windowHandle returns zero

Question

I need a QWindow to capture screenChanged signal in object A.
I have QWidget B that has Qt::Window flag.
B is a parent of A.
After object of class A was created I am trying to call connect like this:

bool isOk = connect(b->window()->windowHandle(), SIGNAL(screenChanged(...)), a, SLOT(...)));
assert(isOk);

I am getting an error that tells:
QObject::connect: Cannot connect (null)::screenChanged(QScreen*) to CFloatingEdit::onScreenChanged()

Why b->window()->windowHandle() is zero?
Am I trying to make with screenChanged the right way?

Take a look at the implementation of `windowForWidget` function in Ming-Ming Cheng's answer in [this](http://stackoverflow.com/questions/14048565/get-hwnd-on-windows-with-qt5-from-wid) question. — Steeve, Feb 02 '17 at 09:41
The nullptr will be returned if your widget is NOT native. Are you sure that you use appropriate widget? — arturx64, Feb 02 '17 at 09:52
By the way, you can set the Qt::WA_NativeWindow attribute on widgets to make it native. — arturx64, Feb 02 '17 at 09:55

score 2 · Accepted Answer · answered Feb 02 '17 at 11:31

Merged comments to the answer:

Steeve: Take a look at the implementation of windowForWidget function in Ming-Ming Cheng's answer in this question.

arturx64: The nullptr will be returned if your widget is NOT native. Are you sure that you use appropriate widget? By the way, you can set the Qt::WA_NativeWindow attribute on widgets to make it native.

score 0 · Answer 2 · edited Apr 17 '23 at 15:33

The working code should look like below. Clarification is into the comments.

// Connect a slot to handle the screen change eve
auto wid = this->winId(); // important!!! it forces "windowHandle()" to get valid "QWindow*" 

QWindow* window = this->windowHandle();
connectionsuccess = connect(window, SIGNAL(screenChanged(QScreen *)), this, SLOT(myScreenChanged(QScreen *)));
Q_ASSERT(connectionsuccess);

...

// capture when app window moved to other display
void MainWindow::myScreenChanged(QScreen* screen)
{
    qDebug() << screen->name() << screen->geometry()<< screen->name();
}

windowHandle returns zero

2 Answers2