I need a tool which displays the sum of selected lines as I go in QGIS. Something like the Measure line-tool. I´m new to PyQGIS. The logic seems quite simple, I just don´t know where to start. I guess this could be a good starting point https://gis.stackexchange.com/a/211162/161290?
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2Since you post a link to the answer to your question, will you close your question yourself? Or will you describe, how that answer doesn't suite your needs? – Erik Apr 8 '20 at 8:11
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1That´s an old answer and to my knowledge there has been changes in the API. I do not know what changes need to be made to the code to make it work. The linked answer does not work as it is. Thank you. – Topi V Apr 8 '20 at 8:38
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A possible solution is using the Field Calculator to create a label that show the sum of the selected lines.
In the lines layer create a label using the expression
array_sum(aggregate(
layer:= 'line',
aggregate:='array_agg',
expression:=$length,
filter:=is_selected()))
NB: To use array_sum in the oldest QGIS versions you need to install the plugin arrayPlus.
This will create a label with the sum of the selected lines. It will appear in all the lines.
Since QGIS 3.10 in the label is integrated a Geometry Generator: you can use it to move the label to the centre of the screen or in a preferred position.
In the Layer Styling > Label Panel go to the Geometry Generator and use the variable
@map_extent_center and select the Point / Multipoint geometry option
this will move a single label to the centre of you Canvas. 
If you want you can play with colour , background and offset to move your label. This is an example integrating the original expression with a text.
Here the modified expression:
concat( 'Total length: ',
array_sum(aggregate(
layer:= 'line',
aggregate:='array_agg',
expression:=(format_number($length,3)),
filter:=is_selected()))
,' m')
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There is no function named array_sum. Which version are you using? Otherwise this is exactly the solution I'm looking for. – Topi V Apr 8 '20 at 9:53
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I am using 3.10. Sorry, my bad. To use
array_sumyou need to install the plugin ArrayPlus that integrated some extra array to your system. I will correct the answer. – Val P Apr 8 '20 at 9:56 -
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You're welcome. If you are happy with the answer can you please vote and accept it. Thanks. – Val P Apr 8 '20 at 10:29
