Subclass of QPlainText does not expand to fill the layout - c++

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);
}