Prevent event loop blocking in modal dialog

Question

I'm developing an application where a user may parse some binary files. Once he clicks the "parse"-button, he first may select some files, which are parsed afterwards. While the application is processing the files, I'd like to display a modal dialog, which informs the user about the progress (QProgressBar bar) and the already parsed files (QListView list / listModel).

My current approach is to override the exec()-method of a QDialog-sublcass. This way I could just call

parseAssistant.exec()

The current implementation looks like this:

class ParseAssistant : public QDialog { public: int exec(); };

int ParseAssistant::exec()
{
    bar->setMaximum(files.size());

    this->show();
    this->setModal(true);

    for (int i = 0; i < files.size(); i++) {
        PluginTable* table = parser.parse(files[i]);

        // do something with the table
        // saveTableintoDB();

        // update GUI
        // bar->setValue(i);
        // listModel->insertRow(0, new QStandardItem(files[i]));
    }
    this->hide();

    return QDialog::Accepted;
}

After this (blocking) method has run, the user either has parsed all files or canceled the progress somewhere. To achieve this I attempted to use QApplication::processEvents in the while-loop (which feels kinda laggy as it's only progressed when a file has finished parsing) or to outsource the heavy calculation(s) to some QConcurrent implementation (::run, ::mapped). Unfortunately, I don't know how to return the program flow back to the exec() method once the QFuture has finished without relying on some CPU-intense loop like:

while (!future.isFinished()) { QApplication::processEvents(); }

Is there a smarter approach to having a modal dialog, which runs a heavy calculation (which may be canceled by the user) without blocking the eventloop?

p-a-o-l-o · Answer 1 · 2018-03-15T15:51:00.840

I wouldn't subclass Qdialog, in the first place, but just use a QFutureWatcher and connect the watcher finished signal to the dialog close slot, this way:

QDialog d;

QFutureWatcher<void> watcher;
QObject::connect(&watcher, &QFutureWatcher<void>::finished, &d, &QDialog::close);

QFuture<void> future = QtConcurrent::run(your_parse_function);
watcher.setFuture(future);

d.exec();

//control returns here when your_parse_function exits

The parse function could be a method in a QObject derived class, like this:

class Parser : public QObject
{
  Q_OBJECT
public:
    void parse()
    {
        for (int i = 0; i < files.size(); i++) {

            PluginTable* table = parser.parse(files[i]);

            emit fileParsed(i, files.size);

            // ...
        }
    }    
signals:
    void fileParsed(int id, int count);
};

You can connect the fileParsed signal to a slot of choice, and from there set the progress bar value accordingly.

How do I pass the function Parser::parse to QtConcurrent::run, while still having the files and the parser (for parser.parse)? — Jonas Möller, Mar 15 '18 at 16:04
Having a `Parser parser` object: `QtConcurrent::run(parser, &Parser::parse);` — p-a-o-l-o, Mar 15 '18 at 16:07

score 0 · Answer 2 · answered Mar 15 '18 at 15:32

My personal approach would be:

create a separate thread and do the processing there (QThread; std::thread should do the trick as well)
provide a signal that informs about the file currently being processed
possibly another signal informing about progress in %
another signal informs that processing is done, emitted just before the thread ends
provide your dialog with appropriate slots and connect them to the signals (as different threads involved, make sure connection type is Qt::QueuedConnection)

Prevent event loop blocking in modal dialog

2 Answers2