QQuickWindow::grabWindow: scene graph already in use

Question

I tried the code shown here: How to take ScreenShot Qt/QML

On execution I am getting the error written in the title.

My main.cpp is:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickWindow>
#include <QImage>
#include <QDebug>
#include "screenshot.h"
#include <QQuickView>
#include <QQmlContext>

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

    const char* drigUi = "DrigUI"; 
    qmlRegisterType <screenCapture> (drigUi, 1, 0, "ScreenShot");

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

    return app.exec();
}

I used this capture function:

void screenCapture::capture(QString const &path) const
{
    QImage img = currentView_->grabWindow();
    img.save(path);
}

and added the following in the constructor:

currentView_ = new QQuickView;

My main.qml :

import QtQuick 2.4
import QtQuick.Window 2.2

import DrigUI 1.0

Window
{
    visible: true
    height: 370
    width: 370

    ScreenShot { id: screenShot }

    Rectangle
    {
        id: ll
        height: 30
        width: 50
        x: 180; y: 0; color: "red"
        MouseArea
        {
            anchors.fill: parent
            onClicked: screenShot.capture ("l.png")
        }
    }
}

What does that error mean? What is the way to solve it? What else info can I provide here?

You can provide your main.qml. Minimized to a SSCCE, that reproduce that problem. — Dmitry Sazonov, Oct 16 '15 at 08:27
@SaZ I delebrately didn't provide . It doesn't contain anything very important. Now, I will it here as you say. — Aquarius_Girl, Oct 16 '15 at 08:27
I'm not sure, but try to put an assert to your `screenCapture::capture` method: `Q_ASSERT( QThread::currentThread() == qApp->thread() );`. There are some comments in documentation: http://doc.qt.io/qt-5/qquickwindow.html#grabWindow — Dmitry Sazonov, Oct 16 '15 at 08:54
@SaZ How will I get qApp object in capture method. That object is in main.cpp. Isn't it? — Aquarius_Girl, Oct 16 '15 at 09:00
`qApp` is on an object. It is a macro for `QApplication::instance` singleton. Try to use search before asking such questions ;) — Dmitry Sazonov, Oct 16 '15 at 09:24
@SaZ Well, I am not very wise. Your code gives me error: "qApp was not decalred in this scope". Why should I search Google on this error? That error does not indicate that it is a macro of anything. — Aquarius_Girl, Oct 16 '15 at 09:28

Aquarius_Girl · Accepted Answer · 2015-10-17T07:41:36.213

I had posted this question on QtCenter.org too, and got the enlightenment from there.

My I ask why you want to create a screenshot of an empty QQuickView? Wouldn't you want to take the screenshot of the Window you create in QML?

I realized that I wasn't using the QQuickView anywhere in main.cpp. so that meant that QQuickView, I was using, was empty.

My other part of program is using QQuickWindow instead of QQuickView, so I replaced QQuickView with QQuickWindow as follows:

ScreenCapture.h

#ifndef SCREENSHOT_H
#define SCREENSHOT_H

#include <QObject>

class QString;
class QQuickWindow;

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

public slots:
    void capture (QString const &path) const;

private:
    QQuickWindow *currentWindow;
};

#endif // SCREENSHOT_H

ScreenCapture.cpp

#include <QPixmap>
#include <QQuickView>
#include <QDebug>
#include "screenshot.h"

screenCapture::screenCapture(QQuickWindow *currentWindow) :
    QObject(0), currentWindow (currentWindow)
{
}

void screenCapture::capture (QString const &path) const
{
    QImage p = currentWindow->grabWindow ();
    bool kk = p.save (path);
    qDebug () << kk;
}

main.cpp

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

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

    QObject      *topLevel = engine.rootObjects().value(0);
    QQuickWindow *window   = qobject_cast<QQuickWindow *>(topLevel);

    screenCapture launcher (window);

    engine.rootContext()->setContextProperty("ScreenShot", &launcher);

    window->show();

    return app.exec();
}

Now, in QML side, we can directly use

ScreenShot.capture ("/home/*****/Desktop/l.png")

anywhere we want. It works.

QQuickWindow::grabWindow: scene graph already in use

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