DevelopingPlugins

Developing Mapnik Plugins

Understanding the Architecture

Plugins can be used from C++ to read in different kinds of files. Here's an example of C++ code that implicitly uses the shape plugin to read in a ESRI Shapefile:

    std::string mapnik_dir(argv[1]); // assume mapnik home directory, such as "~/src/mapnik" passed in
    std::string plugins_dir(mapnik_dir + "/plugins/input/");
    datasource_cache::instance()->register_datasources(plugins_dir + "shape"); // ESRI SHP support
    datasource_cache::instance()->register_datasources(plugins_dir + "postgis"); // PostGIS integration

and used like so,

     {
         parameters p;
         p["type"]="shape";
         p["file"]="../data/statesp020"; // State Boundaries of the United States [SHP]
    
         Layer lyr("Cali");
         lyr.set_datasource(datasource_cache::instance()->create(p)); // Note use of datasource_cache factory method here!
         lyr.add_style("cali"); // style.xml
         lyr.add_style("elsewhere"); // this file
         m.addLayer(lyr);
     }

Let's drill-down into what's actually going on. We constructed a parameter object and passed it to a factory method (datasource_cache::instance()->create(p)). It then returned a shared pointer to a datasource object, which was then passed on a new Layer object.

In C++, the parameters object is-a param_map (a std::map from string keys to "value_holder"s, where a value_holder is a boost::variant that can either hold a double or a string. In other words, "parameters p" is a semi-generic parameter hash, then passed to and read by a factory method.

To specify the kind of datasource, note that we specified p's "type" as "shape". Other options might be "osm", "postgis", or "raster".

Examples

See the hello world input plugin for an example to get started.