Auto merge of #2312 - micbou:on-buffer-visit-changed, r=puremourning

[READY] Do not send BufferVisit and FileReadyToParse notification events when current allowed buffer has not changed

See discussion in PR #2265 and the commit message.

Since only the `BufEnter` autocommand event is concerned, we are splitting `s:OnBufferVisit` in two functions:
 - `s:OnBufferRead`, identical to current `s:OnBufferVisit`, for the `BufRead` and `FileType` autocommand events;
 - `s:OnBufferEnter` for the `BufEnter` event.

We only send the `BufferVisit` and `FileReadyToParse` event notifications to `ycmd` in `s:OnBufferEnter` when the entered buffer, in which we are allowed to complete, is different from the last one.

Closes #2265.

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autoload/youcompleteme.vim

@@ -28,6 +28,7 @@ let s:old_cursor_position = []
 let s:cursor_moved = 0
 let s:moved_vertically_in_insert_mode = 0
 let s:previous_num_chars_on_current_line = strlen( getline('.') )
+let s:previous_allowed_buffer_number = 0
 
 
 function! s:UsingPython2()
@@ -90,7 +91,8 @@ function! youcompleteme#Enable()
     " the user does :enew and then :set ft=something, we need to run buf init
     " code again.
     autocmd BufReadPre * call s:OnBufferReadPre( expand( '<afile>:p' ) )
-    autocmd BufRead,BufEnter,FileType * call s:OnBufferVisit()
+    autocmd BufRead,FileType * call s:OnBufferRead()
+    autocmd BufEnter * call s:OnBufferEnter()
     autocmd BufUnload * call s:OnBufferUnload( expand( '<afile>:p' ) )
     autocmd CursorHold,CursorHoldI * call s:OnCursorHold()
     autocmd InsertLeave * call s:OnInsertLeave()
@@ -118,7 +120,7 @@ function! youcompleteme#Enable()
   " triggering for the first loaded file. This should be the last commands
   " executed in this function!
   call s:OnBufferReadPre( expand( '<afile>:p' ) )
-  call s:OnBufferVisit()
+  call s:OnBufferRead()
 endfunction
 
 
@@ -324,7 +326,7 @@ function! s:TurnOffSyntasticForCFamily()
 endfunction
 
 
-function! s:AllowedToCompleteInCurrentFile()
+function! s:AllowedToCompleteInCurrentBuffer()
   if empty( &filetype ) ||
         \ getbufvar( winbufnr( winnr() ), "&buftype" ) ==# 'nofile' ||
         \ &filetype ==# 'qf'
@@ -343,6 +345,19 @@ function! s:AllowedToCompleteInCurrentFile()
 endfunction
 
 
+function! s:VisitedBufferRequiresReparse()
+  if !s:AllowedToCompleteInCurrentBuffer()
+    return 0
+  endif
+
+  if bufnr( '' ) ==# s:previous_allowed_buffer_number
+    return 0
+  endif
+  let s:previous_allowed_buffer_number = bufnr( '' )
+  return 1
+endfunction
+
+
 function! s:SetUpCommands()
   command! YcmRestartServer call s:RestartServer()
   command! YcmShowDetailedDiagnostic call s:ShowDetailedDiagnostic()
@@ -427,13 +442,13 @@ function! s:OnBufferReadPre(filename)
   endif
 endfunction
 
-function! s:OnBufferVisit()
+function! s:OnBufferRead()
   " We need to do this even when we are not allowed to complete in the current
-  " file because we might be allowed to complete in the future! The canonical
+  " buffer because we might be allowed to complete in the future! The canonical
   " example is creating a new buffer with :enew and then setting a filetype.
   call s:SetUpYcmChangedTick()
 
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -449,8 +464,18 @@ function! s:OnBufferVisit()
 endfunction
 
 
+function! s:OnBufferEnter()
+  if !s:VisitedBufferRequiresReparse()
+    return
+  endif
+
+  exec s:python_command "ycm_state.OnBufferVisit()"
+  call s:OnFileReadyToParse()
+endfunction
+
+
 function! s:OnBufferUnload( deleted_buffer_file )
-  if !s:AllowedToCompleteInCurrentFile() || empty( a:deleted_buffer_file )
+  if !s:AllowedToCompleteInCurrentBuffer() || empty( a:deleted_buffer_file )
     return
   endif
 
@@ -460,7 +485,7 @@ endfunction
 
 
 function! s:OnCursorHold()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -509,7 +534,7 @@ function! s:SetOmnicompleteFunc()
 endfunction
 
 function! s:OnCursorMovedInsertMode()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -546,7 +571,7 @@ endfunction
 
 
 function! s:OnCursorMovedNormalMode()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -556,7 +581,7 @@ endfunction
 
 
 function! s:OnInsertLeave()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -573,7 +598,7 @@ endfunction
 function! s:OnInsertEnter()
   let s:previous_num_chars_on_current_line = strlen( getline('.') )
 
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif