gistfile1.txt

#import <QuartzCore/QuartzCore.h>
#import "PHBallChainViewController.h"

@interface PHBallChainViewController ()

@property (nonatomic) UIDynamicAnimator* animator;
@property (nonatomic, weak) UIView *ballView;
@property (nonatomic) UIAttachmentBehavior* touchAttachmentBehavior;

@end


@implementation PHBallChainViewController

- (void)viewDidLoad
{

    [super viewDidLoad];

    UIDynamicAnimator* animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

    int numberOfLinks = 9;
    CGSize linkSize = CGSizeMake(5, 10);
    float spaceBetweenLinks = 2.f;
    NSMutableArray *linksArray = [NSMutableArray array];
    float currentY = self.postView.frame.origin.y + self.postView.bounds.size.height;


    UIView *previousLink = self.postView;

    for (int i = 0; i < numberOfLinks; i++) {
        UIView *link = [[UIView alloc] initWithFrame:CGRectMake(self.view.bounds.size.width/2 - linkSize.width/2, currentY + spaceBetweenLinks, linkSize.width, linkSize.height)];
        link.backgroundColor = [UIColor blueColor];
        [self.view addSubview:link];

        UIAttachmentBehavior *attachmentBehavior = nil;
        if (i == 0) {
            attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:link
                                                           attachedToAnchor:self.postView.center];
        } else {
            attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:link
                                                                      point:CGPointMake(0, -1)
                                                             attachedToItem:previousLink
                                                                      point:CGPointMake(0, 1)];
        }


        // You can mess with these if you want
//        [attachmentBehavior setFrequency:50.0];
//        [attachmentBehavior setDamping:1.0];

        [animator addBehavior:attachmentBehavior];
        
        currentY += linkSize.height + spaceBetweenLinks;
        [linksArray addObject:link];
        previousLink = link;
    }


    // Create the ball
    CGSize ballSize = CGSizeMake(66, 66);
    UIView *ballView = [[UIView alloc] initWithFrame:CGRectMake(self.view.bounds.size.width/2 - ballSize.width/2, currentY + spaceBetweenLinks, ballSize.width, ballSize.height)];
    ballView.backgroundColor = [UIColor greenColor];
    ballView.layer.cornerRadius = ballSize.width/2; // Note: there'll be errors in the collision since this is still a square, but this is just an example.
    [self.view addSubview:ballView];

    self.ballView = ballView;


    // Connect the ball to the chain
    UIAttachmentBehavior *attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:previousLink
                                                                         attachedToItem:ballView];
    [animator addBehavior:attachmentBehavior];

    // Apply gravity and collision
    [linksArray addObject:ballView];
    UIGravityBehavior *gravityBeahvior = [[UIGravityBehavior alloc] initWithItems:linksArray];
    UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc] initWithItems:linksArray];
    collisionBehavior.translatesReferenceBoundsIntoBoundary = YES;
    [animator addBehavior:gravityBeahvior];
    [animator addBehavior:collisionBehavior];

    self.animator = animator;
}


// Let the user pull the ball around
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UIAttachmentBehavior *touchAttachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:self.ballView
                                                                              attachedToAnchor:self.ballView.center];
    [self.animator addBehavior:touchAttachmentBehavior];
    [touchAttachmentBehavior setFrequency:1.0];
    [touchAttachmentBehavior setDamping:0.1];
    self.touchAttachmentBehavior = touchAttachmentBehavior;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    self.touchAttachmentBehavior.anchorPoint = [touch locationInView:self.view];;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self.animator removeBehavior:self.touchAttachmentBehavior];
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self.animator removeBehavior:self.touchAttachmentBehavior];
}