Move collapsed partial into a diff viewer

We have a concept of diff viewers, which renders diff
file differently depending on the type of this diff file

Following this concept, we can move rendering of a
collapsed diff file into a diff viewer and avoid
unnecessary calculation of a diff file type

app/models/diff_viewer/collapsed.rb

+# frozen_string_literal: true
+
+module DiffViewer
+  class Collapsed < Base
+    include Simple
+    include Static
+
+    self.partial_name = 'collapsed'
+  end
+end

app/views/projects/diffs/_viewer.html.haml

@@ -3,8 +3,6 @@
 .diff-viewer{ data: { type: viewer.type }, class: ('hidden' if hidden) }
   - if viewer.render_error
     = render 'projects/diffs/render_error', viewer: viewer
-  - elsif viewer.collapsed?
-    = render 'projects/diffs/collapsed', viewer: viewer
   - else
     - viewer.prepare!
 

lib/gitlab/diff/file.rb

@@ -358,6 +358,10 @@ module Gitlab
         end
       end
 
+      def modified_file?
+        new_file? || deleted_file? || content_changed?
+      end
+
       # We can't use Object#try because Blob doesn't inherit from Object, but
       # from BasicObject (via SimpleDelegator).
       def try_blobs(meth)
@@ -393,33 +397,16 @@ module Gitlab
       end
 
       def simple_viewer_class
+        return DiffViewer::Collapsed if collapsed?
         return DiffViewer::NotDiffable unless diffable?
+        return DiffViewer::Text if modified_file? && text?
+        return DiffViewer::NoPreview if content_changed?
+        return DiffViewer::Added if new_file?
+        return DiffViewer::Deleted if deleted_file?
+        return DiffViewer::Renamed if renamed_file?
+        return DiffViewer::ModeChanged if mode_changed?
 
-        if content_changed?
-          if text?
-            DiffViewer::Text
-          else
-            DiffViewer::NoPreview
-          end
-        elsif new_file?
-          if text?
-            DiffViewer::Text
-          else
-            DiffViewer::Added
-          end
-        elsif deleted_file?
-          if text?
-            DiffViewer::Text
-          else
-            DiffViewer::Deleted
-          end
-        elsif renamed_file?
-          DiffViewer::Renamed
-        elsif mode_changed?
-          DiffViewer::ModeChanged
-        else
-          DiffViewer::NoPreview
-        end
+        DiffViewer::NoPreview
       end
 
       def rich_viewer_class
@@ -427,8 +414,9 @@ module Gitlab
       end
 
       def viewer_class_from(classes)
+        return if collapsed?
         return unless diffable?
-        return unless new_file? || deleted_file? || content_changed?
+        return unless modified_file?
         return if different_type? || external_storage_error?
 
         verify_binary = !stored_externally?

spec/lib/gitlab/diff/file_spec.rb

@@ -347,6 +347,16 @@ describe Gitlab::Diff::File do
   end
 
   describe '#simple_viewer' do
+    context 'when the file is collapsed' do
+      before do
+        allow(diff_file).to receive(:collapsed?).and_return(true)
+      end
+
+      it 'returns a Collapsed viewer' do
+        expect(diff_file.simple_viewer).to be_a(DiffViewer::Collapsed)
+      end
+    end
+
     context 'when the file is not diffable' do
       before do
         allow(diff_file).to receive(:diffable?).and_return(false)

spec/views/projects/diffs/_viewer.html.haml_spec.rb

@@ -9,15 +9,7 @@ describe 'projects/diffs/_viewer.html.haml' do
   let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
   let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
 
-  let(:viewer_class) do
-    Class.new(DiffViewer::Base) do
-      include DiffViewer::Rich
-
-      self.partial_name = 'text'
-    end
-  end
-
-  let(:viewer) { viewer_class.new(diff_file) }
+  let(:viewer) { diff_file.simple_viewer }
 
   before do
     assign(:project, project)
@@ -53,7 +45,7 @@ describe 'projects/diffs/_viewer.html.haml' do
     it 'renders the collapsed view' do
       render_view
 
-      expect(view).to render_template('projects/diffs/_collapsed')
+      expect(view).to render_template('projects/diffs/viewers/_collapsed')
     end
   end