Set asset finder from callable

It fixes jamesmartin#151 by allowing the asset finder to be a callable object. This is
useful when the asset pipeline is not ready when the initializer is run, for
example, when using Propshaft.

I don't know if this is the best way to solve this problem. On the one hand,
Propshaft could be done when its initializer is run, but on the other hand,
this gem's railtie assumes the asset pipeline is ready.

lib/inline_svg.rb

@@ -11,14 +11,15 @@
 
 require "inline_svg/railtie" if defined?(Rails)
 require 'active_support'
+require 'active_support/core_ext/object/blank'
 require 'active_support/core_ext/string'
 require 'nokogiri'
 
 module InlineSvg
   class Configuration
     class Invalid < ArgumentError; end
 
-    attr_reader :asset_file, :asset_finder, :custom_transformations, :svg_not_found_css_class
+    attr_reader :asset_file, :custom_transformations, :svg_not_found_css_class
 
     def initialize
       @custom_transformations = {}
@@ -40,8 +41,15 @@ def asset_file=(custom_asset_file)
       end
     end
 
+    def asset_finder
+      set_asset_finder_from_callable
+      @asset_finder
+    end
+
     def asset_finder=(finder)
-      @asset_finder = if finder.respond_to?(:find_asset)
+      @asset_finder = if finder.respond_to?(:call)
+                        finder
+                      elsif finder.respond_to?(:find_asset)
                         finder
                       elsif finder.class.name == "Propshaft::Assembly"
                         InlineSvg::PropshaftAssetFinder
@@ -51,7 +59,6 @@ def asset_finder=(finder)
                         # See: https://github.com/jamesmartin/inline_svg/issues/25
                         InlineSvg::StaticAssetFinder
                       end
-      asset_finder
     end
 
     def svg_not_found_css_class=(css_class)
@@ -81,6 +88,11 @@ def incompatible_transformation?(klass)
       !klass.is_a?(Class) || !klass.respond_to?(:create_with_value) || !klass.instance_methods.include?(:transform)
     end
 
+    def set_asset_finder_from_callable
+      while @asset_finder&.respond_to?(:call)
+        self.asset_finder = @asset_finder.call
+      end
+    end
   end
 
   @configuration = InlineSvg::Configuration.new

lib/inline_svg/railtie.rb

@@ -10,14 +10,12 @@ class Railtie < ::Rails::Railtie
 
     config.after_initialize do |app|
       InlineSvg.configure do |config|
-        # Configure the asset_finder:
-        # Only set this when a user-configured asset finder has not been
-        # configured already.
-        if config.asset_finder.nil?
-          # In default Rails apps, this will be a fully operational
-          # Sprockets::Environment instance
-          config.asset_finder = app.instance_variable_get(:@assets)
-        end
+        # Configure an asset finder for Rails. This will be evaluated when the
+        # first SVG is rendered, giving time to the asset pipeline to be done
+        # loading.
+        config.asset_finder = proc {
+          app.instance_variable_get(:@assets)
+        }
       end
     end
   end

spec/inline_svg_spec.rb

@@ -19,6 +19,10 @@ def self.named(filename); end
 
 describe InlineSvg do
   describe "configuration" do
+    before do
+      InlineSvg.reset_configuration!
+    end
+
     context "when a block is not given" do
       it "complains" do
         expect do
@@ -29,14 +33,24 @@ def self.named(filename); end
 
     context "asset finder" do
       it "allows an asset finder to be assigned" do
-        sprockets = double('SomethingLikeSprockets', find_asset: 'some asset')
+        sprockets = double("Something like sprockets", find_asset: "some asset")
         InlineSvg.configure do |config|
           config.asset_finder = sprockets
         end
 
         expect(InlineSvg.configuration.asset_finder).to eq sprockets
       end
 
+      it "allows to give a callable object that returns an asset finder" do
+        propshaft = double("Something like propshaft", class: double(name: "Propshaft::Assembly"))
+        callable = -> { propshaft }
+        InlineSvg.configure do |config|
+          config.asset_finder = callable
+        end
+
+        expect(InlineSvg.configuration.asset_finder).to eq InlineSvg::PropshaftAssetFinder
+      end
+
       it "falls back to StaticAssetFinder when the provided asset finder does not implement #find_asset" do
         InlineSvg.configure do |config|
           config.asset_finder = 'Not a real asset finder'