[Layout] Allow use of item variables to change an item's own properties

[qgib]

Author Name: Harrissou Santanna (@DelazJ)
Original Redmine Issue: 18770
Affected QGIS version: 3.5(master)
Redmine category:map_composer/printing

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Let's add a map item and try to change its position using data-defined properties.
If I write expression like @case when @layout_pageheight >210 then 10 else 40 end@ for the X position data-defined button, then anytime i change the page size crossing the 210, I can see the map item moving in the layout. Great!
Now if i write @case when @map_scale >5000 then 10 else 40 end@ for the same property and change the scale of the map item, it does not move... until something else triggers its displacement ie:

• I use the select/move item button to move the item
• I manually change the X position spinbox value
• or I have another call (let's say the one with layout_pageheight) that triggers a movement

In all these case, it will move to the data-defined position.

I checked with other internal variable of the map item (map_rotation, eg) and other items (eg, label item with its rotation data-defined button using item_id variable) and anytime I use an interval variable to set a property of the item, no change occurs dynamically.

This makes the layout item's variables a bit useless (as you can not even call them from another item properties in 2.18).

Affects both 2.18 and 3.1 (master)

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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Other actions trigger a refresh too - e.g. clicking refresh, changing to a new atlas feature, exporting the layout.

[qgib]

Author Name: Harrissou Santanna (@DelazJ)

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I could have bet that I tried the refresh button unsuccessfully but i'd have lost... So yes, my list was not complete.
Anyway, good to see that when output time comes, the expressions are applied.

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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Please test with QGIS 3.4 - QGIS 2.18 reached it's end of life.

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• status_id was changed from Open to Feedback

[qgib]

Author Name: Harrissou Santanna (@DelazJ)

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Confirmed on master 3.5. Using @map_scale does not visually trigger the map item placement in my example; you may need to eg refresh the dialog beforehand.

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• version was changed from 2.18.18 to 3.5(master)
• status_id was changed from Feedback to Open

[nyalldawson]

This is a feature request, not a bug -- it relates to other open feature requests concerning handling of dependencies in layout expressions