Logging a warning, if a file is skipped because it would be empty while generating a loc drop.

README.md

@@ -128,7 +128,7 @@ This command reads in a folder containing many `.strings` or `.xml` files. These
 
 #### `generate-loc-drop`
 
-This command is a convenient way to generate a zip file containing files created by the `generate-string-file` command. It is often used for creating a single zip containing a large number of strings in all languages which you can then hand off to your translation team.
+This command is a convenient way to generate a zip file containing files created by the `generate-string-file` command. If a file would not contain any translated strings, it is skipped and a warning is logged to `stderr`. This command can be used to create a single zip containing a large number of strings in all languages which you can then hand off to your translation team.
 
 	$ twine generate-loc-drop /path/to/strings.txt LocDrop1.zip
 	$ twine generate-loc-drop /path/to/strings.txt LocDrop2.zip --lang en,fr,ja,ko --tags common,app1

lib/twine/runner.rb

@@ -170,9 +170,13 @@ module Twine
               file_name = lang + formatter.extension
               temp_path = File.join(temp_dir, file_name)
               zip_path = File.join('Locales', file_name)
+
               output = formatter.format_file(lang)
-              next unless output
-              # TODO: report warning unless output
+              unless output
+                Twine::stderr.puts "Skipping file #{file_name} since it would not contain any strings."
+                next
+              end
+              
               IO.write(temp_path, output, encoding: encoding)
               zipfile.add(zip_path, temp_path)
             end

test/test_generate_loc_drop.rb

@@ -1,30 +1,30 @@
 require 'command_test_case'
 
 class TestGenerateLocDrop < CommandTestCase
-  def setup
-    super
-
+  def new_runner(twine_file = nil)
     options = {}
     options[:output_path] = @output_path
     options[:format] = 'apple'
 
-    @twine_file = build_twine_file 'en', 'fr' do
-      add_section 'Section' do
-        add_row key: 'value'
+    unless twine_file
+      twine_file = build_twine_file 'en', 'fr' do
+        add_section 'Section' do
+          add_row key: 'value'
+        end
       end
     end
 
-    @runner = Twine::Runner.new(options, @twine_file)
+    Twine::Runner.new(options, twine_file)
   end
 
   def test_generates_zip_file
-    @runner.generate_loc_drop
+    new_runner.generate_loc_drop
 
     assert File.exists?(@output_path), "zip file should exist"
   end
 
   def test_zip_file_structure
-    @runner.generate_loc_drop
+    new_runner.generate_loc_drop
 
     names = []
     Zip::File.open(@output_path) do |zipfile|
@@ -39,7 +39,13 @@ class TestGenerateLocDrop < CommandTestCase
     formatter = prepare_mock_formatter Twine::Formatters::Apple
     formatter.expects(:format_file).twice
 
-    @runner.generate_loc_drop
+    new_runner.generate_loc_drop
+  end
+
+  def test_prints_empty_file_warnings
+    empty_twine_file = build_twine_file('en') {}
+    new_runner(empty_twine_file).generate_loc_drop
+    assert_match "Skipping file", Twine::stderr.string
   end
 
   class TestValidate < CommandTestCase