Need to understand setUpdatesEnabled behaviour while updating UI elements

starkm42

Hi,

i have simple program that reads GPS values from NMEA log file and displays on MainWindow(subclass of QWidget).

here is how it is setup.

we have a class GPSSource that inherits QGeoPositionInfoSource and defines all pure virtual functions.

it provides a signal newGPSPositionUpdate every time a new GPS value is read from Log file

signals:
    void newGPSPositionUpdate(void);

main widget gets its update from connecting its update slot using this signal

connect(source, &GPSSource::newGPSPositionUpdate, this, QOverload<>::of(&MainWindow::update));

within paintEvent method of main Widget we have.

void MainWindow::paintEvent(QPaintEvent* /* *eventObj */) {
    // access last know position
    QGeoPositionInfo lastPosition = source->lastKnownPosition();

    displayForm->latitude->setText(QString::asprintf("%fd", lastPosition.coordinate().latitude()));
    displayForm->longitude->setText(QString::asprintf("%fd", lastPosition.coordinate().longitude()));
    displayForm->timestamp->setText(lastPosition.timestamp().toString());
qDebug() << lastPosition.timestamp().toString();
}

GPSSource logs timestamp as it reads from log file and main Widget logs timestamp as it receives the signal and i was expecting Application output to be

"Thu Jan 1 12:36:05 1970"
"12h 36m 5.57s  November 2 2023"
"Thu Nov 2 12:36:05 2023"
"12h 36m 6.57s  November 2 2023"
"Thu Nov 2 12:36:06 2023"
"12h 36m 6.57s  November 2 2023"

what i am getting

"12h 36m 5.57s  January 1 1970"
"Thu Jan 1 12:36:05 1970"
"Thu Jan 1 12:36:05 1970"
"12h 36m 5.57s  January 1 1970"
"Thu Jan 1 12:36:05 1970"
"12h 36m 5.57s  November 2 2023"
"Thu Nov 2 12:36:05 2023"
"Thu Nov 2 12:36:05 2023"
"12h 36m 6.57s  November 2 2023"

using setUpdatesEnabled to toggle updates to bulk update UI elements

    setUpdatesEnabled(false);
    displayForm->latitude->setText(QString::asprintf("%fd", lastPosition.coordinate().latitude()));
    displayForm->longitude->setText(QString::asprintf("%fd", lastPosition.coordinate().longitude()));
    displayForm->timestamp->setText(lastPosition.timestamp().toString());
    setUpdatesEnabled(true);

I get

"12h 37m 8.57s  November 2 2023"
"Thu Nov 2 12:37:08 2023"
"Thu Nov 2 12:37:08 2023"
"Thu Nov 2 12:37:08 2023"
"Thu Nov 2 12:37:08 2023"
"Thu Nov 2 12:37:08 2023"
"Thu Nov 2 12:37:08 2023"

my questions.

i need to understand how QT is performing updates to UI, any article explaining this particular scenario or answer is much appreciated.
how can i do tasks like these, periodic updates to UI (often bulk) efficiently.

ps:
i am new to QT.

jsulm

@starkm42 said in Need to understand setUpdatesEnabled behaviour while updating UI elements:

QGeoPositionInfo lastPosition = source->lastKnownPosition();

displayForm->latitude->setText(QString::asprintf("%fd", lastPosition.coordinate().latitude()));
displayForm->longitude->setText(QString::asprintf("%fd", lastPosition.coordinate().longitude()));
displayForm->timestamp->setText(lastPosition.timestamp().toString());

qDebug() << lastPosition.timestamp().toString();

Why are you doing this in paintEvent? You're not painting here, right? You just set some labels. Just connect a slot to newGPSPositionUpdate and set the labels in that slot. No need for update() and paintEvent().

starkm42

@jsulm

yup, that fixed it, thanks !

do you have any advice on my second question regarding bulk updates, do i do them the same as i am doing it here ?

jsulm

@starkm42 If you want to do something periodically then use QTimer.