated Tiles":http://qt-project.org/doc/qt-4.8/animation-animatedtiles.html" example. This one uses different library like QtWidgets, QtCore/qstate.h and main.moc. here not QML. This is an old version qt-4.8 as well.

I'm now wondering, that am I using the right QT tool for my project, which uses 4K video playing, editing, analytics tools with the support of OpenGL to be implimented using Cpp.

I chose QML, because of built in OpenGL, and Cpp and JS support. Now Qt Widgets, Qt Quick Application, Qt Quick UI and old version, new version are confusion to me.

It would be great, If you could please help me with these.

Thank you

p3c0

Have the files iobject.cpp and .h included in the project structure from Qt Creator?
I have created a small test app based upon the example.
Here's the "Link":https://docs.google.com/file/d/0B3rkZH6q1E7ocTdnOFQwNVBDM2c/edit?pli=1
Please check if it works.

Also Animated Tiles is a completely different example. It doesnot use Qt Quick.
Please do not go over to Qt 4.8 or less. It's lot different from Qt 5.3

You can refer Qt 5 docs for more information.
Following are good links
"http://qt-project.org/doc/qt-5/qtqml-index.html":http://qt-project.org/doc/qt-5/qtqml-index.html
"http://qt-project.org/doc/qt-5/qtqml-cppintegration-topic.html":http://qt-project.org/doc/qt-5/qtqml-cppintegration-topic.html

Let me know if the Sample Application works.

Vignes

Thanks, your sample works. I got main working after deleting the build folder. Some reason cleaning didn't work, even when I copy and pasted your code.

I was thinking like this, creating object and connecting it to QML types:
iObject A
A.screenSize (connect to Window : visibility)
A.textApped (connect to Text : text)
A.animantionStart (connect to Animation : start)

Then changing it in cpp and then emitting signal or even creating, changing & destroying QML types in cpp. Like JavaScript in HTML application.

If you can provide this type of sample works on would be very grateful.

*Note: I learn using simple working example rather than reading text with pieces of code, because of my DYSLEXIA.

p3c0

You can also use "QMetaObject::invokeMethod":http://qt-project.org/doc/qt-5/qmetaobject.html#invokeMethod to call a javascript function inside a QML and in that function you can do the operations.
For eg.
Test.qml
@
Window {
visible: true
width: 400
height: 400

function changeColor() {
    rect.color="blue"
}

Rectangle {
    id: rect
    anchors.fill: parent
    color: "red"
}

}
@

and to call it from main.cpp

@
QMetaObject::invokeMethod(engine.rootObjects().first(), "changeColor");
@

You can also keep these calls in separate cpp methods of the iObject class.

Vignes

Say I have following main.qml
@
Window {
visible: true
width: 400
height: 400

Rectangle {
id: rect
anchors.fill: parent
color: "red"

    Text {
    id: mytext
    text: qsTr("<b>Hello World!</b>")
    color: "blue"
    }
    MouseArea {
    anchors.fill: parent
    onClicked: {
        Qt.quit();}
    }
}

}
@

How do I use this "const char * QMetaObject::className() const" to get Text object name.

How do I use "QMetaProperty::write(QObject * object, const QVariant & value) const" to change text value

Thank you

p3c0

bq. How do I use this “const char * QMetaObject::className() const” to get Text object name.

As shown earlier you can find the exact Item using findChild and then use "metaObject()":http://qt-project.org/doc/qt-5/qobject.html#metaObject to get it's corresponding metaObject and then use it to get "className()":http://qt-project.org/doc/qt-5/qmetaobject.html#className;

bq. How do I use “QMetaProperty::write(QObject * object, const QVariant & value) const” to change text value

This kind of bit tedious, you have to first findChild then get it's metaObject and then iterate over it to find the exact index of the "property":http://qt-project.org/doc/qt-5/qmetaobject.html#property which you want to set and then change the property for that particular index using "write":http://qt-project.org/doc/qt-5/qmetaproperty.html#write

Alternatively you use more user friendly "setProperty":http://qt-project.org/doc/qt-5/qobject.html#setProperty to directly set it instead of first iterating,finding and then changing.