Add support for as_leaflet_layer in Map.add

docs/source/layers/index.rst

@@ -29,4 +29,4 @@ Layers
     choropleth
     vector_tile.rst
     wkt_layer
-
+    layer_like

docs/source/layers/layer_like.rst

@@ -0,0 +1,68 @@
+Layer-Like Objects
+==================
+
+The :func:`ipyleaflet.Map.add` method supports
+"layer-like" objects; meaning any object with an ``as_leaflet_layer`` method.
+This interface can be especially useful for downstream developers who want
+their users to more easily be able to add their objects to an
+:class:`ipyleaflet.Map`.
+
+Example
+-------
+
+Downstream objects should implement an ``as_leaflet_layer`` method that returns
+an ``ipyleaflet`` type capable of being added to the ``Map``.
+
+Here is a simple example of creating a custom data class to hold heatmap data
+(coordinates with some numerical value).
+
+
+.. jupyter-execute::
+
+  import numpy as np
+
+
+  class MyHeatMap:
+      def __init__(self, points, values, radius=20):
+          self.points = points
+          self.values = values
+          self.radius = radius
+
+      @property
+      def data(self):
+          return np.column_stack((self.points, self.values))
+
+      def as_leaflet_layer(self):
+          from ipyleaflet import Heatmap
+          return Heatmap(
+              locations=self.data.tolist(),
+              radius=self.radius,
+          )
+
+We can now use that custom data class and because it has an
+``as_leaflet_layer`` interface, we can pass the object directly to
+:func:`ipyleaflet.Map.add`.
+
+
+.. jupyter-execute::
+
+  from ipyleaflet import Map
+
+  n = 1000
+  data = MyHeatMap(
+      np.random.uniform(-80, 80, (n, 2)),
+      np.random.uniform(0, 1000, n),
+  )
+
+  m = Map(center=(0, 0), zoom=2)
+  m.add(data)
+  m
+
+
+External Examples
+-----------------
+
+The following external libraries are working to implement this new interface
+
+- `localtileserver <https://github.com/banesullivan/localtileserver>`_: a dynamic tile server built for visualizing large geospatial images/rasters with ipyleaflet.
+- `xarray-leaflet <https://github.com/xarray-contrib/xarray_leaflet>`_: an xarray extension for tiled map plotting, based on ipyleaflet.

ipyleaflet/leaflet.py

@@ -1165,11 +1165,15 @@ def add(self, layer):
         Parameters
         ----------
         layer: layer instance
-            The new layer to include in the group.
+            The new layer to include in the group. This can also be an object
+            with an ``as_leaflet_layer`` method which generates a compatible
+            layer type.
         """
 
         if isinstance(layer, dict):
             layer = basemap_to_tiles(layer)
+        elif hasattr(layer, 'as_leaflet_layer'):
+            layer = layer.as_leaflet_layer()
         if layer.model_id in self._layer_ids:
             raise LayerException('layer already in layergroup: %r' % layer)
         self.layers = tuple([layer for layer in self.layers] + [layer])