Registers a callback that's fired when a drawing mode shape is changed.
Returns an ID which can be passed to unlisten() to unregister the callback.
The ui.Map.DrawingTools instance.
The callback to fire when the shape is changed. The callback is passed two parameters: the drawing mode shape as a string (or null for cancel) and the ui.Map.DrawingTools widget that the event listener is bound to. The shape values are: