Add completefunc (#283)

* Add vsnip#completefunc()

* Add test for vsnip#completefunc()

* Add vsnip#completefunc to document

Author: hokorobi

README.md

@@ -23,6 +23,9 @@ VSCode(LSP)'s snippet feature in vim/nvim.
     - [vimcomplete](https://github.com/girishji/vimcomplete)
     - [ddc.vim](https://github.com/Shougo/ddc.vim)
     - [vim-easycompletion](https://github.com/jayli/vim-easycomplete)
+- Support built-in completion
+  - A function can be registered for finding completions.
+    `set complete+=Fvsnip#completefunc`
 - Vim script interpolation
   - You can use Vim script interpolation as `${VIM:...Vim script expression...}`.
 - SnipMate-like syntax support

autoload/vsnip.vim

@@ -166,6 +166,25 @@ function! vsnip#get_context() abort
   return {}
 endfunction
 
+"
+" vsnip#completefunc
+"
+function! vsnip#completefunc(findstart, base) abort
+  if !a:findstart
+    if a:base ==# ''
+      return []
+    endif
+    return vsnip#get_complete_items(bufnr('%'))
+  endif
+
+  let line = getline('.')
+  let start = col('.') - 2
+  while start >= 0 && line[start] =~# '\k'
+    let start -= 1
+  endwhile
+  return start + 1
+endfunction
+
 "
 " vsnip#get_complete_items
 "

doc/vsnip.txt

@@ -117,7 +117,12 @@ FUNCTION                                                      *vsnip-function*
 
     Register your own custom variable resolver.
 
+  vsnip#completefunc()
 
+    Register for finding completions.
+>
+    set complete+=Fvsnip#completefunc
+<
 
 ==============================================================================
 MAPPING                                                        *vsnip-mapping*

spec/autoload/vsnip.vimspec

@@ -145,5 +145,50 @@ Describe vsnip
 
   End
 
+  Describe #completefunc
+
+    It should return start position
+      enew!
+      set filetype=basic_spec
+      call setline(1, ' if')
+      call cursor([1, 3])
+      call s:expect(vsnip#completefunc(1, '')).to_equal(1)
+    End
+
+    It should return complete items
+      enew!
+      set filetype=basic_spec
+      call s:expect(vsnip#completefunc(0, 'if')[-1]).to_equal({
+      \   'word': 'if',
+      \   'abbr': 'if',
+      \   'kind': 'Snippet',
+      \   'menu': '[v] if',
+      \   'dup': 1,
+      \   'user_data': json_encode({
+      \     'vsnip': {
+      \       'snippet': [
+      \         "if ${1:condition}",
+      \         "\t$0",
+      \         "endif",
+      \       ]
+      \     }
+      \   })
+      \ }, {
+      \   'word': 'inline-fn',
+      \   'abbr': 'inline-fn',
+      \   'kind': 'Snippet',
+      \   'menu': '[v] inline-fn',
+      \   'dup': 1,
+      \   'user_data': json_encode({
+      \     'vsnip': {
+      \       'snippet': [
+      \         "{ -> $1 }$0"
+      \       ]
+      \     }
+      \   })
+      \ })
+    End
+  End
+
 End