A draggable circle that snaps back into place when released, just like the "3kb" circle on http://moofx.mad4milk.net

SnapViewController.m

//
//  SnapViewController.m
//  DynamicsExample
//
//  Created by Bryan Irace on 3/8/14.
//  Copyright (c) 2014 Bryan Irace. All rights reserved.
//

#import "SnapViewController.h"

static CGFloat const CircleWidth = 50;
static CGFloat const SnapDamping = 0.2;

@interface SnapViewController()

@property (nonatomic) UIView *circle;
@property (nonatomic) UIDynamicAnimator *animator;
@property (nonatomic) UISnapBehavior *snapBehavior;

@end

@implementation SnapViewController

#pragma mark - UIViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.circle = [[UIView alloc] initWithFrame:CGRectMake(0, 0, CircleWidth, CircleWidth)];
    self.circle.backgroundColor = [UIColor grayColor];
    self.circle.layer.cornerRadius = CGRectGetWidth(self.circle.frame)/2;
    [self.view addSubview:self.circle];
    
    [self.circle addGestureRecognizer:[[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)]];
    
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    
    self.circle.center = CGPointMake(CGRectGetWidth(self.view.bounds)/2,
                                     CGRectGetHeight(self.view.bounds)/2);
    
    self.snapBehavior = [[UISnapBehavior alloc] initWithItem:self.circle snapToPoint:self.circle.center];
    self.snapBehavior.damping = SnapDamping;
}

#pragma mark - Gestures

- (void)pan:(UIPanGestureRecognizer *)gestureRecognizer {
    if (gestureRecognizer.state == UIGestureRecognizerStateBegan) {
        [self.animator removeBehavior:self.snapBehavior];
    }
    else if (gestureRecognizer.state == UIGestureRecognizerStateChanged) {
        self.circle.center = ^{
            CGPoint center = self.circle.center;
            
            CGPoint translation = [gestureRecognizer translationInView:self.circle];
            center.x += translation.x;
            center.y += translation.y;
            
            [gestureRecognizer setTranslation:CGPointZero inView:self.circle];
            
            return center;
        }();
    }
    else if (gestureRecognizer.state == UIGestureRecognizerStateEnded || gestureRecognizer.state == UIGestureRecognizerStateCancelled) {
        [self.animator addBehavior:self.snapBehavior];
    }
}

@end