[BUGFIX] Regression in Composer legend, layer's title was not the default legend's title

[rldhont]

Description

In release-2_14, the layer's title was the default legend's title. This
capabilities has been lost with the use of the QgsLegendModelV2 in
QgsLegendRenderer.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[nyalldawson]

Why would we use the title here? From the dox for QgsMapLayer::title(): "Returns the title of the layer used by QGIS Server in GetCapabilities request."

That doesn't sound related to legend titles to me.

[rldhont]

@nyalldawson I probably have to update the dox. But in release-2_14 and before, the layer's title was used as the layer legend's title.

[wonder-sk]

But this change would also change behavior of non-server code, right? That could be regarded as a regression by QGIS app users...

[rldhont]

@wonder-sk Yes, this PR is not only for server it is also to fix a QGIS Desktop regression.

[nyalldawson]

I'm not too strong on server use, but too me this would be unexpected behaviour. The server metadata title and the name used in a legend may have no relation to each other, and it's standard qgis behaviour to take the layer's name for legends. I think this is quite an edge case which doesn't warrant changing the default behaviour for.

[rldhont]

The title was firstly designed for server, but it's a metadata and has been used as a title in legend. The name is for Desktop use, it can be the title or a simplify name to easily identify layers in the interface.

This PR is a fix for a regression compare to the old LTR. This feature has been introduce in desktop by #824.

[rldhont]

This feature is in QGIS since 2.0

[nyalldawson]

I guess ultimately I'm a 0 here because I'm not a server end user so don't have the required knowledge to comment.

[m-kuhn]

For my understanding, what's wrong with using QgsMapLayer::name()?
QgsMapLayer::title() is explicitly tagged in the API as being "used by QGIS Server in GetCapabilities request", so a bit unexpected to influence desktop behavior.

I have no strong opinion (yet), just wondering

• about the clean way to deal with those two potentially conflicting QgsMapLayer properties
• about the risk of breaking previously working 2.18 projects (compatibility with these is more important than compatibility with 2.14 behavior)

[wonder-sk]

In #824 the logic seems more complicated than what I see here... it only applied to single symbol renderer, and it tried to use a text in these priorities: 1. symbol's custom text, 2. layer's custom text, 3. layer's title(), 4. layer's name()

I guess I am also 0 on this...

[rldhont]

@m-kuhn the documentation about QgsMapLayer::title() is not complete. Since the first release of QGIS 2.0, if a title is defined it is used as the default layer name in the Composer legend.

Actually QGIS 2.18 breaks project build with 2.14, because the legend rendering is not based on the same model. Some user does not update to 2.18 because of some regressions in Composer, that I am looking to fix like #7250

[rldhont]

@wonder-sk the old legend model was more complex.

[nyalldawson]

the documentation about QgsMapLayer::title() is not complete

But it's not just the documentation - it's how this setting is exposed within the qgis UI too.

[m-kuhn]

I'm still struggling to see the difference between title and name. What's the use-case where you need them to be different and - as a user - how do I know how this works?

[mdouchin]

I agree with @rldhont : as a QGIS user, when I load layers in my project, from Shapefiles, Gpkg or PostgreSQL, I do not rename them in the layer panel (the "legend") because I like to see the real source name. For example I keep "main_roads" instead of renaming to "Main roads". In this context, I like to use the metadata "title" property as a replacement. Inside the composer, I would like to have the titles used, because my exported PDF is for my colleague and not me. And I do not want to rename all the item in my composer legend manually.
The behaviour has changed between 2.14 (ex LTR) and 2.18, and I think it is a regression

[nyalldawson]

@mdouchin

That sounds like an edge case to me - the original layer source is already visible in the layer tooltips and from the layer properties window.

My issue here is the misleading reuse of an unrelated setting - the title field is quite clearly marked as applying only to qgis server.

[nyalldawson]

Ping @probot

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