When the active edit session is modified, and its file is then modified by the file system, it displays an alert.

spec/app/editor-spec.coffee

@@ -110,6 +110,27 @@ describe "Editor", ->
       expect(editor.isFocused).toBeFalsy()
       expect(editor).not.toHaveClass('focused')
 
+  describe "when the activeEditSession's file is modified on disk", ->
+    it "triggers an alert", ->
+      path = "/tmp/atom-changed-file.txt"
+      fs.write(path, "")
+      editSession = project.buildEditSessionForPath(path)
+      editor.edit(editSession)
+      editor.insertText("now the buffer is modified")
+
+      fileChangeHandler = jasmine.createSpy('fileChange')
+      editSession.buffer.file.on 'contents-change', fileChangeHandler
+
+      spyOn($native, "alert")
+
+      fs.write(path, "a file change")
+
+      waitsFor "file to trigger contents-change event", ->
+        fileChangeHandler.callCount > 0
+
+      runs ->
+        expect($native.alert).toHaveBeenCalled()
+
   describe ".remove()", ->
     it "removes subscriptions from all edit session buffers", ->
       previousEditSession = editor.activeEditSession
@@ -227,6 +248,26 @@ describe "Editor", ->
         editor.activeEditSession.selectToEndOfLine()
         expect(editor.getSelectionView().find('.selection')).toExist()
 
+      it "triggers alert if edit session's file changed on disk", ->
+        path = "/tmp/atom-changed-file.txt"
+        fs.write(path, "")
+        editSession = project.buildEditSessionForPath(path)
+        editSession.insertText("a buffer change")
+
+        fileChangeHandler = jasmine.createSpy('fileChange')
+        editSession.buffer.file.on 'contents-change', fileChangeHandler
+
+        spyOn($native, "alert")
+
+        fs.write(path, "a file change")
+
+        waitsFor "file to trigger contents-change event", ->
+          fileChangeHandler.callCount > 0
+
+        runs ->
+          editor.edit(editSession)
+          expect($native.alert).toHaveBeenCalled()
+
     describe ".loadNextEditSession()", ->
       it "loads the next editor state and wraps to beginning when end is reached", ->
         expect(editor.activeEditSession).toBe session2

src/app/buffer.coffee

@@ -81,6 +81,13 @@ class Buffer
     @file?.off()
     @file = new File(path)
     @subscribeToFile()
+    @file.on "contents-change", =>
+      if @isModified()
+        @modifiedOnDisk = true
+        @trigger "contents-change-on-disk"
+      else
+        @setText(fs.read(@file.getPath()))
+        @modified = false
     @trigger "path-change", this
 
   getExtension: ->

src/app/edit-session.coffee

@@ -44,7 +44,10 @@ class EditSession
 
     @buffer.retain()
     @buffer.on "path-change.edit-session-#{@id}", =>
-      @trigger 'buffer-path-change'
+      @trigger "buffer-path-change"
+
+    @buffer.on "contents-change-on-disk.edit-session-#{@id}", =>
+      @trigger "buffer-contents-change-on-disk"
 
     @buffer.on "update-anchors-after-change.edit-session-#{@id}", =>
       @mergeCursors()

src/app/editor.coffee

@@ -386,10 +386,26 @@ class Editor extends View
 
     @activeEditSession = @editSessions[index]
 
-    @activeEditSession.on "buffer-path-change", => @trigger 'editor-path-change'
+    if @activeEditSession.buffer.isModifiedOnDisk() and @activeEditSession.buffer.isModified()
+      @alertEditSessionChangedOnDisk(@activeEditSession)
+
+    @activeEditSession.on "buffer-contents-change-on-disk", =>
+      @alertEditSessionChangedOnDisk(@activeEditSession)
+
+    @activeEditSession.on "buffer-path-change", =>
+      @trigger 'editor-path-change'
+
     @trigger 'editor-path-change'
     @renderWhenAttached()
 
+  alertEditSessionChangedOnDisk: (editSession) ->
+    message = editSession.getPath()
+    detailedMessage = "Has changed on disk. Do you want to reload it?"
+    Native.alert message, detailedMessage, [
+      ["Reload", => editSession.buffer.reload()]
+      ["Cancel", => ],
+    ]
+
   activateEditSessionForPath: (path) ->
     for editSession, index in @editSessions
       if editSession.buffer.getPath() == path