Just use the already existing change grammar event.

src/display-buffer.coffee

@@ -136,9 +136,6 @@ class DisplayBuffer extends Model
   onDidChangeGrammar: (callback) ->
     @tokenizedBuffer.onDidChangeGrammar(callback)
 
-  onDidUseGrammar: (callback) ->
-    @tokenizedBuffer.onDidUseGrammar(callback)
-
   onDidTokenize: (callback) ->
     @tokenizedBuffer.onDidTokenize(callback)
 

src/text-editor.coffee

@@ -323,9 +323,6 @@ class TextEditor extends Model
   onDidChangeGrammar: (callback) ->
     @emitter.on 'did-change-grammar', callback
 
-  onDidUseGrammar: (callback) ->
-    @displayBuffer.onDidUseGrammar(callback)
-
   # Extended: Calls your `callback` when the result of {::isModified} changes.
   #
   # * `callback` {Function}

src/tokenized-buffer.coffee

@@ -125,12 +125,8 @@ class TokenizedBuffer extends Model
     @disposables.add(@configSubscriptions)
 
     @retokenizeLines()
-    @emitter.emit 'did-use-grammar', grammar
     @emitter.emit 'did-change-grammar', grammar
 
-  onDidUseGrammar: (callback) ->
-    @emitter.on 'did-use-grammar', callback
-
   getGrammarSelectionContent: ->
     @buffer.getTextInRange([[0, 0], [10, 0]])
 

src/workspace.coffee

@@ -550,7 +550,7 @@ class Workspace extends Model
     @project.bufferForPath(filePath, options).then (buffer) =>
       editor = @buildTextEditor(_.extend({buffer, largeFileMode}, options))
       disposable = atom.textEditors.add(editor)
-      grammarSubscription = editor.onDidUseGrammar(@activateGrammar.bind(this))
+      grammarSubscription = editor.onDidChangeGrammar(@activateGrammar.bind(this))
       editor.onDidDestroy ->
         grammarSubscription.dispose()
         disposable.dispose()