Why does my window not show? - c++

Question

I need to write a GrabWindow, so I derived my class GrabWindow from QQuickWindow:
#include <QtQuickWidgets/QtQuickWidgets>
#include <QString>
class GrabWindow : public QQuickWindow {
Q_OBJECT
public:
explicit GrabWindow(QQuickWindow *parent = nullptr);
public slots:
void capture(QString const &path);
};
// .CPP
#include "grab_window.h"
#include <QImage>
GrabWindow::GrabWindow(QQuickWindow *parent) : QQuickWindow(parent) {
}
void GrabWindow::capture(const QString &path) {
QImage img = this->grabWindow();
img.save(path);
}
After I registered it in QML: qmlRegisterType<GrabWindow>("myapp", 1, 0, "GrabWindow");
And after I defined my window in QML:
import QtQuick 2.4
import QtQuick.Controls 2.2
import QtQuick.Window 2.3
import myapp 1.0
GrabWindow {
id : translationWindow
width : 1024
height : 768
color: "transparent"
visibility: "FullScreen"
visible: true;
signal capture(string path)
MouseArea {
anchors.fill: parent
onClicked: translationWindow.capture("/home/user/saveTest.jpg")
}
}
But it doesn't show on start (I know it's transparent, I mean the grab window doesn't start to show). If instead of GrabWindow I use Window or ApplicationWindow then all works perfectly I am seeing a transparent full-screen window.
What's wrong?

Answer 1

Your GrabWindow is not shown because when you are setting the visible property it's not same as when you use Window's visible property.
Yours is just the visible property of QWindow.
Window does not directly instantiate QQuickWindow, it instantiates a private Qt class QQuickWindowImpl which overrides the visible property with a custom one.
It seems to delay the actual call of the QWindow::setVisible at a later time.
As such, I don't think QQuickWindow is meant to be inherited from. You could try doing visible = true in your Component.onCompleted but I'm not sure it will resolve your problem.
What I would advise you instead is not subclassing QQuickWindow but just creating a new type and pass it the existing Window.
Possible API could be :
Window {
id: myWindow
//...
MouseArea {
anchors.fill: parent
onClicked: WindowGrabber.grab(myWindow, path) //singleton type
}
}
or
Window {
id: myWindow
//...
WindowGrabber { // regular type
id: windowGrabber
window: myWindow
}
MouseArea {
anchors.fill: parent
onClicked: windowGrabber.grab(path) // you could even add a path property in WindowGrabber and not have it as a function parameter if that makes sense for your use case
}
}