[Qgis-user] Event loop in QGIS3

[Tudorache, Marian]

Hello community,

A while ago I sent a question about a problem with QMessageBox and message bar does not get updated during a long process.
In my example I create a script like this.

from qgis.PyQt import QtWidgets
message = "Wait to open the airspace project..."
msg = QtWidgets.QMessageBox()
msg.setText(message)
msg.show()
qgis.utils.iface.messageBar().pushMessage("Wait", message)


for i in range(1000):
    print(i)
qgis.utils.iface.messageBar().pushMessage("Done", "The process is done")
msg.setText("The process is done")
msg.show()
qgis.utils.iface.messageBar().pushMessage("Done", "The process is done"))


When I run the scrip in QGIS 2.18 the QMessageBox displays first the message "Wait to open the airspace project...". The same message is pushed to messageBar.
Then during the loop it displays the variable i.
At the end of the loop the QMessageBox displays the message "The process is done" which is also pushed to the messageBar.

When I run the same scrip on QGIS 3 the QMessageBox is empty. The message "Wait to open the airspace project..." is not displayed by QMessageBox nor by messageBar.
The print inside the loop displays the I and when the loop is done the QMessageBox displays the message "The process is done".
The messageBar displays also the message "The process is done". And after a while it displays the initial message "Wait to open the airspace project..."

I modified the scrip by adding QApplication.processEvents() inside the loop and now everything is fine.
It seems QGIS3 no longer triggers this event loop and I have to do this explicitly by calling processEvents from QApplication.

Why QGIS 3 needs to call QApplication.processEvents() and QGIS 2 does not need?

Thank you,
Marian

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