UIKit Dynamics collision - keeping the Barriers elements stationary

Question

How do I prevent the "Barrier"s which are the red elements from moving - while allowing the "falling" elements (Gray / UIView* square) to fall and interact with the barrier(s)?

There are many gray elements, many red barriers...

@interface MainViewController ()
{
            UIDynamicAnimator* _animator;
            UIGravityBehavior* _gravity;
            UICollisionBehavior* _collision;
            //
            NSMutableArray *collisionArray;
}

@end

@implementation MainViewController

 - (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
            self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
            if (self) {
                //
            }
            return self;
}



- (void)viewDidLoad
{
            [super viewDidLoad];
            //

            collisionArray = [[NSMutableArray alloc] init];
            _animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
            [self addTheBarrier];


}


-(void)addTheBarrier
{
            for (int i=0; i<20; i++) {
                UIView* barrier = [[UIView alloc] initWithFrame:CGRectMake(i*16, 250, 10, 10)];
                barrier.backgroundColor = [UIColor redColor];
                [self.view addSubview:barrier];


                CGPoint rightEdge = CGPointMake(barrier.frame.origin.x +
                                                barrier.frame.size.width, barrier.frame.origin.y);
                [_collision addBoundaryWithIdentifier:@"barrier"
                                            fromPoint:barrier.frame.origin
                                              toPoint:rightEdge];
                [collisionArray addObject:barrier];
            }
            [self startDynamic];
        }



    #pragma mark - Dynamics

    -(void)startDynamic
        {
                UIView* square = [[UIView alloc] initWithFrame:CGRectMake((arc4random()%250)+10, 100, 10, 10)];
                square.backgroundColor = [UIColor grayColor];

                [self.view addSubview:square];
                [collisionArray addObject:square];

               _collision = [[UICollisionBehavior alloc] initWithItems:collisionArray];


                _gravity = [[UIGravityBehavior alloc] initWithItems:@[square]];
                [_animator addBehavior:_gravity];
                //
                UIDynamicItemBehavior* itemBehaviour = [[UIDynamicItemBehavior alloc] initWithItems:@[square]];
                itemBehaviour.elasticity = 0.85;
                [_animator addBehavior:itemBehaviour];

                //
                UIPushBehavior *push = [[UIPushBehavior alloc] initWithItems:@[ square ] mode:UIPushBehaviorModeContinuous];
               [push setPushDirection:CGVectorMake(0, 0)];

                [_animator addBehavior:push];
                [_collision setTranslatesReferenceBoundsIntoBoundary:NO];
                [_animator addBehavior:_collision];

            [self performSelector:@selector(startDynamic) withObject:nil afterDelay:0.3 ];

}


@end

Answer 1

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.