Close #2285 - Add a function for use with omnifunc

autoload/ale/completion.vim

@@ -169,7 +169,7 @@ function! s:ReplaceCompletionOptions() abort
             let b:ale_old_omnifunc = &l:omnifunc
         endif
 
-        let &l:omnifunc = 'ale#completion#OmniFunc'
+        let &l:omnifunc = 'ale#completion#AutomaticOmniFunc'
     endif
 
     if l:source is# 'ale-automatic'
@@ -235,7 +235,7 @@ function! ale#completion#GetCompletionResult() abort
     return v:null
 endfunction
 
-function! ale#completion#OmniFunc(findstart, base) abort
+function! ale#completion#AutomaticOmniFunc(findstart, base) abort
     if a:findstart
         return ale#completion#GetCompletionPosition()
     else
@@ -279,6 +279,7 @@ function! s:CompletionStillValid(request_id) abort
     \&& (
     \   b:ale_completion_info.column == l:column
     \   || b:ale_completion_info.source is# 'deoplete'
+    \   || b:ale_completion_info.source is# 'ale-omnifunc'
     \)
 endfunction
 
@@ -570,7 +571,7 @@ function! ale#completion#GetCompletions(source) abort
     let l:prefix = ale#completion#GetPrefix(&filetype, l:line, l:column)
 
     if a:source is# 'ale-automatic' && empty(l:prefix)
-        return
+        return 0
     endif
 
     let l:line_length = len(getline('.'))
@@ -584,16 +585,47 @@ function! ale#completion#GetCompletions(source) abort
     \   'request_id': 0,
     \   'source': a:source,
     \}
+    unlet! b:ale_completion_response
+    unlet! b:ale_completion_parser
     unlet! b:ale_completion_result
 
     let l:buffer = bufnr('')
     let l:Callback = function('s:OnReady')
 
+    let l:started = 0
+
     for l:linter in ale#linter#Get(&filetype)
         if !empty(l:linter.lsp)
-            call ale#lsp_linter#StartLSP(l:buffer, l:linter, l:Callback)
+            if ale#lsp_linter#StartLSP(l:buffer, l:linter, l:Callback)
+                let l:started = 1
+            endif
         endif
     endfor
+
+    return l:started
+endfunction
+
+function! ale#completion#OmniFunc(findstart, base) abort
+    if a:findstart
+        let l:started = ale#completion#GetCompletions('ale-omnifunc')
+
+        if !l:started
+            " This is the special value for cancelling completions silently.
+            " See :help complete-functions
+            return -3
+        endif
+
+        return ale#completion#GetCompletionPosition()
+    else
+        let l:result = ale#completion#GetCompletionResult()
+
+        while l:result is v:null && !complete_check()
+            sleep 2ms
+            let l:result = ale#completion#GetCompletionResult()
+        endwhile
+
+        return l:result isnot v:null ? l:result : []
+    endif
 endfunction
 
 function! s:TimerHandler(...) abort