Auto merge of #2797 - micbou:echo-diagnostic-async, r=Valloric

[READY] Echo diagnostic asynchronously

If there is a diagnostic on the current line while updating diagnostics, echo it on the command line. Here's a demo:

![echo-diagnostic-async](https://user-images.githubusercontent.com/10026824/31182114-4e784200-a923-11e7-831b-e613d126fd8b.gif)

Without this change, users have to move the cursor up and down to see the message on the command line.

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python/ycm/diagnostic_interface.py

@@ -38,17 +38,15 @@ class DiagnosticInterface( object ):
     self._line_to_diags = defaultdict( list )
     self._placed_signs = []
     self._next_sign_id = 1
-    self._previous_line_number = -1
+    self._previous_diag_line_number = -1
     self._diag_message_needs_clearing = False
 
 
   def OnCursorMoved( self ):
+    if self._user_options[ 'echo_current_diagnostic' ]:
       line, _ = vimsupport.CurrentLineAndColumn()
       line += 1  # Convert to 1-based
-    if line != self._previous_line_number:
-      self._previous_line_number = line
-
-      if self._user_options[ 'echo_current_diagnostic' ]:
+      if line != self._previous_diag_line_number:
         self._EchoDiagnosticForLine( line )
 
 
@@ -72,6 +70,9 @@ class DiagnosticInterface( object ):
                             self._ApplyDiagnosticFilter( diags ) ]
     self._ConvertDiagListToDict()
 
+    if self._user_options[ 'echo_current_diagnostic' ]:
+      self._EchoDiagnostic()
+
     if self._user_options[ 'enable_diagnostic_signs' ]:
       self._UpdateSigns()
 
@@ -88,7 +89,15 @@ class DiagnosticInterface( object ):
     return filter( diag_filter.IsAllowed, diags )
 
 
+  def _EchoDiagnostic( self ):
+    line, _ = vimsupport.CurrentLineAndColumn()
+    line += 1  # Convert to 1-based
+    self._EchoDiagnosticForLine( line )
+
+
   def _EchoDiagnosticForLine( self, line_num ):
+    self._previous_diag_line_number = line_num
+
     diags = self._line_to_diags[ line_num ]
     if not diags:
       if self._diag_message_needs_clearing:
@@ -163,6 +172,9 @@ class DiagnosticInterface( object ):
     new_signs = []
     obsolete_signs = list( self._placed_signs )
     for line, diags in iteritems( self._line_to_diags ):
+      if not diags:
+        continue
+
       # We always go for the first diagnostic on line,
       # because it is sorted giving priority to the Errors.
       diag = diags[ 0 ]

python/ycm/tests/__init__.py

@@ -44,6 +44,7 @@ DEFAULT_CLIENT_OPTIONS = {
   'keep_logfiles': 0,
   'extra_conf_vim_data': [],
   'show_diagnostics_ui': 1,
+  'echo_current_diagnostic': 1,
   'enable_diagnostic_signs': 1,
   'enable_diagnostic_highlighting': 0,
   'always_populate_location_list': 0,

python/ycm/tests/youcompleteme_test.py

@@ -551,6 +551,11 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
       ycm.OnFileReadyToParse()
       ycm.HandleFileParseRequest( block = True )
 
+    # The error on the current line is echoed, not the warning.
+    post_vim_message.assert_called_once_with(
+      "expected ';' after expression (FixIt)",
+      truncate = True, warning = False )
+
     # Error match is added after warning matches.
     assert_that(
       test_utils.VIM_MATCHES,
@@ -566,13 +571,25 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
       call( 'sign place 1 name=YcmError line=3 buffer=5' ),
     ] )
 
-    # When moving the cursor on the diagnostics, the error is displayed to the
-    # user, not the warning.
+  # The error is not echoed again when moving the cursor along the line.
+  with MockVimBuffers( [ current_buffer ], current_buffer, ( 3, 2 ) ):
+    post_vim_message.reset_mock()
     ycm.OnCursorMoved()
-    post_vim_message.assert_has_exact_calls( [
-      call( "expected ';' after expression (FixIt)",
+    post_vim_message.assert_not_called()
+
+  # The error is cleared when moving the cursor to another line.
+  with MockVimBuffers( [ current_buffer ], current_buffer, ( 2, 2 ) ):
+    post_vim_message.reset_mock()
+    ycm.OnCursorMoved()
+    post_vim_message.assert_called_once_with( "", warning = False )
+
+  # The error is echoed when moving the cursor back.
+  with MockVimBuffers( [ current_buffer ], current_buffer, ( 3, 2 ) ):
+    post_vim_message.reset_mock()
+    ycm.OnCursorMoved()
+    post_vim_message.assert_called_once_with(
+      "expected ';' after expression (FixIt)",
       truncate = True, warning = False )
-    ] )
 
     vim_command.reset_mock()
     with patch( 'ycm.client.event_notification.EventNotification.Response',