Inserting text into QTextEdit freezes UI

tzorake

I'm working on my project. I want to make something like tree-sitter playground.
I'm parsing code from one QTextEdit and tokens insert parsing output into another QTextEdit. When typing an the first QTextEdit UI is freezing because of text insertion (imo, mb it's something different clear operation maybe the reason also). How to fix it and make UI responsive due typing or text insertion action?
video demo (YouTube)

SGaist

Hi,

What processing are you applying ?
How big is the content your generate and add to the QTextEdit ?
How many times per second does it happen ?

tzorake

@SGaist Hi. I have text 17,508 bytes (524 lines in editor). I use tree sitter parser for the text then I'm trying to set the text in the right QTextEdit. It happens when text in the first QTextEdit changes.

tzorake

@tzorake

Also there is some implementation details:

class Playground : public QTextEdit
{
    Q_OBJECT
public:
    explicit Playground(QWidget *parent = nullptr) : QTextEdit(parent)
    {
        setTextInteractionFlags(Qt::NoTextInteraction);
   
        m_worker = new Worker();
    
        connect(this, &Playground::updateNodes, m_worker, &Worker::process);
        connect(m_worker, &Worker::started, this, &QTextEdit::clear);
        connect(m_worker, &Worker::success, this, &Playground::handle);
    }

    void setSource(QTextEdit *textEdit);

signals:
    void updateNodes(QString text);

private slots:
    void onTextChanged()
    {
        emit updateNodes(m_source->toPlainText());
    }

    void handle(QString text);

private:
    Worker *m_worker;
    QTextEdit *m_source = nullptr;
};

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker(QObject *parent = nullptr)
    {
        m_thread.reset(new QThread);
        moveToThread(m_thread.get());
        m_thread->start();
    }
    ~Worker();
    ...

public slots:
    void process(QString text);

private slots:
    void cleanup();

signals:
    void started();
    void success(QString result);
    void finished();

private:
    ...
    std::unique_ptr<QThread> m_thread;
};

Playground is the widget on the right side of the app. Worker is located within Playground and has its own QThread. They are communicate through signals and slots.

SGaist

I would venture that you are connecting your "source" textChanged signal to onTextChanged slot which will trigger your worker processing on each and every change. This can be quite a lot. Am I correct ?

tzorake

@SGaist Yeah, you right that connect textChanged signal to onTextChanged slot is a bit of oopsy doopsy. So I add debounce for key presses.

Add timer and reimplement onTextChanged slot.

class Playground : public QTextEdit
{
    Q_OBJECT
public:
    explicit Playground(QWidget *parent = nullptr) : QTextEdit(parent)
    {
        setTextInteractionFlags(Qt::NoTextInteraction);
   
        m_timer = new QTimer(this);
        m_timer->setSingleShot(true);
        m_worker = new Worker();
    
        connect(m_timer, &QTimer::timeout, this, &Playground::process);
        connect(this, &Playground::updateNodes, m_worker, &Worker::process);
        connect(m_worker, &Worker::started, this, &QTextEdit::clear);
        connect(m_worker, &Worker::success, this, &Playground::handle);  
    }

    void setSource(QTextEdit *textEdit);

signals:
    void updateNodes(QString text);

private slots:
    void onTextChanged()
    {
        if (m_timer->isActive()) {
            m_timer->stop();
        }
        
        m_timer->start(500);  
    }

    void handle(QString text);

private:
    QTimer *m_timer;
    Worker *m_worker;
    QTextEdit *m_source = nullptr;
};

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker(QObject *parent = nullptr)
    {
        m_thread.reset(new QThread);
        moveToThread(m_thread.get());
        m_thread->start();
    }
    ~Worker();
    ...

public slots:
    void process(QString text);

private slots:
    void cleanup();

signals:
    void started();
    void success(QString result);
    void finished();

private:
    ...
    std::unique_ptr<QThread> m_thread;
};

But It does not solve the problem that the UI is freezed by setPlainText or clear (or something like that I don't really know what is the problem in here).
Demo

Also there are input and output from my program:
input.txt
output.txt

JonB

@tzorake
You should be able to test both these quickly, with your existing data, in a standalone program outside your current code:

Does it behave badly if you use a QPlainTextEdit instead of a QTextEdit?
Does it behave badly if you remove any code you have for threads and just test in single-threaded with a QPlainTextEdit?

tzorake

@JonB Okey. I tested each of the cases.
The results:

Single threaded application behaves the same way as before (nothing changed).
QPlainTextEdit works fine. It does not freeze while data is setting in it. Demo

It works exactly how I would like QTextEdit to work. Thanks a lot, everyone! I really appreciate your answer @JonB.

Just in case, do you know why QTextEdit and QPlainTextEdit behave itself differently in this case?

JonB

@tzorake said in Inserting text into QTextEdit freezes UI:

Just in case, do you know why QTextEdit and QPlainTextEdit behave itself differently in this case?

Well, QTextEdit accepts Qt's rich text, a subset of HTML. It has potentially a lot of work to do to parse/format what you hand it. Depends what your input was. And I don't know which method of QTextEdit you are using to set its content?

QTextEdit has a setPlainText() method. Try that? It's not the same as QPlainTextEdit though, that is its own dedicated plain text widget. Use that if you know you will only have plain text. If your text has HTML this won't do you, but it looks like whatever you have is taking some time in QTextEdit.

tzorake

@JonB Yes, I used setPlainText to set up the text in QTextEdit. Thanks for the explanation!

JonB

@tzorake said in Inserting text into QTextEdit freezes UI:

@JonB Yes, I used setPlainText to set up the text in QTextEdit. Thanks for the explanation!

Then I don't know why you see QPlainTextEdit much better performant than QTextEdit::setPlainText(). Maybe QTextEdit is not optimized for plain text, it just accepts it as a convenience. But really it is supporting rich text and more complex formatting.

smithanna20901

Then I'm not sure why you think QTextEdit::setPlainText() is significantly less performant than QPlainTextEdit. It's possible that QTextEdit only supports plain text as a convenience and isn't optimized for it. In actuality, though, it is supporting more intricate formatting and rich text.

CPPUIX