Refreshing map canvas in for loop in QGIS 3

Question

I have a console script where I'm trying to iterate through saved bookmarks from the open project to have the map pan/zoom to the bookmark extent. I would like to eventually save each map canvas as pdf file. I can manually set the right parameters to cycle through the bookmarks and the canvas refreshes each time I set the extent.

However, if I cycle through the saved bookmarks and set the canvas extent in a for loop (as below), the canvas does not refresh with each extent change, it only refreshes once the script has run it's course. I've added sleep commands in the for loop to allow for redraw time, but that didn't work. Am I missing something simple?

bmarks = QgsApplication.bookmarkManager()

bmarks.bookmarks() 

for b in bmarks.bookmarks():
    canv = iface.mapCanvas()
    canv.setExtent(b.extent())
    iface.mapCanvas().refresh()

Answer 1

I see that your request is for the canvas, but it seems the ultimate goal is a file based output, so I'll answer that one instead.

You don't need to update the canvas itself. You can also very well do this in jobs that render to images.

bmarks = QgsApplication.bookmarkManager()
bmarks.bookmarks() 

settings = iface.mapCanvas().mapSettings()

for i, b in enumerate(bmarks.bookmarks()):
    settings.setExtent(b.extent())
    renderer = QgsMapRendererParallelJob(settings)

    event_loop = QEventLoop()
    renderer.finished.connect(event_loop.quit)
    renderer.start()
    event_loop.exec_()

    img = renderer.renderedImage()
    img.save('/tmp/img{}.png'.format(i))

The advantage of this approach is also that you are able to fine tune parameters in the settings, e.g. width and height of the rendered image (by default it depends on the window size) or that you can have a transparent background. See https://qgis.org/api/classQgsMapSettings.html for details.

You could even launch several background tasks in parallel for this, this is left as an exercise for the reader.

Answer 2

First of all, it looks like using time.sleep() might not give desired effect, see: https://stackoverflow.com/questions/55301747/how-to-make-python-wait-while-qgis-renders-a-shapefile

So in order to prove that solution works, I've introduced saving images (please don't forget to adjust the path) The key difference is using waitWhileRendering(), which apparently, well, waits until rendering is finished, so that canvas' refresh does make its job.

bmarks = QgsApplication.bookmarkManager()
canv = iface.mapCanvas()
i=0
for b in bmarks:
    i+=1
    canv.setExtent(b.extent(),False)
    canv.refresh()
    canv.waitWhileRendering() #probably that makes the difference
    canv.saveAsImage('/home/username/Documents/j{0}'.format(i)) ##adjust the path