Qt: editor with line numbers

linenumberarea.h

#ifndef LINENUMBERAREA_H
#define LINENUMBERAREA_H

#include <QWidget>
#include "numberededit.h"

class LineNumberArea : public QWidget
{
public:
    LineNumberArea(NumberedEdit *editor) : QWidget(editor) {
        numberedEdit = editor;
        setParent(editor);
    }

    QSize sizeHint() const override {
        return QSize(numberedEdit->lineNumberAreaWidth(), 0);
    }

protected:
    void paintEvent(QPaintEvent *event) override {
        numberedEdit->lineNumberAreaPaintEvent(event);
    }

private:
    NumberedEdit *numberedEdit;
};

#endif // LINENUMBERAREA_H

lineNumbers.pro

QT += widgets

HEADERS         = \
                  mainwindow.h \
                  linenumberarea.h \
    numberededit.h
SOURCES         = \
                  mainwindow.cpp \
                  main.cpp \
    numberededit.cpp

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow window;
    window.resize(640, 512);
    window.show();
    return app.exec();
}

mainwindow.cpp

#include <QtWidgets>
#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    setupFileMenu();
    editor = new NumberedEdit;
    setCentralWidget(editor);
    setWindowTitle(tr("line numbers"));
}

void MainWindow::openFile(const QString &path)
{
    QString fileName = path;

    if (fileName.isNull())
        fileName = QFileDialog::getOpenFileName(this, tr("Open File"), "", "All Files (*)");

    if (!fileName.isEmpty()) {
        QFile file(fileName);
        if (file.open(QFile::ReadOnly | QFile::Text))
        {
            QString text = file.readAll();
            editor->setPlainText(text);
        }
    }
}

void MainWindow::setupFileMenu()
{
    QMenu *fileMenu = new QMenu(tr("&File"), this);
    menuBar()->addMenu(fileMenu);
    fileMenu->addAction(tr("&Open..."), this, SLOT(openFile()), QKeySequence::Open);
    fileMenu->addAction(tr("E&xit"), qApp, SLOT(quit()), QKeySequence::Quit);
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "numberededit.h"

QT_BEGIN_NAMESPACE
class QTextEdit;
QT_END_NAMESPACE

//! [0]
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);

public slots:    
    void openFile(const QString &path = QString());

private:
    void setupFileMenu();
    NumberedEdit *editor;
};
//! [0]

#endif // MAINWINDOW_H

numberededit.cpp

#include "numberededit.h"
#include "linenumberarea.h"

NumberedEdit::NumberedEdit(QWidget *parent) : QTextEdit(parent)
{
    lineNumberArea = new LineNumberArea(this);
    connect(this, SIGNAL(blockCountChanged(int)), this, SLOT(updateLineNumberAreaWidth(int)));
    connect(this, SIGNAL(updateRequest(QRect,int)), this, SLOT(updateLineNumberArea(QRect,int)));
    connect(this, SIGNAL(cursorPositionChanged()), this, SLOT(highlightCurrentLine()));

    updateLineNumberAreaWidth(0);    
}

NumberedEdit::~NumberedEdit()
{
    delete lineNumberArea;
}

int NumberedEdit::lineNumberAreaWidth()
{
    int digits = 1;
    int max = qMax(1, document()->blockCount());
    while (max >= 10) {
        max /= 10;
        ++digits;
    }

    int space = 3 + fontMetrics().width(QLatin1Char('9')) * digits;

    return space;
}

void NumberedEdit::updateLineNumberAreaWidth(int /* newBlockCount */)
{
    setViewportMargins(lineNumberAreaWidth(), 0, 0, 0);
}

void NumberedEdit::updateLineNumberArea(const QRect &rect, int dy)
{
    if (dy)
        lineNumberArea->scroll(0, dy);
    else
        lineNumberArea->update(0, rect.y(), lineNumberArea->width(), rect.height());

    if (rect.contains(viewport()->rect()))
        updateLineNumberAreaWidth(0);
}

void NumberedEdit::resizeEvent(QResizeEvent *e)
{
    QTextEdit::resizeEvent(e);

    QRect cr = contentsRect();
    lineNumberArea->setGeometry(QRect(cr.left(), cr.top(), lineNumberAreaWidth(), cr.height()));
}

FirstLineInfo NumberedEdit::getFirstLineInfo()
{
    FirstLineInfo result;
    QFontMetrics fm(font());
    result.height = fm.ascent() + fm.descent() + fm.leading();
    result.firstVisible = verticalScrollBar()->value()/result.height;
    result.top = result.firstVisible*result.height - verticalScrollBar()->value();
    return result;
}

void NumberedEdit::lineNumberAreaPaintEvent(QPaintEvent *event)
{
    QPainter painter(lineNumberArea);
    painter.fillRect(event->rect(), Qt::lightGray);

    FirstLineInfo fli = getFirstLineInfo();
    QTextBlock block = document()->findBlockByLineNumber(fli.firstVisible);
    int blockNumber = fli.firstVisible;
    int top = fli.top;
    int bottom = top + fli.height;

    while (block.isValid() && top <= event->rect().bottom()) {
        if (block.isVisible() && bottom >= event->rect().top()) {
            QString number = QString::number(blockNumber + 1);
            painter.setPen(Qt::black);
            painter.drawText(0, top+4, lineNumberArea->width(), fontMetrics().height(),
                             Qt::AlignRight, number);
        }

        block = block.next();
        top = bottom;
        bottom = top + fli.height;
        blockNumber++;
    }
}

numberededit.h

#ifndef NUMBEREDEDIT_H
#define NUMBEREDEDIT_H

#include <QtWidgets>

class LineNumberArea;

struct FirstLineInfo
{
    int height;
    int firstVisible;
    int top;
};

class NumberedEdit : public QTextEdit
{
    Q_OBJECT    
public:
    explicit NumberedEdit(QWidget *parent = 0);
    ~NumberedEdit();
    void lineNumberAreaPaintEvent(QPaintEvent *event);
    int lineNumberAreaWidth();
protected:
    void resizeEvent(QResizeEvent *event) override;
private slots:
    void updateLineNumberAreaWidth(int newBlockCount);
    void updateLineNumberArea(const QRect &, int);
private:
    QWidget *lineNumberArea;
    FirstLineInfo getFirstLineInfo();
};

#endif // NUMBEREDEDIT_H