A host's generators: styles and colors a design names and the frame computes at render (spec/14 §3.13, D-177).

A design stores a call, generate(:name, args), where a style or a color goes; a host module defines the generator as ordinary Elixir:

defmodule MyApp.Generators do
  use Visualize.Generators

  defgenerator tenant_style(tenant_id, ctx) do
    MyApp.Tenants.theme(tenant_id, ctx.assigns).message_style
  end
end

use Visualize.Generators imports defgenerator/2 and compiles __generators__/0, the {name, arity} of every generator the module defines. A generator's last parameter is the context, %{theme: theme, vars: vars, assigns: assigns}; the others are the design's arguments, literal terms or a datum's readings of its field bindings.

The host registers the module — config :visualize, generators: [MyApp.Generators], or generators: [MyApp.Generators] on Visualize.Chart.from_map/2, Visualize.Chart.apply/2 and the other reads and renders — and a name no registered module defines is a validator fault when the chart is built. A generator in a style position returns a style (a name, a node or a list), one in a color position a typed color (Visualize.Color); any other result applies nothing and is logged. A generator that raises is not rescued.

Examples

iex> import Visualize.Generators, only: [generate: 2]
iex> generate(:tenant_style, [{:field, :tenant}])
{:generate, :tenant_style, [{:field, :tenant}]}