Subclass of QPlainText does not expand to fill the layout

Question

I do not understand why the CodeEditor example from the Qt website does not appear to work as expected. Every time I run the code it displays it like this, really small and not expanding to take up all the available space. Does anyone have any idea why? I have even tried to set a fixed size, sizepolicy and minimum sizing. Not sure what I am missing here.

main.cpp:

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QVBoxLayout>
#include <QLabel>
#include "codeeditor.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    // controls
    auto *widget_main = new QWidget(this);
    auto *lay_main = new QVBoxLayout(widget_main);
    auto *label = new QLabel("Test");
    auto *editor = new CodeEditor();

    // layout
    lay_main->addWidget(label);
    lay_main->addWidget(editor);
    lay_main->setContentsMargins(5, 5, 5, 5);
}

MainWindow::~MainWindow()
{
}

codeeditor.h:

#ifndef CODEEDITOR_H
#define CODEEDITOR_H

#include <QPlainTextEdit>
#include <QObject>

class QPaintEvent;
class QResizeEvent;
class QSize;
class QWidget;

class LineNumberArea;

// Main text editor
class CodeEditor : public QPlainTextEdit
{
    Q_OBJECT

    public:
        CodeEditor(QWidget *parent = 0);

        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;
};

// Line number gutter
class LineNumberArea : public QWidget
{
    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 "codeeditor.h"
#include <QtWidgets>
#include <QFontMetrics>


CodeEditor::CodeEditor(QWidget *parent) : QPlainTextEdit(parent)
{
    setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

    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();
    setFixedSize(200, 200);
    setMinimumSize(200,200);
}


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

    //int space = 3 + fontMetrics().horizontalAdvance(QLatin1Char('9')) * digits;
    int space = 3 + 12 * digits;

    return space;
}


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


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);
}


void CodeEditor::resizeEvent(QResizeEvent *e)
{
    QPlainTextEdit::resizeEvent(e);

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


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);
}


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


    QTextBlock block = firstVisibleBlock();
    int blockNumber = block.blockNumber();
    int top = (int) blockBoundingGeometry(block).translated(contentOffset()).top();
    int bottom = top + (int) blockBoundingRect(block).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, lineNumberArea->width(), fontMetrics().height(),
                             Qt::AlignRight, number);
        }

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

Answer 1

QMainWindow is a special widget since it has a preset layout

So you must set the main widget through setCentralWidget():

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    auto *widget_main = new QWidget;
    auto *lay_main = new QVBoxLayout(widget_main);
    auto *label = new QLabel("Test");
    auto *editor = new CodeEditor();

    setCentralWidget(widget_main); // <-- +++

    // layout
    lay_main->addWidget(label);
    lay_main->addWidget(editor);
    lay_main->setContentsMargins(5, 5, 5, 5);
}

On the other hand if you are going to use layouts then you should not set a fixed size to the widget, in your case remove setFixedSize(200, 200) on the other hand it is recommended that you make the connections with the new syntax:

CodeEditor::CodeEditor(QWidget *parent) : QPlainTextEdit(parent)
{
    setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

    lineNumberArea = new LineNumberArea(this);

    connect(this, &QPlainTextEdit::blockCountChanged, this, &CodeEditor::updateLineNumberAreaWidth);
    connect(this, &QPlainTextEdit::updateRequest, this, &CodeEditor::updateLineNumberArea);
    connect(this, &QPlainTextEdit::cursorPositionChanged, this, &CodeEditor::highlightCurrentLine);

    updateLineNumberAreaWidth(0);
    highlightCurrentLine();
    // setFixedSize(200, 200); <-- ---
    setMinimumSize(200,200);
}