Why the slot function cannot insert text even if it's successfully called

I'm writing a code editor in Qt with QScintilla.

I want to automatically complete the back brackets when I enter the front brackets. So that I connect the cursorPositionChanged(int, int) signal to complete_braces() slot and the connection works. But the insert() statement doesn't work even if the slot function is called.

This is for a Windows 10 PC, running Qt 5.13.0(MinGW 7.3.0 32-bit), Qt Creator 4.9.2, QScintilla 2.11.2.
The connection of signal and slot works because the qDebug() statement output correctly.
When I connect a toolbar button trigger to the slot function and push the button. The back brace is placed correctly.
The insert(QString) function works when I call it in normal functions.

Header:

/* codeeditor.h */
class CodeEditor : public QsciScintilla
{
    Q_OBJECT
...

public slots:
    void complete_brackets();
...
};

Code:

/* codeeditor.cpp */
CodeEditor::CodeEditor()
{
...
    // Slots
    connect(this, SIGNAL(textChanged()),
            this, SLOT(complete_brackets()));
...
}
...
void CodeEditor::complete_brackets()
{
    int line, index;
    getCursorPosition(&line, &index);
    if (text(line)[index] == '(')
    {
        qDebug() << "Get a bracket"; // This statement works.
        insert(QString(")")); // This statement doesn't work.
    }
}
...

I expected the insert(QString) function in the slot function to be called correctly, but it doesn't.

How can I do to make the insert statement effective or, is there any other method to auto complete the brackets?

Solution

It seems that QsciScintilla does not allow adding text in the slot connected to the textChanged signal, a possible solution is to add it a moment later using a QTimer::singleShot():

void CodeEditor::complete_brackets(){
    int line, index;
    getCursorPosition(&line, &index);
    if (text(line)[index] == '(')
        QTimer::singleShot(0, [this, line, index](){
          insert(")");
          setCursorPosition(line, index+2);
        });
}

On the other hand it is recommended that you use the new connection syntax:

connect(this, &QsciScintilla::textChanged, this, &CodeEditor::complete_brackets);