Add a User/Preferences API with update service

* Adds documentation surrounding the new parameter
* Reveals the parameter so frontend is able to access it
* Adds a very basic update service to run inside the endpoint

app/controllers/projects/merge_requests_controller.rb

@@ -110,6 +110,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
         @show_whitespace_default = current_user.nil? || current_user.show_whitespace_in_diffs
         @file_by_file_default = current_user&.view_diffs_file_by_file
         @coverage_path = coverage_reports_project_merge_request_path(@project, @merge_request, format: :json) if @merge_request.has_coverage_reports?
+        @update_current_user_path = expose_path(api_v4_user_preferences_path)
         @endpoint_metadata_url = endpoint_metadata_url(@project, @merge_request)
 
         set_pipeline_variables

app/services/user_preferences/update_service.rb

+# frozen_string_literal: true
+
+module UserPreferences
+  class UpdateService < BaseService
+    def initialize(user, params = {})
+      @preferences = user.user_preference
+      @params = params.to_h.dup.with_indifferent_access
+    end
+
+    def execute
+      if @preferences.update(@params)
+        ServiceResponse.success(
+          message: 'Preference was updated',
+          payload: { preferences: @preferences })
+      else
+        ServiceResponse.error(message: 'Could not update preference')
+      end
+    end
+  end
+end

app/views/projects/merge_requests/show.html.haml

@@ -88,6 +88,7 @@
         endpoint_coverage: @coverage_path,
         help_page_path: suggest_changes_help_path,
         current_user_data: @current_user_data,
+        update_current_user_path: @update_current_user_path,
         project_path: project_path(@merge_request.project),
         changes_empty_state_illustration: image_path('illustrations/merge_request_changes_empty.svg'),
         is_fluid_layout: fluid_layout.to_s,

changelogs/unreleased/284116-sync-single-file-mode-user-preference.yml

+---
+title: Create UserPreferences API
+merge_request: 55033
+author:
+type: added

doc/api/users.md

@@ -677,6 +677,28 @@ Example response:
 }
 ```
 
+## User preference modification
+
+Update the current user's preferences.
+
+```plaintext
+PUT /user/preferences
+```
+
+```json
+{
+  "id": 1,
+    "user_id": 1
+      "view_diffs_file_by_file": true
+}
+```
+
+Parameters:
+
+| Attribute                    | Required | Description                                                 |
+| :--------------------------- | :------- | :---------------------------------------------------------- |
+| `view_diffs_file_by_file`    | Yes      | Flag indicating the user sees only one file diff per page.  |
+
 ## Set user status
 
 Set the status of the current user.

lib/api/entities/user_preferences.rb

+# frozen_string_literal: true
+
+module API
+  module Entities
+    class UserPreferences < Grape::Entity
+      expose :id, :user_id, :view_diffs_file_by_file
+    end
+  end
+end

lib/api/users.rb

@@ -998,6 +998,29 @@ module API
         present paginate(current_user.emails), with: Entities::Email
       end
 
+      desc "Update the current user's preferences" do
+        success Entities::UserPreferences
+        detail 'This feature was introduced in GitLab 13.10.'
+      end
+      params do
+        requires :view_diffs_file_by_file, type: Boolean, desc: 'Flag indicating the user sees only one file diff per page'
+      end
+      put "preferences", feature_category: :users do
+        authenticate!
+
+        preferences = current_user.user_preference
+
+        attrs = declared_params(include_missing: false)
+
+        service = ::UserPreferences::UpdateService.new(current_user, attrs).execute
+
+        if service.success?
+          present preferences, with: Entities::UserPreferences
+        else
+          render_api_error!('400 Bad Request', 400)
+        end
+      end
+
       desc 'Get a single email address owned by the currently authenticated user' do
         success Entities::Email
       end

spec/requests/api/users_preferences_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe API::Users do
+  let_it_be(:user) { create(:user) }
+
+  describe 'PUT /user/preferences/' do
+    context "with correct attributes and a logged in user" do
+      it 'returns a success status and the value has been changed' do
+        put api("/user/preferences", user), params: { view_diffs_file_by_file: true }
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response['view_diffs_file_by_file']).to eq(true)
+        expect(user.reload.view_diffs_file_by_file).to be_truthy
+      end
+    end
+
+    context "missing a preference" do
+      it 'returns a bad request status' do
+        put api("/user/preferences", user), params: {}
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+      end
+    end
+
+    context "without a logged in user" do
+      it 'returns an unauthorized status' do
+        put api("/user/preferences"), params: { view_diffs_file_by_file: true }
+
+        expect(response).to have_gitlab_http_status(:unauthorized)
+      end
+    end
+
+    context "with an unsupported preference" do
+      it 'returns a bad parameter' do
+        put api("/user/preferences", user), params: { jawn: true }
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+      end
+    end
+
+    context "with an unsupported value" do
+      it 'returns a bad parameter' do
+        put api("/user/preferences", user), params: { view_diffs_file_by_file: 3 }
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+      end
+    end
+
+    context "with an update service failure" do
+      it 'returns a bad request' do
+        bad_service = double("Failed Service", success?: false)
+
+        allow_next_instance_of(::UserPreferences::UpdateService) do |instance|
+          allow(instance).to receive(:execute).and_return(bad_service)
+        end
+
+        put api("/user/preferences", user), params: { view_diffs_file_by_file: true }
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+      end
+    end
+  end
+end

spec/services/user_preferences/update_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe UserPreferences::UpdateService do
+  let(:user) { create(:user) }
+  let(:params) { { view_diffs_file_by_file: false } }
+
+  describe '#execute' do
+    subject(:service) { described_class.new(user, params) }
+
+    context 'successfully updating the record' do
+      it 'updates the preference and returns a success' do
+        result = service.execute
+
+        expect(result.status).to eq(:success)
+        expect(result.payload[:preferences].view_diffs_file_by_file).to eq(params[:view_diffs_file_by_file])
+      end
+    end
+
+    context 'unsuccessfully updating the record' do
+      before do
+        allow(user.user_preference).to receive(:update).and_return(false)
+      end
+
+      it 'returns an error' do
+        result = service.execute
+
+        expect(result.status).to eq(:error)
+      end
+    end
+  end
+end