Elided Label Example¶
This example creates a widget similar to
QLabel
, that elides the last visible line, if the text is too long to fit the widget’s geometry.When text of varying length has to be displayed in a uniformly sized area, for instance within a list or grid view where all list items have the same size, it can be useful to give the user a visual clue when not all text is visible.
QLabel
can elide text that doesn’t fit within it, but only in one line. TheElidedLabel
widget shown in this example word wraps its text by its width, and elides the last visible line if some text is left out.TestWidget
gives control to the features ofElidedWidget
and forms the example application.
ElidedLabel Class Definition¶
Like
QLabel
,ElidedLabel
inherits fromQFrame
. Here’s the definition of theElidedLabel
class:class ElidedLabel : public QFrame { Q_OBJECT Q_PROPERTY(QString text READ text WRITE setText) Q_PROPERTY(bool isElided READ isElided) public: explicit ElidedLabel(const QString &text, QWidget *parent = nullptr); void setText(const QString &text); const QString & text() const { return content; } bool isElided() const { return elided; } protected: void paintEvent(QPaintEvent *event) override; signals: void elisionChanged(bool elided); private: bool elided; QString content; };The
isElided
property depends the font, text content and geometry of the widget. Whenever any of these change, theelisionChanged()
signal might trigger. We cache the current elision value inelided
, so that it doesn’t have to be recomputed every time it’s asked for.
ElidedLabel Class Implementation¶
Except for initializing the member variables, the constructor sets the size policy to be horizontally expanding, since it’s meant to fill the width of its container and grow vertically.
ElidedLabel::ElidedLabel(const QString &text, QWidget *parent) : QFrame(parent) , elided(false) , content(text) { setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Preferred); }Changing the
content
require a repaint of the widget.void ElidedLabel::setText(const QString &newText) { content = newText; update(); }
QTextLayout
is used in thepaintEvent()
to divide thecontent
into lines, that wrap on word boundaries. Each line, except the last visible one, is drawnlineSpacing
pixels below the previous one. Thedraw()
method ofQTextLine
will draw the line using the coordinate point as the top left corner.void ElidedLabel::paintEvent(QPaintEvent *event) { QFrame::paintEvent(event); QPainter painter(this); QFontMetrics fontMetrics = painter.fontMetrics(); bool didElide = false; int lineSpacing = fontMetrics.lineSpacing(); int y = 0; QTextLayout textLayout(content, painter.font()); textLayout.beginLayout(); forever { QTextLine line = textLayout.createLine(); if (!line.isValid()) break; line.setLineWidth(width()); int nextLineY = y + lineSpacing; if (height() >= nextLineY + lineSpacing) { line.draw(&painter, QPoint(0, y)); y = nextLineY;Unfortunately,
QTextLayout
does not elide text, so the last visible line has to be treated differently. This last line is elided if it is too wide. ThedrawText()
method ofQPainter
draws the text starting from the base line, which isascecnt()
pixels below the last drawn line.Finally, one more line is created to see if everything fit on this line.
} else { QString lastLine = content.mid(line.textStart()); QString elidedLastLine = fontMetrics.elidedText(lastLine, Qt::ElideRight, width()); painter.drawText(QPoint(0, y + fontMetrics.ascent()), elidedLastLine); line = textLayout.createLine(); didElide = line.isValid(); break; } } textLayout.endLayout();If the text was elided and wasn’t before or vice versa, cache it in
elided
and emit the change.if (didElide != elided) { elided = didElide; emit elisionChanged(didElide); } }
TestWidget Class Definition¶
TestWidget
is aQWidget
and is the main window of the example. It contains anElidedLabel
which can be resized with twoQSlider
widgets.class TestWidget : public QWidget { Q_OBJECT public: TestWidget(QWidget *parent = nullptr); protected: void resizeEvent(QResizeEvent *event) override; private slots: void switchText(); void onWidthChanged(int width); void onHeightChanged(int height); private: int sampleIndex; QStringList textSamples; ElidedLabel *elidedText; QSlider *heightSlider; QSlider *widthSlider; };
TestWidget Class Implementation¶
The constructor initializes the whole widget. Strings of different length are stored in
textSamples
. The user is able to switch between these.TestWidget::TestWidget(QWidget *parent) : QWidget(parent) { const QString romeo = tr( "But soft, what light through yonder window breaks? / " "It is the east, and Juliet is the sun. / " "Arise, fair sun, and kill the envious moon, / " "Who is already sick and pale with grief / " "That thou, her maid, art far more fair than she." ); const QString macbeth = tr( "To-morrow, and to-morrow, and to-morrow, / " "Creeps in this petty pace from day to day, / " "To the last syllable of recorded time; / " "And all our yesterdays have lighted fools / " "The way to dusty death. Out, out, brief candle! / " "Life's but a walking shadow, a poor player, / " "That struts and frets his hour upon the stage, / " "And then is heard no more. It is a tale / " "Told by an idiot, full of sound and fury, / " "Signifying nothing." ); const QString harry = tr("Feeling lucky, punk?"); textSamples << romeo << macbeth << harry;An
ElidedLabel
is created to contain the first of the sample strings. The frame is made visible to make it easier to see the actual size of the widget.sampleIndex = 0; elidedText = new ElidedLabel(textSamples[sampleIndex], this); elidedText->setFrameStyle(QFrame::Box);The buttons and the elision label are created. By connecting the
elisionChanged()
signal to thesetVisible()
slot of thelabel
, it will act as an indicator to when the text is elided or not. This signal could, for instance, be used to make a “More” button visible, or similar.QPushButton *switchButton = new QPushButton(tr("Switch text")); connect(switchButton, &QPushButton::clicked, this, &TestWidget::switchText); QPushButton *exitButton = new QPushButton(tr("Exit")); connect(exitButton, &QPushButton::clicked, this, &TestWidget::close); QLabel *label = new QLabel(tr("Elided")); label->setVisible(elidedText->isElided()); connect(elidedText, &ElidedLabel::elisionChanged, label, &QLabel::setVisible);The
widthSlider
andheightSlider
specify the size of theelidedText
. Since the y-axis is inverted, theheightSlider
has to be inverted to act appropriately.widthSlider = new QSlider(Qt::Horizontal); widthSlider->setMinimum(0); connect(widthSlider, &QSlider::valueChanged, this, &TestWidget::onWidthChanged); heightSlider = new QSlider(Qt::Vertical); heightSlider->setInvertedAppearance(true); heightSlider->setMinimum(0); connect(heightSlider, &QSlider::valueChanged, this, &TestWidget::onHeightChanged);The components are all stored in a
QGridLayout
, which is made the layout of theTestWidget
.QGridLayout *layout = new QGridLayout; layout->addWidget(label, 0, 1, Qt::AlignCenter); layout->addWidget(switchButton, 0, 2); layout->addWidget(exitButton, 0, 3); layout->addWidget(widthSlider, 1, 1, 1, 3); layout->addWidget(heightSlider, 2, 0); layout->addWidget(elidedText, 2, 1, 1, 3, Qt::AlignTop | Qt::AlignLeft); setLayout(layout);The
widthSlider
andheightSlider
have the exact same length as the dimensions of theelidedText
. The maximum value for both of them is thus their lengths, and each tick indicates one pixel.void TestWidget::resizeEvent(QResizeEvent *event) { Q_UNUSED(event) int maxWidth = widthSlider->width(); widthSlider->setMaximum(maxWidth); widthSlider->setValue(maxWidth / 2); int maxHeight = heightSlider->height(); heightSlider->setMaximum(maxHeight); heightSlider->setValue(maxHeight / 2); elidedText->setFixedSize(widthSlider->value(), heightSlider->value()); }The
switchText()
slot simply cycles through all the available sample texts.void TestWidget::switchText() { sampleIndex = (sampleIndex + 1) % textSamples.size(); elidedText->setText(textSamples.at(sampleIndex)); }These slots set the width and height of the
elided
text, in response to changes in the sliders.
The
main()
Function¶
The
main()
function creates an instance ofTestWidget
fullscreen and enters the message loop.int main( int argc, char *argv[] ) { #ifdef Q_OS_ANDROID QApplication::setAttribute(Qt::AA_EnableHighDpiScaling); #endif QApplication application( argc, argv ); TestWidget w; w.showFullScreen(); return application.exec(); }
© 2022 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.