UIDynamics : Spring

Question

I am trying to create some animation in an iOS app I am developing. I have a box that falls until it collides with a bar. I also added a bounce to the box for the impact on the bar. What I am trying to add now is a behavior on the bar so when the box hits the bar the reaction is a slight spring. I tried adding a UIAttachmentBehavior but couldn't figure out how to implement it correctly. I have seen the WWDC videos and other but I can not get it to work in this setting. If you could show me in this example how to implement it that would be great.

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    _mainView = [[UIView alloc] initWithFrame:[[self view] bounds]];
    [_mainView setBackgroundColor:[UIColor clearColor]];
    [[self view] addSubview: _mainView];

    _objectView = 
     [[UIView alloc] initWithFrame:CGRectMake(_mainView.bounds.size.width/2-40, 40,
                                              80, 80)];
    [_objectView setBackgroundColor:[UIColor redColor]];
    [_mainView addSubview: _objectView];

    _barView = 
     [[UIView alloc] initWithFrame:CGRectMake(_mainView.bounds.size.width/2-50, 
                                              (_mainView.bounds.size.height/5) * 4, 
                                              100, 3)];
    [_barView setBackgroundColor:[UIColor blackColor]];
    [_mainView addSubview: _barView];

    //----------------------------------

    _animator = [[UIDynamicAnimator alloc] initWithReferenceView:_mainView];

    BounceCustomBehavior *bouncyBehavior = 
     [[BounceCustomBehavior alloc] 
                     initWithItems:@[_objectView] 
                            objects:[NSArray arrayWithObjects:_barView, nil]];
    [_animator addBehavior:bouncyBehavior];

    //-----------------------------------


}

---

#import "BounceCustomBehavior.h"

@implementation BounceCustomBehavior
-(instancetype)initWithItems:(NSArray *)items objects:(NSArray *)collisionObjs {
    if (!(self = [super init])) return nil;

    UIGravityBehavior* gravityBehavior = [[UIGravityBehavior alloc] initWithItems:items];
    [self addChildBehavior:gravityBehavior];

    UICollisionBehavior* collisionBehavior = [[UICollisionBehavior alloc] initWithItems:items];
    collisionBehavior.translatesReferenceBoundsIntoBoundary = YES;

    for (UIView * view in collisionObjs) {
        CGPoint rightEdge = CGPointMake(view.frame.origin.x +
                                        view.frame.size.width, view.frame.origin.y);
        [collisionBehavior addBoundaryWithIdentifier:@""
                                    fromPoint:view.frame.origin
                                      toPoint:rightEdge];
    }
    [self addChildBehavior:collisionBehavior];

    UIDynamicItemBehavior *elasticityBehavior = [[UIDynamicItemBehavior alloc] initWithItems:items];
    elasticityBehavior.elasticity = 0.3f;
    [self addChildBehavior:elasticityBehavior];

    return self;
}
@end

Answer 1

While attachment behaviors are very powerful, I first suggest you try attaching the line to a snap behavior and see if it works for you. Typically you need several attachment behaviors (often 4) in order to stabilize an item. Snap gives similar effects with a single, often easier to use, behavior.

As a separate issue, you're setting up your collisions incorrectly. Just add all of your views to a collision behavior (using addItem:). You don't need to create a bunch of boundaries.