带行号的QPlainTextEdit

//CodeEditor.h
#ifndef CODEEDITOR_H
#define CODEEDITOR_H
#include <QPlainTextEdit>
#include <QObject>
#include <QSize>
#include <QPaintEvent>
#include <QResizeEvent>
QT_BEGIN_NAMESPACE
class QPaintEvent;
class QResizeEvent;
class QSize;
class QWidget;
QT_END_NAMESPACE
class LineNumberArea;
class CodeEditor : public QPlainTextEdit
{
    Q_OBJECT
public:
    CodeEditor(QWidget *parent);
    void lineNumberAreaPaintEvent(QPaintEvent *event);
    int lineNumberAreaWidth();
protected:
    void resizeEvent(QResizeEvent *event) override;
private slots:
    void updateLineNumberAreaWidth(int newBlockCount);
    void highlightCurrentLine();
    void updateLineNumberArea(const QRect &, int);
private:
    QWidget *lineNumberArea;
};
class LineNumberArea : public QWidget
{
  Q_OBJECT
public:
    LineNumberArea(CodeEditor *editor) : QWidget(editor) {
        codeEditor = editor;
    }
    QSize sizeHint() const override {
        return QSize(codeEditor->lineNumberAreaWidth(), 0);
    }
protected:
    void paintEvent(QPaintEvent *event) override {
        codeEditor->lineNumberAreaPaintEvent(event);
    }
private:
    CodeEditor *codeEditor;
};
#endif

//CodeEditor.cpp
#include <QtWidgets>
#include "codeeditor.h"
//![constructor]
CodeEditor::CodeEditor(QWidget *parent) : QPlainTextEdit(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);
    highlightCurrentLine();
}
//![constructor]
//![extraAreaWidth]
int CodeEditor::lineNumberAreaWidth()
{
    int digits = 1;
    int max = qMax(1, blockCount());
    while (max >= 10) {
        max /= 10;
        ++digits;
    }
    int space = 3 + fontMetrics().width(QLatin1Char('9')) * digits;
    return space;
}
//![extraAreaWidth]
//![slotUpdateExtraAreaWidth]
void CodeEditor::updateLineNumberAreaWidth(int /* newBlockCount */)
{
    setViewportMargins(lineNumberAreaWidth(), 0, 0, 0);
}
//![slotUpdateExtraAreaWidth]
//![slotUpdateRequest]
void CodeEditor::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);
}
//![slotUpdateRequest]
//![resizeEvent]
void CodeEditor::resizeEvent(QResizeEvent *e)
{
    QPlainTextEdit::resizeEvent(e);
    QRect cr = contentsRect();
    lineNumberArea->setGeometry(QRect(cr.left(), cr.top(), lineNumberAreaWidth(), cr.height()));
}
//![resizeEvent]
//![cursorPositionChanged]
void CodeEditor::highlightCurrentLine()
{
    QList<QTextEdit::ExtraSelection> extraSelections;
    if (!isReadOnly()) {
        QTextEdit::ExtraSelection selection;
        QColor lineColor = QColor(Qt::yellow).lighter(160);
        selection.format.setBackground(lineColor);
        selection.format.setProperty(QTextFormat::FullWidthSelection, true);
        selection.cursor = textCursor();
        selection.cursor.clearSelection();
        extraSelections.append(selection);
    }
    setExtraSelections(extraSelections);
}
//![cursorPositionChanged]
//![extraAreaPaintEvent_0]
void CodeEditor::lineNumberAreaPaintEvent(QPaintEvent *event)
{
    QPainter painter(lineNumberArea);
    painter.fillRect(event->rect(), Qt::gray);
//![extraAreaPaintEvent_0]
//![extraAreaPaintEvent_1]
    QTextBlock block = firstVisibleBlock();
    int blockNumber = block.blockNumber();
    int top = (int) blockBoundingGeometry(block).translated(contentOffset()).top();
    int bottom = top + (int) blockBoundingRect(block).height();
//![extraAreaPaintEvent_1]
//![extraAreaPaintEvent_2]
    while (block.isValid() && top <= event->rect().bottom()) {
        if (block.isVisible() && bottom >= event->rect().top()) {
            QString number = QString::number(blockNumber + 1);
            painter.setPen(Qt::white);
            painter.drawText(0, top, lineNumberArea->width(), fontMetrics().height(),
                             Qt::AlignRight, number);
        }
        block = block.next();
        top = bottom;
        bottom = top + (int) blockBoundingRect(block).height();
        ++blockNumber;
    }
}
//![extraAreaPaintEvent_2]