I did so:
Controller.prototype.ComponentSelectionPageCallback = function()
{
var targetDir = gui.pageById(QInstaller.TargetDirectory).TargetDirectoryLineEdit.text;
var files = QDesktopServices.findFiles(targetDir, "*.*");
if (files.length!=0) {
installer.setValue("TargetDir", targetDir + "\\" + installer.value("ProductName"));
}
}
