Fix geojson issue with marker cluster

[fullonic]

This PR fixed #1129
My understating of the cause of this issue is because we are currently adding a geojson with a null value to cluster marker layer and only later adding the geojson data.

folium/folium/features.py

Line 416 in 2122625

var {{ this.get_name() }} = L.geoJson(null, {

And because of this, the following code produces a empty map:

import folium
folium.__version__
  '0.10.0'
from folium.plugins import MarkerCluster
m = folium.Map()
mc = MarkerCluster()
geojson = folium.GeoJson("random_points.geojson")
mc.add_child(geojson)
mc.add_to(m)
m.save("cluster.html")

The cluster.html html code:

(....)

  var marker_cluster_e6eb5734160c404c8ba80bee9e023ff6 = L.markerClusterGroup(
      {}
  );
  map_555133c3ff134e328f848608cd78fce6.addLayer(marker_cluster_e6eb5734160c404c8ba80bee9e023ff6);

function geo_json_93c25184bf6049d2a7b7b92750ddd587_onEachFeature(feature, layer) {
  layer.on({
      click: function(e) {
          map_555133c3ff134e328f848608cd78fce6.fitBounds(e.target.getBounds());
      }
  });
};
var geo_json_93c25184bf6049d2a7b7b92750ddd587 = L.geoJson(null, {
      onEachFeature: geo_json_93c25184bf6049d2a7b7b92750ddd587_onEachFeature,

}).addTo(marker_cluster_e6eb5734160c404c8ba80bee9e023ff6);  # **Here we are adding the empty geojson layer**

geo_json_93c25184bf6049d2a7b7b92750ddd587.addData({GEOJSON_DATA});
(....)

However, with the same python code, if we change the html code and add the geojson data at the same time we create the geojson layer, as bellow, the markers are displayed on map.

var geo_json_93c25184bf6049d2a7b7b92750ddd587 = L.geoJson({**GEOJSON_DATA**}, {
      onEachFeature: geo_json_93c25184bf6049d2a7b7b92750ddd587_onEachFeature,

}).addTo(marker_cluster_e6eb5734160c404c8ba80bee9e023ff6);

This PR fixed this issue and at the same time makes it possible have data not embed on html file when an url is passed as data source.

[Conengmo]

Indeed, linking geojson to marker cluster is broken at the moment. To be honest I didn't know we offered that functionality. Thanks for the well-written problem description.

I tried your fix, but it seems it breaks LayerControl when using GeoJson normally and with non-embedded data. While looking into this I came up with an alternative approach. I'll make a PR, maybe you can check it out, let me know what you think? #1190 Then we can discuss how to move on.

[Conengmo]

The problem with this is that this breaks LayerControl for regular geojson use cases. This variable has to be declared and defined outside of the ajax call.

[fullonic]

Yes, in fact it breaks. I wasn't aware of it. I just tried your PR and works as expected 👍

[Conengmo]

Why do you remove the indentation here?

[fullonic]

May be automatic linting, I didn't see it. Sorry

[Conengmo]

You can link this to the ajax call directly: $.ajax().done(function() {});

[fullonic]

Have you tried? Because the problem I had was related with async ajax request and because of that I used a variable to check if the request was finished. I don't have experience with JS. I just searched about it and I found that solution in a blog post.

[Conengmo]

It seems to work here, and is also listed in the examples in the jQuery documentation, so assume it should be fine.

[Conengmo]

This problem has been fixed in #1190.