Get the current filename from a Visual Studio text adornment extension

Question

I'm new to VS extension development. I'm currently working with the text adornment sample in VS 2015 and have been able to get coloured boxes showing correctly. Now I want to extend the sample so the adornment only appears on certain file names.

Googling has said I can use ITextDocumentFactoryService.TryGetTextDocument interface with the IWpfTextView.TextBuffer property to get a filename. This sounds great. But I can't seem to actually get the interface.

In my class I have:

    [Import]
    public ITextDocumentFactoryService TextDocumentFactoryService = null;

But it is always NULL.

How can I get ITextDocumentFactoryService?

namespace Test
{
    internal sealed class TestAdornment
    {
        [Import]
        public ITextDocumentFactoryService TextDocumentFactoryService = null;

        public TestAdornment(IWpfTextView view)
        {
        }

        /// <summary>
        /// Adds the scarlet box behind the 'a' characters within the given line
        /// </summary>
        /// <param name="line">Line to add the adornments</param>
        private void CreateVisuals(ITextViewLine line)
        {
            // TextDocumentFactoryService is NULL
        }
    }
}

Answer 1

TextAdornmentTextViewCreationListener.cs

[Export(typeof(IWpfTextViewCreationListener))]
[ContentType("text")]
[TextViewRole(PredefinedTextViewRoles.Document)]
internal sealed class TextAdornmentTextViewCreationListener : IWpfTextViewCreationListener
{
    [Import]
    public ITextDocumentFactoryService textDocumentFactory { get; set; }

    //...

    public void TextViewCreated(IWpfTextView textView)
    {
        new TextAdornment(textView, textDocumentFactory);
    }
}

TextAdornment.cs

internal sealed class TextAdornment
    {
        private readonly ITextDocumentFactoryService textDocumentFactory;
        private ITextDocument TextDocument;

        //...    

        public TextAdornment(IWpfTextView view, ITextDocumentFactoryService textDocumentFactory)
        {
            //...

            this.textDocumentFactory = textDocumentFactory;

            //...
        }

     internal void OnLayoutChanged(object sender, TextViewLayoutChangedEventArgs e)
        {
            var res = this.textDocumentFactory.TryGetTextDocument(this.view.TextBuffer, out this.TextDocument);
            if (res)
            {
                //this.TextDocument.FilePath;
            }
            else
            {
                //ERROR
            }
        }
    }