Switching to `QWidget::activateWindow()` from the deprecated `QApplication::setActiveWindow` yields null widget for `QApplication::activeWindow`

[Sankhesh Jhaveri]

When switching away from the deprecated API: QApplication::setActiveWindow to the recommended QWidget::activateWindow, the widget/window never gets set as the active window. QApplication::activeWindow returns a nullptr.

Is there an alternate API for fetching the active widget as set by QWidget::activateWindow?

Here's a small representative example:

#include <QApplication>
#include <QDebug>
#include <QMainWindow>

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

  Q_ASSERT(QApplication::activeWindow() == nullptr);
  QMainWindow w;
  w.show();
  w.raise();
  w.activateWindow();
  // QApplication::setActiveWindow(&w); Deprecated API
  qDebug() << QApplication::activeWindow() << w.isActiveWindow();
  Q_ASSERT(QApplication::activeWindow() != nullptr);
  if (!QApplication::activeWindow())
  {
    // in case if this is not a debug build
    qCritical() << "Couldn't activate window.";
  }

  return app.exec();
}

The output is

$ ./QtActiveWindow
QWidget(0x0) false
ASSERT: "QApplication::activeWindow() != nullptr" in file /home/sankhesh/Projects/TestProjects/QtActiveWindow/QtActiveWindow/QtActiveWindow.cxx, line 16
[1]    256514 IOT instruction (core dumped)  ./QtActiveWindow

[SGaist]

Hi and welcome to devnet,

You are calling that function way too early.

Check it's documentation again. The key word is: visible. Calling show on a widget does not make it instantly visible. It schedules its appearance at the earliest possible time when the event loop is running.

[Sankhesh Jhaveri]

Thank you for the quick response, @SGaist. Your answer helped me understand the issue.

It appears that QApplication::setActiveWindow updates the internal cached active_window pointer to the specified widget here: https://codebrowser.dev/qt6/qtbase/src/widgets/kernel/qapplication.cpp.html#1848. This enables subsequent code to call QApplication::activeWindow immediately afterward, successfully.

However, QWidget::activateWindow schedules this action on the platform window (https://codebrowser.dev/qt6/qtbase/src/gui/kernel/qwindow.cpp.html#1292), which means that the activeWindow will not be available until the underlying windowing event system has processed that request.

So, in my case, I could do something like:

  w.show();
  QTimer::singleShot(100, [&w](){w.activateWindow(); qDebug() << QApplication::activeWindow();});

which gives the native windowing system a chance to process the event. And it works indeed.

[JonB]

@Sankhesh-Jhaveri
I have not tested but you may find the activation first takes place (rather than waiting on a timer), and activeWindow() becomes valid, if you subclass your widget and override the showEvent()

[Sankhesh Jhaveri]

Thanks @JonB. Is there a signal I can connect to, for when the window is actually activated?

FWIW, this issue is for the ParaView client where the window is already visible but not the active one. The showEvent override wouldn't help there.

[JonB]

@Sankhesh-Jhaveri
Heave a read through https://stackoverflow.com/questions/16721557/how-to-detect-if-a-window-has-been-activated from 11 years ago :) Does anything there like QEvent::WindowActivate help your situation?

[Sankhesh Jhaveri]

The QWindow::activeChanged signal allows me to call the delegate function when the window is actually activated.