Editor.addCommand

Registers a command object with the editor. You can reference the command from a user interface component such as a custom toolbar button.

//declare only once
Editor.getScope().declare("show", false);

var ShowDraftCommentsCommand = 
{
  execute: function (scope) {
    scope.set("show", !scope.get("show")); 
    var canvas = Editor.Canvas.getActiveCanvas();
    canvas.setViewParam("show-draft-comments", scope.get("show")); 
  },

  getEnabled: function(scope)
  {
    var canvas = Editor.Canvas.getActiveCanvas();
    return !!canvas;
  },

  getChecked: function (scope) {
    return scope.get("show"); //the button shows a 'pressed' state
  },
  
  getLabel: function (scope) {
    if (scope.get("show")) 
      return "Hide Draft Comments";
    else           
      return "Show Draft Comments";
  }
}

Editor.addCommand("showDraftCommentsCommand", ShowDraftCommentsCommand);

Another often used version of getEnabled, using selection can be:

getEnabled: function(scope) {
  var selection = scope.get("xmlSelection");
  return selection.getRange().getCommonAncestorContainer().getLocalName() == "para")
}

For a button that is active in paragraphs (or elements named "para" in this case) only.