Working with unique value rules

Usually, we do not only style features by what they represent, for example a city or a village, but we style depending on their attributes, such as the number of citizens, year of foundation, and number of squares.

To help on these cases, OpenLayers offers us the possibility to define rules to decide how to style features. For example, we can define a rule that for all features of a city with a population greater than 100,000 a point with radius 20 and color brown can be rendered, while for cities with a population less than 100,000, a point with radius 10, color orange, and semi transparent can be rendered.

Beginning in the world of the rules, the concept of unique value rules are the simplest case we can find. The idea is simple, apply one style or another depending on the value of a feature's attribute.

In this recipe, we are going to load a GeoJSON file, with some cities of the world, and apply a rule that will set the radius of the points depending on the...