add the ability to create map pool from the array of layers #12

[stepankuzmin]

This PR will add fromLayers(arr, initOptions) method, wich allows to create map pool from array of Mapnik Layers like this:

var mapnik = require('mapnik'),
    mapnikPool = require('mapnik-pool')(mapnik),
    fs = require('fs');

var datasource = new mapnik.Datasource(...);
var layer = new mapnik.Layer(...);
layer.datasource = datasource;

var pool = mapnikPool.fromLayers([layer], { size: 256, srs: '...' });

pool.acquire(function(err, map) {
    // pooled map
    pool.release(map);
});

See #12 for issue.

[springmeyer]

👋 @stepankuzmin - thanks for providing this PR. I think I understand your usecase, but to make sure let me try to describe what you are doing:

• You want to build up layers dynamically on a map
• You want to leverage a pool for best, threadsafe performance
• One you create a map, you don't need to change it anymore but you want to render from it

If all of those things are true, then this PR looks good. However another alternative would be to:

• Build up the map object like you are doing
• Call map.toXML() to serialize the config to XML
• Then use the existing mapnik-pool (without modifications)

I think this latter idea might sound odd and you might not like XML. However it has the advantage that is is fast and well tested. The node-mapnik API for dynamically adding layers is not a commonly used code pathway whereas the fromString is bombproof.

[stepankuzmin]

Wow, thanks @springmeyer! That's actually what I need! I did the trick like this: https://github.com/stepankuzmin/tilelive-postgis/blob/master/index.js#L40-L47

I didn't find the map.toXML() method in documentation http://mapnik.org/documentation/node-mapnik/, maybe we should add it? Nevertheless, you can close this PR. Thanks!

[springmeyer]

@stepankuzmin glad that helps. I've added docs for map.toXML in https://github.com/mapnik/node-mapnik/commit/89c0d43ce55329cd4bf32887717ad5612e16bbaf.