Method theme() of QgsMapCanvas not in sync?

[qgib]

Author Name: Darius G. (Darius G.)
Original Redmine Issue: 19674
Affected QGIS version: 3.4.3
Redmine category:python_bindings_/_sipify

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iface.mapCanvas().theme() refers always to an empty string while user set map themes in the QGIS GUI interactively.

In addition you can also check this:

https://gis.stackexchange.com/questions/293507/how-to-read-out-which-map-theme-is-used-in-qgis-3

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Related issue(s): #27594 (relates)
Redmine related issue(s): 19769

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[qgib]

Author Name: Adam Szieberth (@SzieberthAdam)

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Connects to #27594

It is weird that @qgc.QgsProject.instance().mapThemeCollection()@ can provide the list of map themes and can emit a signal when map theme changed but can not give the current/active/actual map theme. Also I found @applyTheme()@ overcomplicated as I would expect to give the name as a single argument and see the theme changed.

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• relates was configured as Add a variable returning the current map theme #27594

[qgib]

Author Name: Darius G. (Darius G.)

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I have installed latest QGIS 3.4 and I have noticed that the bug is still not solved in this version?

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from 3.2.2 to 3.4.0

[qgib]

Author Name: Darius G. (Darius G.)

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Bug still in QGIS 3.4.2 not solved.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from 3.4.0 to 3.4.2
• category_id was changed from Python plugins to Python bindings / sipify

[qgib]

Author Name: Darius G. (Darius G.)

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Bug still in QGIS 3.4.3 not solved.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from 3.4.2 to 3.4.3

[qgib]

Author Name: Darius G. (Darius G.)

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Bug still in QGIS 3.4.4 not solved...

[qgib]

Author Name: Martin Dobias (@wonder-sk)

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The main map canvas is "special" - it does not have map theme assigned. Instead when you pick a map theme, styles of map layers are modified according to the theme's settings. The theme attribute in QgsMapCanvas is used for other map canvases where user want to follow a specific map theme which is different from the current one.

So in fact this is not a bug, but I understand there's some confusion due to the way how the implementation works.

To figure out the current map theme (and that's how the map themes menu in qgis does it) is like this:

1. get the current state using QgsMapThemeCollection::createThemeFromCurrentState( root, model ) (root and model come from QgsProject::instance())
2. go through the map themes QgsProject::instance()->mapThemeCollection()->mapThemes() and for each have a look at their content QgsProject::instance()->mapThemeCollection()->mapThemeState( name ) and compare it to the current theme's content

[qgib]

Author Name: Darius G. (Darius G.)

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Thank you very much for the feedback. I have implemented your procedure and it works.

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• status_id was changed from Open to Closed

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• resolution was changed from to invalid

[alexMuellerKela]

Hi all, thanks for the described solution on Feb 26, 2019 - only this appears to be a riddle to me. I just do not understand where to go and what to do. I would be thankful if there would be some explanations, best based on the QGIS UI.
thanks so far