QFutureWatcher

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QFutureWatcher

QFutureWatcher is useful in keeping track of when an asynchronous task complete. Consider a scenario where a dialog pops up while some operation runs in the background. The first step is to show the dialog:

  msgDialog = new MessageDialog();
  msgDialog->show();
  QApplication::processEvents(); //needed to properly show the dialog

Having a QFuture m_OperationFuture as a field, a naive approach to keeping the GUI responsive while the background operation takes place is to keep calling QApplication::processEvents() while the operation is still executing:

m_OperationFuture = QtConcurrent::run(runBackgroundOperation);
while(m_OperationFuture.isRunning())
{
       QApplication::processEvents();
}
performNextOperation();

However, QFutureWatcher provides a better alternative:

QFutureWatcher<QString> *watcher = new QFutureWatcher<QString>(this);
QObject::connect(watcher,SIGNAL(finished()),this,SLOT(operationFinished()));
m_OperationFuture = QtConcurrent::run(runBackgroundOperation);
watcher->setFuture(m_OperationFuture);

In the operationFinished() function, the result can be retrieved and the dialog can be closed:

QString err = m_OperationFuture.result();
closeDialog();

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