Text scrolling (Marquee) in QLabel - qt

Question

I'm studying WidgetMarqueeLabel class:
#include "WidgetMarqueeLabel.h"
#include <QPainter>
#include <QWidget>
WidgetMarqueeLabel::WidgetMarqueeLabel(QWidget *parent)//*parent)
{
px = 0;
py = 10;
speed = 1;
direction = RightToLeft;
connect(&timer3, SIGNAL(timeout()), this, SLOT(refreshLabel()));
timer3.start(10);
}
void WidgetMarqueeLabel::refreshLabel()
{
repaint();
}
WidgetMarqueeLabel::~WidgetMarqueeLabel()
{}
void WidgetMarqueeLabel::show()
{
QLabel::show();
}
void WidgetMarqueeLabel::setAlignment(Qt::Alignment al)
{
m_align = al;
updateCoordinates();
QLabel::setAlignment(al);
}
void WidgetMarqueeLabel::paintEvent(QPaintEvent *evt)
{
QPainter p(this);
if(direction==RightToLeft)
{
px -= speed;
if(px <= (-textLength))
px = width();
}
else
{
px += speed;
if(px >= width())
px = - textLength;
}
p.drawText(px, py+fontPointSize, text());
p.translate(px,0);
}
void WidgetMarqueeLabel::resizeEvent(QResizeEvent *evt)
{
updateCoordinates();
QLabel::resizeEvent(evt);
}
void WidgetMarqueeLabel::updateCoordinates()
{
switch(m_align)
{
case Qt::AlignTop:
py = 10;
break;
case Qt::AlignBottom:
py = height()-10;
break;
case Qt::AlignVCenter:
py = height()/2;
break;
}
fontPointSize = font().pointSize()/2;
textLength = fontMetrics().width(text());
}
void WidgetMarqueeLabel::setSpeed(int s)
{
speed = s;
}
int WidgetMarqueeLabel::getSpeed()
{
return speed;
}
void WidgetMarqueeLabel::setDirection(int d)
{
direction = d;
if (direction==RightToLeft)
px = width() - textLength;
else
px = 0;
refreshLabel();
}
void WidgetMarqueeLabel::close()
{
QLabel::close();
}
I was wondering if it was possible to make the text reappear before the text that reaches the end of the last letter on the right. I want something like this: for example (white space are 25):
WidgetMarqueeLabel
tMarqueeLabel Widge
eLabel WidgetMarque
el WidgetMarqueeLa
WidgetMarqueeLabel
WidgetMarqueeLabel
WidgetMarqueeLabel
WidgetMarqueeLabel
Is this possible?

Answer 1

For this purpose, I once wrote a class.
Example screenshot showing the text "This is an example text. It will be scrolled horizontally.". Note the alpha blending at both sides.
The code:
scrolltext.h:
#ifndef SCROLLTEXT_H
#define SCROLLTEXT_H
#include <QWidget>
#include <QStaticText>
#include <QTimer>
class ScrollText : public QWidget
{
Q_OBJECT
Q_PROPERTY(QString text READ text WRITE setText)
Q_PROPERTY(QString separator READ separator WRITE setSeparator)
public:
explicit ScrollText(QWidget *parent = 0);
public slots:
QString text() const;
void setText(QString text);
QString separator() const;
void setSeparator(QString separator);
protected:
virtual void paintEvent(QPaintEvent *);
virtual void resizeEvent(QResizeEvent *);
private:
void updateText();
QString _text;
QString _separator;
QStaticText staticText;
int singleTextWidth;
QSize wholeTextSize;
int leftMargin;
bool scrollEnabled;
int scrollPos;
QImage alphaChannel;
QImage buffer;
QTimer timer;
private slots:
virtual void timer_timeout();
};
#endif // SCROLLTEXT_H
scrolltext.cpp:
#include "scrolltext.h"
#include <QPainter>
ScrollText::ScrollText(QWidget *parent) :
QWidget(parent), scrollPos(0)
{
staticText.setTextFormat(Qt::PlainText);
setFixedHeight(fontMetrics().height());
leftMargin = height() / 3;
setSeparator(" --- ");
connect(&timer, SIGNAL(timeout()), this, SLOT(timer_timeout()));
timer.setInterval(50);
}
QString ScrollText::text() const
{
return _text;
}
void ScrollText::setText(QString text)
{
_text = text;
updateText();
update();
}
QString ScrollText::separator() const
{
return _separator;
}
void ScrollText::setSeparator(QString separator)
{
_separator = separator;
updateText();
update();
}
void ScrollText::updateText()
{
timer.stop();
singleTextWidth = fontMetrics().width(_text);
scrollEnabled = (singleTextWidth > width() - leftMargin);
if(scrollEnabled)
{
scrollPos = -64;
staticText.setText(_text + _separator);
timer.start();
}
else
staticText.setText(_text);
staticText.prepare(QTransform(), font());
wholeTextSize = QSize(fontMetrics().width(staticText.text()), fontMetrics().height());
}
void ScrollText::paintEvent(QPaintEvent*)
{
QPainter p(this);
if(scrollEnabled)
{
buffer.fill(qRgba(0, 0, 0, 0));
QPainter pb(&buffer);
pb.setPen(p.pen());
pb.setFont(p.font());
int x = qMin(-scrollPos, 0) + leftMargin;
while(x < width())
{
pb.drawStaticText(QPointF(x, (height() - wholeTextSize.height()) / 2) + QPoint(2, 2), staticText);
x += wholeTextSize.width();
}
//Apply Alpha Channel
pb.setCompositionMode(QPainter::CompositionMode_DestinationIn);
pb.setClipRect(width() - 15, 0, 15, height());
pb.drawImage(0, 0, alphaChannel);
pb.setClipRect(0, 0, 15, height());
//initial situation: don't apply alpha channel in the left half of the image at all; apply it more and more until scrollPos gets positive
if(scrollPos < 0)
pb.setOpacity((qreal)(qMax(-8, scrollPos) + 8) / 8.0);
pb.drawImage(0, 0, alphaChannel);
//pb.end();
p.drawImage(0, 0, buffer);
}
else
{
p.drawStaticText(QPointF(leftMargin, (height() - wholeTextSize.height()) / 2), staticText);
}
}
void ScrollText::resizeEvent(QResizeEvent*)
{
//When the widget is resized, we need to update the alpha channel.
alphaChannel = QImage(size(), QImage::Format_ARGB32_Premultiplied);
buffer = QImage(size(), QImage::Format_ARGB32_Premultiplied);
//Create Alpha Channel:
if(width() > 64)
{
//create first scanline
QRgb* scanline1 = (QRgb*)alphaChannel.scanLine(0);
for(int x = 1; x < 16; ++x)
scanline1[x - 1] = scanline1[width() - x] = qRgba(0, 0, 0, x << 4);
for(int x = 15; x < width() - 15; ++x)
scanline1[x] = qRgb(0, 0, 0);
//copy scanline to the other ones
for(int y = 1; y < height(); ++y)
memcpy(alphaChannel.scanLine(y), (uchar*)scanline1, width() * 4);
}
else
alphaChannel.fill(qRgb(0, 0, 0));
//Update scrolling state
bool newScrollEnabled = (singleTextWidth > width() - leftMargin);
if(newScrollEnabled != scrollEnabled)
updateText();
}
void ScrollText::timer_timeout()
{
scrollPos = (scrollPos + 2)
% wholeTextSize.width();
update();
}

Answer 2

Really easy. Simply repaint the text displaced by the width of the control:
void WidgetMarqueeLabel::paintEvent(QPaintEvent *evt)
{
QPainter p(this);
if(direction==RightToLeft)
{
px -= speed;
if(px <= (-textLength))
px = width();
}
else
{
px += speed;
if(px >= width())
px = - textLength;
}
p.drawText(px, py+fontPointSize, text());
__p.drawText(px-width(), py+fontPointSize, text());
p.drawText(px+width(), py+fontPointSize, text());
p.translate(px,0);
}

Answer 3

Something like the following should work. The padding is hard-coded at 25, which is what is sounded like you wanted. If you wanted the label to always be a certain size, you could use something like QString::leftJustified.
class MarqueeLabel : public QLabel {
public:
explicit MarqueeLabel(const QString &text) : QLabel(text), pos_(0) {
QString pad(25, ' ');
actual_text_ = text + pad;
startTimer(100);
}
protected:
void timerEvent(QTimerEvent *) {
pos_ = ++pos_ % actual_text_.length();
setText(actual_text_.mid(pos_).append(actual_text_.left(pos_)));
}
private:
QString actual_text_;
int pos_;
};
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
MarqueeLabel lbl("WidgetMarqueeLabel");
lbl.show();
return a.exec();
}