iOS7—Day by day—Day0:UIKit Dynamics

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UIView *ballBearing = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
ballBearing.backgroundColor = [UIColor lightGrayColor];
ballBearing.layer.cornerRadius = 10;
ballBearing.layer.borderColor = [UIColor grayColor].CGColor;
ballBearing.layer.borderWidth = 2;
ballBearing.center = CGPointMake(200, 300);
[self.view addSubview:ballBearing];

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UIDynamicBehavior *behavior = [[UIDynamicBehavior alloc] init];

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UIGravityBehavior *gravity = [[UIGravityBehavior alloc] initWithItems:@[ballBearing]];
gravity.magnitude = 10;
[behavior addChildBehavior:gravity];

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CGPoint anchor = ballBearing.center;
anchor.y -= 20;
UIAttachmentBehavior *attachment = [[UIAttachmentBeahavior alloc] initWithItem:ballBearing attachedToAnchor:anchor];
[behavior addChildBehavior:attachment];

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_animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
[_animator addBehavior:behavior];

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UIPanGestureRecognizer *gesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleBallBearingPan:)];
[ballBearing addGestureRecognizer:gesture];

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- (void)handleBallBearingPan:(UIPanGestureRecognizer *)recognizer {
// If we're starting the gesture then create a drag force
if (recognizer.state == UIGestureRecognizerStateBegan) {
if (_userDragBehavior) {
[_animator removeBehavior:_userDragBehavior];
}

_userDragBehavior = [[UIPushBehavior alloc] initWithItems:@[recognizer.view] mode:UIPushBehaviorModeContinuous];
[_animator addBehavior:_userDragBehavior];
}

// Set the force to be proportional to distance the gesture has moved
_userDragBehavior.pushDirection = CGVectorMake([recognizer translationInView:self].x / 10.f, 0);

// If we're finishing then cancel the behavior to 'let-go' of the ball
if (recognizer.state = UIGestureRecognizerStateEnded) {
[_animator removeBehavior:_userDragBehavior];
_userDragBehavior = nil;
}
}

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UICollisionBehavior *collision = [[UICollisionBehavior alloc] initWithObjects:_ballBearings];
[behavior addChildBehavior:collision];

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UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:_ballBearings];
// Elasticity governs the efficiency of the collisions
itemBehavior.elasticity = 1.0;
itemBehavior.allowsRotation = NO;
itemBehavior.resistance = 0.5;
[behavior addChildBehavior:itemBehavior];