Implement completer-specific commands

This provides a framework for completer-writers to create
completer-specific commands. I have in mind to use this for the clang
completer to force reloading of a flags module via `:YcmCompleter reload`.

README.md

@@ -446,6 +446,15 @@ This will print out various debug information for the current file. Useful to
 see what compile commands will be used for the file if you're using the semantic
 completion engine.
 
+### The `YcmCompleter` command
+
+This command can be used to invoke completer-specific commands.  If the first
+argument is of the form `ft=...` the completer for that file type will be used
+(for example `ft=cpp`), else the native completer of the current buffer will be
+used.
+Call `YcmCompleter` without further arguments for information about the
+commands you can call for the selected completer.
+
 Options
 -------
 

autoload/youcompleteme.vim

@@ -484,6 +484,40 @@ endfunction
 
 command! YcmDebugInfo call s:DebugInfo()
 
+function! s:CompleterCommand(...)
+  " CompleterCommand will call the OnUserCommand function of a completer.
+  " If the first arguments is of the form "ft=..." it can be used to specify the
+  " completer to use (for example "ft=cpp").  Else the native filetype completer
+  " of the current buffer is used.  If no native filetype completer is found and
+  " no completer was specified this throws an error.  You can use "ft=ycm:omni"
+  " to select the omni completer or "ft=ycm:ident" to select the identifier
+  " completer.  The remaining arguments will passed to the completer.
+  let arguments = copy(a:000)
+
+  if a:0 > 0 && strpart(a:1, 0, 3) == 'ft='
+    if a:1 == 'ft=ycm:omni'
+      py completer = ycm_state.GetOmniCompleter()
+    elseif a:1 == 'ft=ycm:ident'
+      py completer = ycm_state.GetIdentifierCompleter()
+    else
+      py completer = ycm_state.GetFiletypeCompleterForFiletype(
+                   \ vim.eval('a:1').lstrip('ft=') )
+    endif
+    let arguments = arguments[1:]
+  elseif pyeval( 'ycm_state.NativeFiletypeCompletionAvailable()' )
+    py completer = ycm_state.GetFiletypeCompleter()
+  else
+    echohl WarningMsg |
+      \ echomsg "No native completer found for current buffer." |
+      \ echomsg  "Use ft=... as the first argument to specify a completer." |
+      \ echohl None
+    return
+  endif
+
+  py completer.OnUserCommand( vim.eval( 'l:arguments' ) )
+endfunction
+
+command! -nargs=* YcmCompleter call s:CompleterCommand(<f-args>)
 
 function! s:ForceCompile()
   if !pyeval( 'ycm_state.NativeFiletypeCompletionUsable()' )

python/completers/completer.py

@@ -23,6 +23,7 @@ import vimsupport
 import ycm_core
 from collections import defaultdict
 
+NO_USER_COMMANDS = 'This completer does not define any commands.'
 
 class Completer( object ):
   """A base class for all Completers in YCM.
@@ -96,7 +97,15 @@ class Completer( object ):
   their specific events occur. For instance, the identifier completer collects
   all the identifiers in the file in OnFileReadyToParse() which gets called when
   the user stops typing for 2 seconds (Vim's CursorHold and CursorHoldI events).
-  """
+
+  One special function is OnUserCommand. It is called when the user uses the
+  command :YcmCompleter and is passed all extra arguments used on command
+  invocation (e.g. OnUserCommand(['first argument', 'second'])).  This can be
+  used for completer-specific commands such as reloading external
+  configuration.
+  When the command is called with no arguments you should print a short summary
+  of the supported commands or point the user to the help section where this
+  information can be found."""
 
   __metaclass__ = abc.ABCMeta
 
@@ -234,6 +243,10 @@ class Completer( object ):
     pass
 
 
+  def OnUserCommand( self, arguments ):
+    vimsupport.PostVimMessage( NO_USER_COMMANDS )
+
+
   def OnCurrentIdentifierFinished( self ):
     pass
 

python/ycm.py

@@ -53,6 +53,10 @@ class YouCompleteMe( object ):
     return self.identcomp
 
 
+  def GetOmniCompleter( self ):
+    return self.omnicomp
+
+
   def GetFiletypeCompleter( self ):
     filetypes = vimsupport.CurrentFiletypes()