Draw a line in column 80 of QPlainTextEdit

Question

I'm writing a text editor and using Qt for the GUI. I'm a noob in Qt and I'm having trouble to do this.

I need to draw a line in the column 80 of the QPlainTextEdit but I really don't know how. I'm using QPainter but I just can't get it right, any help?

Answer 1

Here's how I'd do it. It's admittedly not entirely trivial. The inputs to determining the 80th column position are:

1. 80 x the average character width in floating point. Using the integer value will magnify the roundoff error by a factor of 80. Thus use QFontMetricsF.

2. The offset due to scrollbars comes from contentOffset(). It'd be bad to use horizontalScrollbar()->value(). The latter currently works, but relies on the implementation-specific detail. QPlainTextEdit happens to map scrollbar values to pixels -- who knows if it won't change tomorrow. It's not documented, thus falls under unspecified behavior.

3. The QTextDocument implements its own margin, available via documentMargin().

Another pitfall: you must paint on the viewport() in any class that derives from QAbstractScrollArea -- and QPlainTextEdit does so. If you don't, your paintEvent becomes a no-op. It's documented, but you must be clever enough to actually look into documentation. I'd consider it a bad corner case of an API that does something unexpected. In every other paintEvent, you simply create QPainter p or QPainter p(this) and it works.

Note: this is tested, compileable code.

//main.cpp
#include <cmath>
#include <QtWidgets>

class Edit : public QPlainTextEdit
{
public:
    Edit(QWidget * parent = 0) : QPlainTextEdit(parent) {}
protected:
    void paintEvent(QPaintEvent * ev)
    {
        QPlainTextEdit::paintEvent(ev);
        const QRect rect = ev->rect();
        const QFont font = currentCharFormat().font();
        int x80 = round(QFontMetricsF(font).averageCharWidth() * 80.0)
                + contentOffset().x()
                + document()->documentMargin();
        QPainter p(viewport());
        p.setPen(QPen("gray"));
        p.drawLine(x80, rect.top(), x80, rect.bottom());
        qDebug() << x80 << contentOffset() << document()->documentMargin() << font << endl;
    }
};

static QString filler()
{
    QString str;
    for (char c = '0'; c < '9'; ++ c) {
        str.append(QString(10, c));
    }
    return str;
}

int main(int argc, char ** argv)
{
    QApplication app(argc, argv);
    Edit ed;
    QTextCharFormat fmt = ed.currentCharFormat();
    fmt.setFontFamily("courier");
    fmt.setFontFixedPitch(true);
    ed.setCurrentCharFormat(fmt);
    ed.setLineWrapMode(QPlainTextEdit::NoWrap);
    qDebug() << fmt.font() << endl;
    ed.setPlainText(filler());
    ed.show();
    app.exec();
}

#include "main.moc"