[PROJ] How to find the active metadata search path in PROJ 6

[Roger Bivand]

On Sun, 8 Sep 2019, Nyall Dawson wrote:

> On Sat, 7 Sep 2019 at 23:20, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> xample
>> holding grid files downloaded on the fly or caches from previous sessions,
>> and where the default data/ directory is not user-writable,
>
> Good move, I think all proj based end-user applications should also
> implement this logic.

It does seem justified, doesn't it? It might be even more sensible if (at 
some time in the future) the user-writable cache directory was shared 
between applications, so that different R or Python packages or QGIS etc. 
didn't each have to have separate dumps of the same grid files ... just 
speculating.

>
>> I think I need
>> to build the ":" or ";" separated list to update the search path. To avoid
>> overwriting it, I need to prepend the active search path to the local
>> user-writable directory.
>>
>> Hope this is just something I've missed, a function returning a string, or
>> equivalent mechanism.
>
> Here's how we do it in QGIS. Excuse the mix of pure c++/proj/qt api (a
> necessary by-product of working in QGIS code):
>
> on startup:
>
> // append local user-writable folder as a proj search path
> QStringList currentProjSearchPaths = QgsProjUtils::searchPaths();
> currentProjSearchPaths.append( qgisSettingsDirPath() + "proj" );
> char **newPaths = new char *[currentProjSearchPaths.length()];
> for ( int i = 0; i < currentProjSearchPaths.count(); ++i )
> {
>  newPaths[i] = CPLStrdup( currentProjSearchPaths.at( i
> ).toUtf8().constData() );
> }
> proj_context_set_search_paths( nullptr,
> currentProjSearchPaths.count(), newPaths );
> for ( int i = 0; i < currentProjSearchPaths.count(); ++i )
> {
>  CPLFree( newPaths[i] );
> }
> delete [] newPaths;
>
> where QgsProjUtils::searchPaths() is:
>
> QStringList QgsProjUtils::searchPaths()
> {
>   const QString path( proj_info().searchpath );

This:                    ^^^^^^^^^^^^^^^^^^^^^

was what I needed for a first prototype, thanks very much! I'd missed this 
because I didn't read the PROJ_INFO struct definition in proj.h (starting 
line 273 in the 6.2.0 source).

>  QStringList paths;
>  #if PROJ_VERSION_MINOR==1 && PROJ_VERSION_PATCH==0
>  // -- see https://github.com/OSGeo/proj.4/pull/1497
>  paths = path.split( ';' );
>  #else
>  #ifdef Q_OS_WIN
>  paths = path.split( ';' );
>  #else
>  paths = path.split( ':' );
>  #endif
>  #endif
>  QSet<QString> existing;
>  // thin out duplicates from paths -- see
> https://github.com/OSGeo/proj.4/pull/1498
>  QStringList res;
>  res.reserve( paths.count() );
>  for ( const QString &p : qgis::as_const( paths ) )
>  {
>    if ( existing.contains( p ) )
>      continue;
>    existing.insert( p );
>    res << p;
>  }
>  return res;
> }
>
> There's quite a bit of logic here which is in place to work around
> earlier PROJ 6 release behavior, see the links commented in the code
> above.

Thanks again,

Roger

>
> Nyall
>
>
>>
>> Roger
>>
>> --
>> Roger Bivand
>> Department of Economics, Norwegian School of Economics,
>> Helleveien 30, N-5045 Bergen, Norway.
>> voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
>> https://orcid.org/0000-0003-2392-6140
>> https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
>> _______________________________________________
>> PROJ mailing list
>> PROJ at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/proj
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
https://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en