How to check which image is set in my QLabel? - c++

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();
}