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

[Roger Bivand]

I think that this thread is related to the later CDN thread here: 
https://lists.osgeo.org/pipermail/proj/2019-September/008858.html

This earlier thread addressed where to put the downloaded grids (adding 
that path to the existing search path), with a suggestion to use an XDG 
discovery mechanism that could be shared across applications but located 
in user space unless otherwise specified. The CDN thread addresses the 
mechanisms for serving the grids. It seems as though a CDN would be 
superior in many settings to manual download and unpacking of selected 
grids from datumgrid zip archives.

Roger

On Sun, 8 Sep 2019, Roger Bivand wrote:

> 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