Well, I didn't find a way to not create a temporary QApplication to get the screen size, so I did the next best thing that I could think of and that is to write the QT_SCALE_FACTOR value out to a file on the Android device, then display a message to the user to "reopen the app", and then exit the app. After the user reopens the app, check to see if the file exists, if it exists then read the scale factor value, call qputenv with the scale factor value read from file. This avoids creating the temporary QApplication once the scale factor has been written to the file and re-read every time the app is opened. Here's the code to do all that:
#ifdef Q_OS_MOBILE
const char *qtScaleFactor = "QT_SCALE_FACTOR";
// Check to see if the QT_SCALE_FACTOR environment variable is set and is not empty
if (!qEnvironmentVariableIsSet(qtScaleFactor) || qEnvironmentVariableIsEmpty(qtScaleFactor))
{
QString scaleFactor;
// Try to read the QT_SCALE_FACTOR from file first
if (!FileIO::readQtScaleFactor(scaleFactor))
{
// Temporary QApplication to get width and height of screen 0
QApplication *temp = new QApplication(argc, argv);
double width = temp->screens().at(0)->availableSize().width();
double height = temp->screens().at(0)->availableSize().height();
double scaleWidth = width / 800.0;
double scaleHeight = height / 480.0;
scaleFactor = QString::number(min(scaleWidth, scaleHeight));
qputenv(qtScaleFactor, scaleFactor.toLocal8Bit());
// Write the QT_SCALE_FACTOR to file
bool writeSuccessful = FileIO::writeQtScaleFactor(scaleFactor);
// If the write was successful, display a message to the user to reopen the "AppName" App
if (writeSuccessful)
{
tempMsgBox("Screen Scale Factor Set\nfor \"AppName\" App.\n\nPlease Reopen \"AppName\" App.", nullptr, 5000, 36);
// Delay and process events to display MessageBox
QElapsedTimer elapsedTimer;
elapsedTimer.start();
do
{
QCoreApplication::processEvents();
} while (elapsedTimer.elapsed() < 5500);
elapsedTimer.invalidate();
delete temp;
return 0; // Exits the App
}
delete temp;
}
else
{
// QT_SCALE_FACTOR was sucessfully read from file, so just set the environment variable
qputenv(qtScaleFactor, scaleFactor.toLocal8Bit());
}
}
#endif
// Primary QApplication for the real application
QApplication app(argc, argv);
This code is only run on a mobile device (e.g. Android and iOS), not Windows or MacOS (Desktop). The deletion of the temporary QApplication was postponed until after the QMessageBox was displayed since it was an error not to have a QApplication before creating a QWidget. And, the QElapsedTimer based do-loop for calling QCoreApplication::processEvents() was necessary to display the QMessageBox, accept the OK button and/or timeout after 5 seconds, and dismiss the QMessageBox automatically.
This was my solution and I'm pretty happy with it. I hope it helps someone else.
