如何检查QLabel中设置了哪个图像？

Question

I have a Qt application where I need to show a blinking LED and for that I need to use some png image of off and on led.I created a Qlabel and used setstylesheet to display the image. I created a timer and connected the signal to a slot.
Now the problem is how do I know if the current displayed image is OFF led or ON led .

I have many led in GUI so is there any better way to check this?

Answer 1

Don't bother trying to compare the image, just store a variable of the state of the LED. When the timer triggers you change the state of the variable and set the QImage accordingly.

// assuming a boolean variable
var = !var;
if(var)
   label->setImage(":/images/imageOn"); 
else
   label->setImage(":/images/imageOff");

This assumes the images imageOn and imageOff have been added to a Qt resource file and are under an 'images' prefix.

It is good practise to separate logic from its visual representation.

Answer 2

You can leverage the property mechanism to store the index of the next image to be used. A QLabel is-a QObject . Objects can have arbitrary properties assigned to them.

You also don't need to use style sheets to set image on a label. It's a premature pessimization because the stylesheet needs to be parsed every time you set it. If you're not using stylesheets for other purposes, to set an image on a label simply use setPixmap .

For example (Qt 5, C++11):

#include <QApplication>
#include <QTimer>
#include <QLabel>
#include <QImage>
#include <QPainter>

void blink(QLabel * label, const QList<QImage> & images)
{
  const char * const prop = "imageIndex";
  Q_ASSERT(!images.isEmpty());
  if (label->property(prop).isNull()) {
    // We're setting the image for the first time
    label->setProperty(prop, images.size());
  }
  int i = (label->property(prop).toInt() + 1) % images.size();
  label->setPixmap(QPixmap::fromImage(images[i]));
  label->setProperty(prop, i);
}

QImage textImage(const QString & text, int size = 64)
{
  QImage image(size, size, QImage::Format_ARGB32_Premultiplied);
  image.fill(Qt::transparent);
  QPainter p(&image);
  p.setFont(QFont("helvetica", 20));
  QTextOption opt;
  opt.setAlignment(Qt::AlignCenter);
  p.drawText(image.rect(), text, opt);
  return image;
}

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  QList<QImage> images;
  QLabel label;
  QTimer timer;

  images << textImage("0") << textImage("1") << textImage("2") << textImage("3");
  blink(&label, images);
  timer.start(250);
  QObject::connect(&timer, &QTimer::timeout, [&]{ blink(&label, images); });

  label.show();
  return a.exec();
}