Smart Indentation for Visual Studio Extensibility projects

I said previously in my Brace Matching post that I want to try to document some of my findings while working on BooLangStudio. Well this is my second post on the subject.

When you’re implementing a custom language in Visual Studio, there’s a very good chance that you’re going to want to handle indentation slightly differently to the defaults. Every language has it’s own rules, after all.

Most of the resources I found online we’re pretty poor for how to get this working. Most people we’re pointing to overriding OnCommand in your derived Source class, or implementing some interop interface (i.e. IVsLanguageLineIndent). I could get none of these working. OnCommand never got raised, and the interfaces we’re useless.

I tried a few different methods for handling indentation, but none of them worked very well. I tried capturing the enter key press, but that didn’t work. Then I tried capturing alterations to the document, but those also got fired for navigating the document, so you’d press the up key and add a new line!

It ended up being pretty simple to implement, once I finally found the correct way to do it. I’ll cover how I did this for BooLangStudio below.

In your LanguageService, there’s a method you can override called CreateViewFilter which sets everything in motion.

Create yourself a class that derives from ViewFilter, and then return an instance of it from the overridden CreateViewFilter method in your language service.

public class BooViewFilter : ViewFilter
{
  public BooViewFilter(CodeWindowManager mgr, IVsTextView view) : base(mgr, view)
  {}
}

public override ViewFilter CreateViewFilter(CodeWindowManager mgr, IVsTextView newView)
{
  return new BooViewFilter(mgr, newView);
}

Note: You need to make sure your project is configured to use Smart Indentation. If your project is complete enough to allow the user to customise this, then you’re fine. However, if not you can hard code this value yourself. In your language service there’s a GetLanguagePreferences method that returns all the preferences for your project. In that method you can set languagePreferences.IndentStyle = IdentingStyle.Smart, which is what I’ve done in BooLangStudio.

You’ll kick yourself for how easy this is. Now override the HandleSmartIndent method in your derived ViewFilter. That’s it really, in there you can access the Source object and do as you wish with smart indentation.

Taking BooLangStudio as an example, you can see in our BooViewFilter.cs that I delegate the work to a HandleSmartIndentAction. This is to make testing easier by having as little dependencies on Visual Studio as possible.

The Execute method of our HandleSmartIndentAction.cs class gets the caret location and passes it to an instance of a LineIndenter class, which determines (based on the previous line to the caret) whether the next line should be indented, or outdented.

public bool Execute()
{
  int line, col;

  view.GetCaretPos(out line, out col);

  indenter.SetIndentationForNextLine(line);

  return false;
}

class MyClass:
  def MethodName(): # indent
    return # outdent

So that’s how to implement Smart Indentation in your Visual Studio Extensibility project, and a little bit of implementation details of BooLangStudio.