You need to add this to your dynamics code:
//Attachment behavior that you figured out:
UIAttachmentBehavior *attachment = [[UIAttachmentBehavior alloc] initWithItem:barrier attachedToAnchor:barrier.center];
[_animator addBehavior:attachment];
// To keep the views from spinning:
UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:@[barrier]];
itemBehavior.allowsRotation = NO;
[_animator addBehavior:itemBehavior];
We're making an attachment behavior to keep the views from bouncing all over the screen, then we're using a UIDynamicItemBehavior
to fine tune the behavior of those items - in particular, to keep them from rotating.