In Xcode 3 we could change file type settings, like indicated below:
There are lots of text editors available on Mac OS X. If you shelled out good money for a text editor, you want to use it to edit your source code instead of Xcode's editor. How do you tell Xcode to use your desired text editor to edit your source code files?
Choose Xcode > Preferences to open Xcode's preferences window. There is a toolbar at the top of the window with many buttons. Click the File Types button. From here you can assign the editor to use for lots of file types.
Initially there will be two items: file and folder. Click the disclosure triangle next to file. Click the disclosure triangle next to text because you're interested in text editing. The two most interesting subcategories are sourcecode and scripts.
When you come across a file type you want to edit with your text editor, select the Preferred Editor column for that file type. A menu will pop up. Choose External Editor. If your text editor does not appear in the submenu, choose Other. Navigate to the location of your text editor, and click the OK button. Repeat these steps for every file type you want to edit with your text editor.
Now when you double-click a source code file in Xcode's project window, it will open in your text editor.
But there are no settings for using external editor with Xcode 4.
Is there any way to use custom text editors with Xcode (such as Emacs.app or MacVim)?