Split out mock backend, fix out-of-data bug

lms/static/coffee/src/staff_grading/staff_grading.coffee

@@ -8,10 +8,57 @@ state_grading = "have_data"
 state_no_data = "no_data"
 state_error = "error"
 
+class StaffGradingBackend
+  constructor: (ajax_url, mock_backend) ->
+    @ajax_url = ajax_url
+    @mock_backend = mock_backend
+    if @mock_backend
+      @mock_cnt = 0
+
+  mock: (cmd, data) ->
+    # Return a mock response to cmd and data
+    @mock_cnt++
+    if cmd == 'get_next'
+      response =
+        success: true
+        submission: 'submission! ' + @mock_cnt
+        rubric: 'A rubric!' + @mock_cnt
+
+    else if cmd == 'save_grade'
+      response =
+        success: true
+        submission: 'another submission! ' + @mock_cnt
+        rubric: 'A rubric!' + @mock_cnt
+    else
+      response =
+        success: false
+        error: 'Unknown command ' + cmd
+
+    if @mock_cnt % 5 == 0
+        response = 
+          success: true
+          message: 'No more submissions'
+
+
+    if @mock_cnt % 7 == 0
+      response =
+        success: false
+        error: 'An error for testing'
+        
+    return response
+
+
+  post: (cmd, data, callback) ->
+    if @mock_backend
+      callback(@mock(cmd, data))
+    else
+      # TODO: replace with postWithPrefix when that's loaded
+      $.post(@ajax_url + cmd, data, callback)
+
+
 class StaffGrading
-  constructor: (mock_backend) ->
-    @el = $('.staff-grading')
-    @ajax_url = @el.data('ajax_url')
+  constructor: (backend) ->
+    @backend = backend
 
     @error_container = $('.error-container')
     @submission_container = $('.submission-container')
@@ -20,42 +67,12 @@ class StaffGrading
     @button.click @clicked
     @state = state_no_data
 
-    @mock_backend = mock_backend
-    if @mock_backend
-      @mock_cnt = 0
-
     @get_next_submission()
 
-  mock: (cmd, data) ->
-    # Return a mock response to cmd and data
-    @mock_cnt++
-    if cmd == 'get_next'
-      response =
-        'success': true
-        'submission': 'submission! ' + @mock_cnt
-        'rubric': 'A rubric!' + @mock_cnt
-    else if cmd == 'save_grade'
-      response =
-        'success': true
-        'submission': 'another submission! ' + @mock_cnt
-        'rubric': 'A rubric!' + @mock_cnt
-    else
-      response =
-        'success': false
-        'error': 'Unknown command ' + cmd
-    return response
-
 
   set_button_text: (text) ->
     @button.prop('value', text)
 
-  _post: (cmd, data, callback) ->
-    if @mock_backend
-      callback(@mock(cmd, data))
-    else
-      # TODO: replace with postWithPrefix when that's loaded
-      $.post(@ajax_url + cmd, data, callback)
-
 
   ajax_callback: (response) =>
       if response.success
@@ -67,11 +84,11 @@ class StaffGrading
         @error(response.error)
   
   get_next_submission: () ->
-    @_post('get_next', {}, @ajax_callback)
+    @backend.post('get_next', {}, @ajax_callback)
 
   submit_and_get_next: () ->
     data = {eval: '123'}
-    @_post('save_grade', data, @ajax_callback)
+    @backend.post('save_grade', data, @ajax_callback)
 
   error: (msg) ->
     @error_container.html(msg)
@@ -85,8 +102,8 @@ class StaffGrading
     @update()
 
   no_more: () ->
-    @submission_container.html(submission)
-    @rubric_container.html(rubric)
+    @submission_container.html('')
+    @rubric_container.html('')
     @state = state_no_data
     @update()
 
@@ -117,5 +134,8 @@ class StaffGrading
 
 
 # for now, just create an instance and load it...
-mock_backend = true 
-$(document).ready(() -> new StaffGrading(mock_backend))
+mock_backend = true
+ajax_url = $('.staff-grading').data('ajax_url')
+backend = new StaffGradingBackend(ajax_url, mock_backend)
+
+$(document).ready(() -> new StaffGrading(backend))