c++ - How to automatically increase/decrease text size in label in Qt

Question

I have a Qt application where I have a textedit and a label. When a user presses the button, the textedit text should be displayed on label. For the label I have set few properties like word wrap is enabled and horizontal and vertically it is aligned center. Below is the screenshot :

Now I have to automatically adjust the size of the text in label so that if someone enters a large string, then it should fit inside the label, that means size of text should decrease. And if the text string is small, then size should increase automatically to fill up the complete label. Currently if I am typing it the large string, it looks like something:

As you can see, in the above image, text is moving out of the label. It should remain inside the label.

How to detect in application if the text is moving out of the label height & width. Then how to reduce the text size. I want the size to automatically increase if the string is small and decrease it string is large to fill up the complete label. Is there any class or something provided in QT. Any help or example please. Thanks.

EDIT: With the below code I am able to reduce the size of text to fit inside the label width but not able to make the text multi line.

QString string = ui->textEdit->toPlainText();   //Getting data from textEdit

ui->label->setAlignment(Qt::AlignCenter);   //Aligning label text to center
QFont f("Arial",50);        //Setting the default font size to 50
QFontMetrics fm(f);
ui->label->setFont(f);      //Setting the font to the label
int width = fm.width(string);   //Getting the width of the string
int size;
while(width >= 870)     //870 is the max width of label
{

    size = ui->label->font().pointSize()-1;     //Reduce font size by 1
    QFont newFont("Arial",size);            
    QFontMetrics nfm(newFont);          
    ui->label->setFont(newFont);        //Set the new font with new size
    width = nfm.width(string);      //Get the new width
}
ui->label->setText(string);

Answer 1

You (S. Andrew) solved it a little bit different like I proposed (just a statement but not critics). You did the word wrapping by yourself.

I wrote a minimal complete application to check how the Qt internal word wrapping can be used for your problem:

// 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>

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();
  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;
      if (rect.height() < rectLbl.height()) break;
    } else {
      if (rect.height() > rectLbl.height()) break;
      size += step;
    }
  }
  // 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.setFrameStyle(Label::Box);
  qLbl.setFrameShadow(Label::Sunken);
  qLbl.setWordWrap(true);
  qBox.addWidget(&qLbl, 1);
  QLineEdit qTxt;
  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();
}

Compiled and tested with VS2013 / Qt 5.6 on Windows 10 (64 bit):

When playing around with this test application, I recognized that the text fits not everytimes perfectly into the QLabel. I tried to improve the code exchanging QRect rectLbl = rect(); with QRect rectLbl = contentsRect();. This made it better but still not perfect. It seems there is some finetuning necessary (where the development starts to become effort). (See update at end of text.)

Actually, it would not be necessary to derive QLabel. In my first implementation, layout() was a function with QLabel& and const QString& as parameters.

After I got the font size management working, I intended to consider resize events also. Googling a little bit, I found the solution to apply event filters. However, event filters are called before the event is processed but I need after. Finally, I decided to inherit QLabel and to overload QLabel::resizeEvent() to keep things simple.

Btw. I noticed it is even not necessary to set

height eventually to a very large value

as I suggested in a comment earlier. It seems that QFontMetrics::boundingRect(const QRect &rect, int flags, ...) increases the height automa[gt]ically to keep required width when Qt::TextWordWrap is enabled.

Update:

@annacarolina encouraged me to investigate a little bit deeper into this issue that font size is sometimes choosen to large. Some debugging in Label::layout() uncovered that sometimes computed rect looked like unwrapped text where visual output was wrapped. This made me suspiciuous about correctness of the rectLbl. Thus, I started in qlabel.cpp on woboq.org but actually the Qt forum QLabel: Resize font to contentsRect provided the final hint which leaded me to QLabelPrivate::documentRect() (actually again on woboq.org where I already had looked for enlightment). Thus, I added a method Label::documentRect() to my class. This makes results much better (although I'm not fully convinced about "perfect").