How to automatically increase/decrease text size in label in Qt - comment on https://stackoverflow.com/a/42690033/1341914

main.cpp

// standard C++ header:
#include <iostream>
#include <string>

// Qt header:
#include <QApplication>
#include <QBoxLayout>
#include <QFrame>
#include <QGroupBox>
#include <QLabel>
#include <QLineEdit>
#include <QMainWindow>
#include <QStyle>
#include <QDebug>

#define DBG(msg) qDebug() << __FILE__ << ":" << __LINE__ << "> " << msg;

//#define CHECK_ONLY_HEIGHT

using namespace std;

class Label: public QLabel {

  public:
    void layout();
    QRect documentRect(); // borrowed from QLabelPrivate
  protected:
    virtual void resizeEvent(QResizeEvent *pQEvent);
};

QRect Label::documentRect()
{
  QRect rect = contentsRect();
  int m = margin(); rect.adjust(m, m, -m, -m);
  layoutDirection();
  const int align
    = QStyle::visualAlignment(layoutDirection(), QLabel::alignment());
  int i = indent();
  if (i < 0 && frameWidth()) { // no indent, but we do have a frame
    m = fontMetrics().width(QLatin1Char('x')) / 2 - m;
  }
  if (m > 0) {
    if (align & Qt::AlignLeft) rect.setLeft(rect.left() + m);
    if (align & Qt::AlignRight) rect.setRight(rect.right() - m);
    if (align & Qt::AlignTop) rect.setTop(rect.top() + m);
    if (align & Qt::AlignBottom) rect.setBottom(rect.bottom() - m);
  }
  return rect;
}

void Label::layout()
{
  // get initial settings
  QString text = this->text();
  if (text.isEmpty()) return;
  QRect rectLbl = documentRect(); // wrong: contentsRect();
  QFont font = this->font();
  int size = font.pointSize();
  QFontMetrics fontMetrics(font);
  QRect rect = fontMetrics.boundingRect(rectLbl,
    Qt::TextWordWrap, text);
  // decide whether to increase or decrease
  int step = rect.height() > rectLbl.height() ? -1 : 1;
  // iterate until text fits best into rectangle of label
  for (;;) {
    font.setPointSize(size + step);
    QFontMetrics fontMetrics(font);
    rect = fontMetrics.boundingRect(rectLbl,
      Qt::TextWordWrap, text);
    if (size <= 1) {
      cout << "Font cannot be made smaller!" << endl;
      break;
    }
    if (step < 0) {
      size += step;
	  #ifdef CHECK_ONLY_HEIGHT
      if (rect.height() < rectLbl.height()) break;
	  #else
      if (rect.height() < rectLbl.height() ||
          rect.width () < rectLbl.width ()) break;
	  #endif
    } else {
	  #ifdef CHECK_ONLY_HEIGHT
      if (rect.height() > rectLbl.height()) break;
	  #else
      if (rect.height() > rectLbl.height() ||
          rect.width () > rectLbl.width ()) break;
	  #endif
      size += step;
    }
    DBG("size:"<<size<<" step:"<<step<<" rect:"<<rect<<" rectLbl:"<<rectLbl<<" contentsRect:"<<contentsRect());
  }

  DBG("size:"<<size<<" rect:"<<rect<<" rectLbl:"<<rectLbl<<" contentsRect:"<<contentsRect());
  // apply result of iteration
  font.setPointSize(size);
  setFont(font);
}

void Label::resizeEvent(QResizeEvent *pQEvent)
{
  QLabel::resizeEvent(pQEvent);
  layout();
}

int main(int argc, char **argv)
{
  cout << QT_VERSION_STR << endl;
  // main application
#undef qApp // undef macro qApp out of the way
  QApplication qApp(argc, argv);
  // setup GUI
  QMainWindow qWin;
  QGroupBox qGBox;
  QVBoxLayout qBox;
  Label qLbl;
  qLbl.setText("aaa abcdefghijklmno bbbb ccccc");
  qLbl.setMinimumSize(QSize(200, 200));
  qLbl.setMaximumSize(QSize(200, 200));
  qLbl.setFrameStyle(Label::Box);
  qLbl.setFrameShadow(Label::Sunken);
  qLbl.setWordWrap(true);
  qBox.addWidget(&qLbl, 1);
  QLineEdit qTxt;
  qTxt.setText(qLbl.text());
  qBox.addWidget(&qTxt, 0);
  qGBox.setLayout(&qBox);
  qWin.setCentralWidget(&qGBox);
  qWin.show();
  // install signal handlers
  QObject::connect(&qTxt, &QLineEdit::editingFinished,
    [&qTxt, &qLbl]() {
      QString text = qTxt.text();
      qLbl.setText(text);
      qLbl.layout();
    });
  return qApp.exec();
}

screenshot_height.png

screenshot_height_and_width.png

On line 79, I replaced the || with an &&. The algorithm works better when the initial font is big and stepping towards a smaller value. But there is still a missing step.

Tried with T, then The quick brown fox jumps over the lazy dog, then a last enter.