class MyWindow: public QDialog
{
    Q_OBJECT
public:
   QPushButton *Btn;
   void Scan();
....
};
extern void StartScan(pfcallback);
void MyWindow::Scan()
{
 Btn->setEnabled(false);
 StartScan(Scanfinished);
}
void static Scanfinished()
{
  Btn->setEnabled(true);
 }