how to use { annotationEvents: { step: callback, stop: callback },

[pawelfus]

Use them as standard callback to update annotation. For example, when adding circle, update radius. When creating rectangle, update width/height/x/y etc.

Here you can find example how it's implemented with built-in buttons:

• https://github.com/blacklabel/annotations/blob/master/js/annotations.js#L72
• https://github.com/blacklabel/annotations/blob/master/js/annotations.js#L88