A HowTo on making a nice little popup selector for a list of markdown files in a given directory

HowTo_CoolEnumPopup_MarkdownFilesInDir.cs

private void DrawMainPageSelector()
{
    bool isMainPageSet = !string.IsNullOrEmpty(setMainPage);
    var mainPageTitle = !isMainPageSet ? "NONE" : Path.GetFileName(setMainPage.ToUpper());
    //Display a Popup selector with default of non if exisitng value is not in the list
    using (var mainPageScope = new EditorGUILayout.VerticalScope(EditorStyles.helpBox))
    {
        GUILayout.Label(string.Format("Main Page Setting\nCurrent: {0}", isMainPageSet?setMainPage.ToUpper():"NONE"), EditorStyles.centeredGreyMiniLabel);

        using (var selectionScope = new EditorGUILayout.HorizontalScope(EditorStyles.toolbar))
        {
            EditorGUILayout.PrefixLabel("Main Page ", EditorStyles.toolbar);
            int selection = 0;
            var existing = mainPageCandidates.Where((x) => x == setMainPage).FirstOrDefault();
            selection = mainPageCandidates.IndexOf(existing);
            if(selection < 0)
                selection = 0;
            Color gCol = GUI.color;
            GUI.color = isMainPageSet ? Color.green : Color.yellow;
            var nextSel = EditorGUILayout.Popup(selection, mainPageCandidates.ToArray());
            GUI.color = gCol;
            if (selection != nextSel)
            {
                if(nextSel == 0)
                    setMainPage = string.Empty;
                else
                    setMainPage = mainPageCandidates[nextSel];
                selection = nextSel;
            }

            if(mainPageCandidates.Count < 0 && !string.IsNullOrEmpty(inputDirectory) || GUILayout.Button("Refresh", EditorStyles.toolbarButton))
            {
                RefreshMainPageCandidates();
            }
        }
    }
}

private void RefreshMainPageCandidates()
{
    if (string.IsNullOrEmpty(inputDirectory))
        return;
    mainPageCandidates.Clear();
    mainPageCandidates = GetMarkDownFiles(inputDirectory);
    mainPageCandidates.Insert(0, "NONE");
}
private List<string> GetMarkDownFiles(string path)
{
    var files = new List<string>();
    var directories = Directory.GetDirectories(path);
    foreach (var directory in directories)
    {
        files.AddRange(GetMarkDownFiles(directory));
    }

    var filePaths = Directory.GetFiles(path);
    foreach (var filePath in filePaths)
    {
        if (filePath.EndsWith(".md"))
        {
            files.Add(filePath);
        }
    }

    return files;
}