Two versions of the demo here.
Dropit Attach is simply the code from lecture.
Dropit Align is a bonus posting which has a rudimentary alignment algorithm in DropitBehavior which causes a block involved in a collision to continue to move horizontally (based on its linear velocity) until it lines up with the grid. This is posted just so you can see how a collision delegate works and how one might get physics information about items out of a UIDynamicItemBehavior. A little bit contrived, we'll admit.