utility method to rescue and replace LoadError

Author: timdiggins

lib/html/pipeline.rb

@@ -44,6 +44,12 @@ class Pipeline
     autoload :TextFilter,            'html/pipeline/text_filter'
 
     class MissingDependencyError < LoadError; end
+    def self.require_dependency(name, requirer)
+      require name
+    rescue LoadError => e
+      raise MissingDependencyError,
+        "Missing dependency '#{name}' for #{requirer}. See README.md for details.\n#{e.class.name}: #{e}"
+    end
 
     # Our DOM implementation.
     DocumentFragment = Nokogiri::HTML::DocumentFragment

test/html/pipeline/require_helper_test.rb

@@ -0,0 +1,29 @@
+# encoding: utf-8
+require "test_helper"
+
+class HTML::Pipeline::RequireHelperTest < Minitest::Test
+
+  def test_works_with_existing
+    HTML::Pipeline.require_dependency('rake', 'SomeClass')
+  end
+
+  def test_raises_mising_dependency_error
+    assert_raises HTML::Pipeline::MissingDependencyError do
+      HTML::Pipeline.require_dependency('non-existant', 'SomeClass')
+    end
+  end
+
+  def test_raises_error_including_message
+    error = assert_raises(LoadError) do
+      HTML::Pipeline.require_dependency('non-existant', 'SomeClass')
+    end
+    assert_includes(error.message, "Missing dependency 'non-existant' for SomeClass. See README.md for details.")
+  end
+
+  def test_raises_error_includes_underlying_message
+    error = assert_raises LoadError do
+      HTML::Pipeline.require_dependency('non-existant', 'SomeClass')
+    end
+    assert_includes(error.message, "LoadError: cannot load such file")
+  end
+end