I recently ran into a weird issue where my QPlainTextEdit::selectionChanged handler function terminates prematurely whenever QTextCursor::mergeCharFormat/setCharFormat/setBlockCharFormat is called. Additionally, after terminating it gets called again and runs into the same issue, leading to an infinite loop.

I'm trying to replicate a feature present in many text editors (such as Notepad ) where upon selecting a word, all similar words in the entire document are highlighted. My TextEditor class is overloaded from QPlainTextEdit.

The minimal reproducible example is as follows:

main.cpp:

#include "mainWindow.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    mainWindow w;
    w.show();
    return a.exec();
}

MainWindow.h:

#pragma once
#include <QtWidgets/QMainWindow>
#include "ui_mainWindow.h"
#include "TextEditor.h"

class mainWindow : public QMainWindow
{
    Q_OBJECT

public:
    mainWindow(QWidget *parent = Q_NULLPTR) : QMainWindow(parent)
    {
        ui.setupUi(this);

        auto textEdit = new TextEditor(this);
        textEdit->setPlainText("test lorem ipsum test\n dolor sit test");
        ui.tabWidget->addTab(textEdit, "Editor"); //Or any other way of adding the widget to the window
    }

private:
    Ui::mainWindowClass ui;
};

TextEditor.h:

The regex highlighter part is based on this SO answer.

#pragma once
#include <QPlainTextEdit>

class TextEditor : public QPlainTextEdit
{
    Q_OBJECT

public:
    TextEditor(QWidget* parent) : QPlainTextEdit(parent)
    {
        connect(this, &QPlainTextEdit::selectionChanged, this, &TextEditor::selectChangeHandler);
    }

private:
    void selectChangeHandler()
    {
        //Ignore empty selections
        if (textCursor().selectionStart() >= textCursor().selectionEnd())
            return;

        //We only care about fully selected words (nonalphanumerical characters on either side of selection)
        auto plaintext = toPlainText();
        auto prevChar = plaintext.mid(textCursor().selectionStart() - 1, 1).toStdString()[0];
        auto nextChar = plaintext.mid(textCursor().selectionEnd(), 1).toStdString()[0];
        if (isalnum(prevChar) || isalnum(nextChar))
            return;

        auto qselection = textCursor().selectedText();
        auto selection = qselection.toStdString();

        //We also only care about selections that do not themselves contain nonalphanumerical characters
        if (std::find_if(selection.begin(), selection.end(), [](char c) { return !isalnum(c); }) != selection.end())
            return;

        //Prepare text format
        QTextCharFormat format;
        format.setBackground(Qt::green);

        //Find all words in our document that match the selected word and apply the background format to them
        size_t pos = 0;
        auto reg = QRegExp(qselection);
        auto cur = textCursor();
        auto index = reg.indexIn(plaintext, pos);
        while (index >= 0)
        {
            //Select matched text and apply format
            cur.setPosition(index);
            cur.movePosition(QTextCursor::EndOfWord, QTextCursor::KeepAnchor, 1);
            cur.mergeCharFormat(format); //This causes the selectChangeHandler function to terminate and then execute again, causing an infinite loop leading to a stack overflow

            //Move to next match
            pos = index   (size_t)reg.matchedLength();
            index = reg.indexIn(plaintext, pos);
        }
    }
};

I suspect the format fails to apply for some reason, possibly causing an exception that gets caught inside Qt and terminates the parent function. I tried adding my own try-catch handler around the problematic area, but it did nothing (as expected).

I'm not sure whether this is my fault or a bug inside Qt. Does anybody know what I'm doing wrong or how to work around this issue?

CodePudding user response：

An infinite loop is being generated because it seems that getting the text changes also changes the selection. One possible solution is to block the signals using QSignalBlocker:

void selectChangeHandler()
{
    const QSignalBlocker blocker(this); // <--- this line
    //Ignore empty selections
    if (textCursor().selectionStart() >= textCursor().selectionEnd())
        return;
    // ...