[gdal-dev] how to implement user specified events and handlers

[Ari Jolma]

On 10/15/2013 09:10 PM, Even Rouault wrote:
> Martin,
>
> The closest equivalent in the GDAL/OGR code base I could think of is the
> possibility of defining a custom error handler from the Python bindings.

For the Perl bindings I wrote a mechanism to call a Perl function via 
the GDALProgressFunc mechanism, which is used quite a lot - it was a bit 
like dark art, but seems to work. Is similar implemented for Python?

Ari

>
> Basically you would need to have a C API to register a C callback,
> "VFKLineCallbackRegister", and you would have a Python C module that would
> define such a C callback "PyVFKCallbackWrapper" (or extend the existing GDAL
> Python bindings), that would be a wrapper that would take a Python callable
> object as argument and call it. And you would have a C function, exposed as
> Python, "RegisterVFKCallback" that would pass PyVFKCallbackWrapper to
> VFKLineCallbackRegister with the Python function as a userdata.
>
> The relevant existing code is in :
>
> * swig/include/port.i :
>
> /* Equivalent of PyVFKCallbackWrapper */
> %{
> void CPL_STDCALL PyCPLErrorHandler(CPLErr eErrClass, int err_no, const char*
> pszErrorMsg)
> {
>      void* user_data = CPLGetErrorHandlerUserData();
>      PyObject *psArgs;
>
>      psArgs = Py_BuildValue("(iis)", eErrClass, err_no, pszErrorMsg );
>      PyEval_CallObject( (PyObject*)user_data, psArgs);
>      Py_XDECREF(psArgs);
> }
> %}
>
> /* Equivalent of RegisterVFKCallback */
> %inline %{
>    CPLErr PushErrorHandler( CPLErrorHandler pfnErrorHandler = NULL, void*
> user_data = NULL )
>    {
> [...]
>          CPLPushErrorHandlerEx(pfnErrorHandler, user_data);  /* equivalent of
> VFKLineCallbackRegister */
>      return CE_None;
>    }
> %}
>
> * swig/include/python/typemaps_python.i :
>
> %typemap(in) ( CPLErrorHandler pfnErrorHandler = NULL, void* user_data = NULL
> )
> {
>      /* %typemap(in) (CPLErrorHandler pfnErrorHandler = NULL, void* user_data =
> NULL) */
>      [...]
>       if (!PyCallable_Check($input))
>      {
>          PyErr_SetString( PyExc_RuntimeError,
>                           "Object given is not a String or a Python function"
> );
>          SWIG_fail;
>      }
>      else
>      {
>          Py_INCREF($input);
>          $1 = PyCPLErrorHandler;
>          $2 = $input;
>      }
> }
>
> Best regards,
>
> Even
>
>> Hi all,
>>
>> I wonder if there is a way how to implement mechanism for
>> user-specified events and handlers in GDAL/OGR.
>>
>> I am implementing a Python script which checks VFK file using OGR
>> driver [1]. This script reads VFK data and stores them in SQLite
>> database which is created by the OGR driver.
>>
>> Beside some geometry checks the script should do some extra
>> user-defined checks on the original VFK file. To avoid double reading
>> of the file (first by the script and then by the OGR driver) I would
>> like to use mechanism of events and handlers. In other words, event
>> for reading a new line from the VFK file [2] and the handler (callback
>> subroutine in Python script) which would do some extra checks on the
>> line [3].
>>
>> Any idea how it should be correctly implemented in VFK driver and the
>> Python script? Thanks a lot for any hits in advance!
>>
>> Martin
>>
>> [1] http://gdal.org/ogr/drv_vfk.html
>> [2]
>> http://trac.osgeo.org/gdal/browser/trunk/gdal/ogr/ogrsf_frmts/vfk/vfkreade
>> r.cpp#L125 [3]
>> https://github.com/landam/ogr-vfk-testing/blob/master/read-vfk-test/read-v
>> fk.py#L52