Showing dialog box even though QGIS is freezing

Question

I have a long script in PyQGIS that contains three processing tasks after another. I want to display a pop up box (like "QProgressDialog"-Box) for each task when it is finished.

Example: When Task 1 (Merge) has finished, I want a box to pop up ("Task 1 (Merge) is finished). Then the Task 2 is executing and when it is finished i want another box to pop up...

Unfortunately only the first "QProgressDialog"-Box is shown and QGIS freezes as it is processing.

How can I achieve to display a pop up box (or even better a progress bar) when QGIS is freezing?

My code is:

bar = QProgressDialog("Operation in progress", "Cancel",0,100)
bar.show()
bar.setRange(0,0)

#the processing tasks are following below
...

Answer 1

QGIS freezes as it is processing because you need to set 'setWindowModality' for QProgressDialog object and a sleep (delay) time for each task. Another issue is referred to 'setRange' method. It was already set in QProgressDialog object (as parameters ..., 0, 100). So, for three processing tasks following code works as expected:

import time

m = ""

bar = QProgressDialog(m, "Cancel", 0, 100)

bar.setWindowModality(Qt.WindowModal)

m = "Operation 1 in progress"
bar.setLabelText(m)

for i in range(101):
    time.sleep(0.05)
    bar.setValue(i)

m = "Operation 2 in progress"
bar.setLabelText(m)

for i in range(101):
    time.sleep(0.05)
    bar.setValue(i)

m = "Operation 3 in progress"
bar.setLabelText(m)

for i in range(101):
    time.sleep(0.05)
    bar.setValue(i)

Following image is showing your "freezing behavior" when 'setWindowModality' line in above code is commented.

When 'setWindowModality' line is not commented, code behavior is as expected:

Answer 2

I don't think this can be achieved with dialog boxes. However, there is something better: the MessageBar. Have a look at: Is it possible to show our custom message in QGIS Status bar using python?