QTextStream read issue

[Cobra91151]

Hi! I want to read some text from file and display in QPlainTextEdit. File is about 2 MB. I have created QObject class and connect signals and slots with thread. The problem is it still freezes window even when the thread is running.

I have decided to use QtConcurrent.

My code:

QFile dbFile;
QTextStream dbTextStream;
dbFile.setFileName(":Test/Test.txt");
dbFile.open(QIODevice::ReadOnly);
dbTextStream.setDevice(&dbFile);
dbTextStream.setCodec("windows-1251");
QFuture<QString> myData = QtConcurrent::run(this, &TestWindow::loadTestData);
ui->plainTextEdit->appendPlainText(myData.result());

QString TestWindow::loadTestData()
{
  QString data = dbTextStream.readAll();
  return data;
}

No errors, no warnings but it blocks window (GUI) for seconds.

So I have to use QFutureWatcher:

QFutureWatcher<QString> watcher;
 connect(&watcher, QFutureWatcher::finished, this, &TestWindow::handleFinished);

QFuture<QString> myData = QtConcurrent::run(this, &TestWindow::loadDatabase);
watcher.setFuture(myData);

void DatabaseManager::handleFinished()
{
    // Append text data to QPlainTextEdit
}

When I compile it throws errors: C2955: 'QFutureWatcher': use of class template requires template argument list 'QFutureWatcherBase::finished': non-standard syntax; use '&' to create a pointer to member

So how to fix it?

[SGaist]

Hi,

I'd avoid emitting 2MB of text at once.

Note that your while loop will only run once since you call readAll in it.

[Cobra91151]

@SGaist

I have used readLine but it displays only one line?

[SGaist]

Because you call emit testData outside of the loop thus only sending the last line read.

[Cobra91151]

@SGaist

I changed code to:

while (!dbTextStream.atEnd()) {
        data = dbTextStream.readLine();
        emit databaseData(data);
    }

But window is still not responding and after a while displays only one line?

[SGaist]

Are you calling setText rather than append ?

Be aware that with such a loop you are basically spamming the event loop that will try to re-render the text widget content for each new line you read. You should rather use batches and give the event loop some time to work.

[Cobra91151]

@SGaist

Yes. The problem was that I use setText rather than append and now it sets all text, but still window not responding. What do you mean about using batches?

[SGaist]

I don't know how many lines your text file contains but basically you are likely requesting tens of thousands of line to be added in a fraction of a second.

Doing batches means doing things by chunks i.e. load a certain amount of data, send it further and start again until everything has been read..

[Cobra91151]

@SGaist

Yes. The file contains a lot of lines.
But why QThread can't handle it? I have the same program written in C# and it opens and displays one file without dividing it by chunks. Are there other classes in Qt to read huge text files?

[Cobra91151]

The main post was updated to display current issue.

[SGaist]

As the error mention, you forgot to put the & and the template type. i.e. &QFutureWatcher<int>::finished