fix #1165: insert mode lag (#1552)

* fix #1165: insert mode lag

Ultisnips was traversing the whole runtimepath every time an expansion
was attempted, something that happens for every key press since there is
autotrigger.

Even without autotrigger, traversal was done at every snippet expansion,
which caused slowdowns in that case.

With this change, traversal happens only the first time, or after
a source must be refreshed (because a snippets file has been updated).

autoload/UltiSnips.vim

@@ -3,6 +3,9 @@ if exists("b:did_autoload_ultisnips")
 endif
 let b:did_autoload_ultisnips = 1
 
+" Ensure snippets are loaded for current buffer
+au UltiSnips_AutoTrigger FileType,BufEnter * call UltiSnips#CheckFiletype()
+
 " Also import vim as we expect it to be imported in many places.
 py3 import vim
 py3 from UltiSnips import UltiSnips_Manager
@@ -188,6 +191,10 @@ function! UltiSnips#TrackChange() abort
     py3 UltiSnips_Manager._track_change()
 endfunction
 
+function! UltiSnips#CheckFiletype() abort
+    py3 UltiSnips_Manager._check_filetype(vim.eval('&ft'))
+endfunction
+
 function! UltiSnips#RefreshSnippets() abort
     py3 UltiSnips_Manager._refresh_snippets()
 endfunction

pythonx/UltiSnips/snippet/source/base.py

@@ -15,6 +15,7 @@ class SnippetSource:
     def __init__(self):
         self._snippets = defaultdict(SnippetDictionary)
         self._extends = defaultdict(set)
+        self.must_ensure = True
 
     def ensure(self, filetypes):
         """Ensures that snippets are loaded."""

pythonx/UltiSnips/snippet/source/file/base.py

@@ -27,9 +27,11 @@ def __init__(self):
         SnippetSource.__init__(self)
 
     def ensure(self, filetypes):
-        for ft in self.get_deep_extends(filetypes):
-            if self._needs_update(ft):
-                self._load_snippets_for(ft)
+        if self.must_ensure:
+            for ft in self.get_deep_extends(filetypes):
+                if self._needs_update(ft):
+                    self._load_snippets_for(ft)
+            self.must_ensure = False
 
     def refresh(self):
         self.__init__()

pythonx/UltiSnips/snippet_manager.py

@@ -130,6 +130,7 @@ def __init__(self, expand_trigger, forward_trigger, backward_trigger):
         self._visual_content = VisualContentPreserver()
 
         self._snippet_sources = []
+        self._filetypes = []
 
         self._snip_expanded_in_action = False
         self._inside_action = False
@@ -984,6 +985,15 @@ def _refresh_snippets(self):
             source.refresh()
 
 
+    @err_to_scratch_buffer.wrap
+    def _check_filetype(self, ft):
+        """Ensure snippets are loaded for the current filetype."""
+        if ft not in self._filetypes:
+            self._filetypes.append(ft)
+            for _, source in self._snippet_sources:
+                source.must_ensure = True
+
+
 UltiSnips_Manager = SnippetManager(  # pylint:disable=invalid-name
     vim.eval("g:UltiSnipsExpandTrigger"),
     vim.eval("g:UltiSnipsJumpForwardTrigger"),