Execute block-based filters within object scope (#142)

* Execute block-based filters within object scope

* Switch to using instance_exec

bridgetown-builder/lib/bridgetown-builder/dsl/liquid.rb

@@ -5,10 +5,18 @@ module Builders
     module DSL
       module Liquid
         def liquid_filter(filter_name, method_name = nil, &block)
-          block = method(method_name) if method_name.is_a?(Symbol)
-
+          builder_self = self
           m = Module.new
-          m.send(:define_method, filter_name, &block)
+
+          if block
+            m.define_method filter_name do |*args|
+              builder_self.instance_exec(*args, &block)
+            end
+          elsif method_name
+            block = method(method_name)
+            m.define_method filter_name, &block
+          end
+
           ::Liquid::Template.register_filter(m)
 
           functions << { name: name, filter: m }

bridgetown-builder/test/test_filters_dsl.rb

@@ -16,6 +16,10 @@ def build
       value = input * multiply_by
       add_by ? value + add_by : value
     end
+
+    liquid_filter "site_config" do |input|
+      input.to_s + " #{site.root_dir}"
+    end
   end
 end
 
@@ -48,5 +52,11 @@ class TestFilterDSL < BridgetownUnitTest
       result = Liquid::Template.parse(content).render
       assert_equal "5 times 10 plus 3 equals 53", result
     end
+
+    should "allow access to local builder scope" do
+      content = "root_dir: {{ 'is' | site_config }}"
+      result = Liquid::Template.parse(content).render
+      assert_equal "root_dir: is #{@site.root_dir}", result
+    end
   end
 end