Ensure required context values are present

lib/html/pipeline/emoji_filter.rb

@@ -31,8 +31,16 @@ def emoji_image_filter(text)
 
       text.gsub EmojiPattern do |match|
         name = $1
-        "<img class='emoji' title=':#{name}:' alt=':#{name}:' src='#{File.join(context[:asset_root], "emoji", "#{name}.png")}' height='20' width='20' align='absmiddle' />"
+        "<img class='emoji' title=':#{name}:' alt=':#{name}:' src='#{File.join(asset_root, "emoji", "#{name}.png")}' height='20' width='20' align='absmiddle' />"
       end
     end
+
+    # The base url to link emoji sprites
+    #
+    # Raises ArgumentError if context option has not been provided.
+    # Returns the context's asset_root.
+    def asset_root
+      context[:asset_root] or raise ArgumentError, "Missing context :asset_root"
+    end
   end
 end

lib/html/pipeline/version.rb

@@ -1,5 +1,5 @@
 module HTML
   module Pipeline
-    VERSION = "0.0.2"
+    VERSION = "0.0.3"
   end
 end

test/html/pipeline/emoji_filter_test.rb

@@ -6,4 +6,11 @@ def test_emojify
     doc = filter.call
     assert_match "https://foo.com/emoji/shipit.png", doc.search('img').attr('src').value
   end
+
+  def test_missing_context
+    filter = HTML::Pipeline::EmojiFilter.new("<p>:shipit:</p>", {})
+    assert_raises ArgumentError do
+      filter.call
+    end
+  end
 end