QT show different images depending on input on serial interface - c++

Question

I'm new to QT and I'm trying to create a program that shows different images depending on input from a serial interface.
I realize that there are a number of ways to achieve this and I'm looking for input from someone with experience in QT.
My idea was to send new data to a compare function that returns an integer to the main function, this integer will determine what picture will be shown. However, using a while loop results in the picture being re-drawn and not static.
My question is, should i start another thread for the image viewer, or use a different class for it?
Or is this approach hideous and should i start over?
Thankful for any input!
if(!serial.open(QIODevice::ReadOnly))
qDebug() << serial.errorString();
QObject::connect(&serial, &QSerialPort::readyRead, [&]
{
int comp=0;
int landscape =1;
int total_data = serial.bytesAvailable();
qDebug() << "New data Available: " << serial.bytesAvailable();
QByteArray datas = serial.readAll();
comp= compare(total_data,datas);
while(comp == landscape){
qDebug() << "I Picture";
QLabel label("<img src='landscape.jpg' /> ");
label.show();
}
qDebug() << datas;
});
This is the compare function that reads data from serial interface
int compare(int x, QByteArray y)
{
int r=0;
for(int i = 0; i <= x ; i++){
if (strncmp (y, "\x00",1) ==0)
{
//picture();
r=1;
return r;
}
}
return r;
}

Answer 1

By doing this:
while(comp == landscape){
qDebug() << "I Picture";
QLabel label("<img src='landscape.jpg' /> ");
label.show();
}
You are creating a local QLabel on the stack. It will be destroyed at each iteration.
Qt uses its own mechanism to update its objects (the event loop) and you just have to change the picture in your QLabel when needed.
So, what you can do, it's creating a QLabel in your widget and change the image in your slot:
class Window: public QWidget
{
Q_OBJECT
public:
enum ImageType {
landscape = 1,
};
Window(QObject* parent=nullptr): QWidget(parent),
myLabel(new QLabel(this))
{
if(!serial.open(QIODevice::ReadOnly))
qDebug() << serial.errorString();
connect(&serial, &QSerialPort::readyRead, this, &Window::updateImage);
}
public slots:
void updateImage()
{
int total_data = serial.bytesAvailable();
qDebug() << "New data Available: " << serial.bytesAvailable();
QByteArray datas = serial.readAll();
int const comp = compare(total_data,datas);
if (comp == Window::landscape)
myLabel->setPixmap("landscape.png");
else
myLabel->setPixmap("anotherImg.png");
qDebug() << datas;
}
private:
QLabel* myLabel;
QSerialPort serial;
};

Answer 2

You can use signals/slots and a thread (only if you want to do more actions during the program execution). It is very typical when using the serial line, as it is an asynchronus protocol that has to be pulled to know if there exist new data.
To do so, emit a signal every time you receive data from serial line and create a slot that process it. In this way you will use similar than an event driven system.