This is not really a dynamics problem. Autolayout is incompatible with any view animation, or any manual setting of the frame: when layout comes along, it is the constraints that will be obeyed. It is up to you, if you move a view manually in any way, to update the constraints to match its new position/size/whatever.
Having said that: with UIKit Dynamics, when the animation ends, the animator will pause, and the animator's delegate is notified:
So that is the moment to update the constraints.