Why arent the views being added to this UIAnimator? - ios

Question

Im adding views to an animator like so:
-(void)drawCellLikeViews{
//2. Add Animator
_animator = [[UIDynamicAnimator alloc] initWithReferenceView:self];
//Alloc array
viewsArray = [[NSMutableArray alloc] initWithCapacity:6];
for (int numberOfViews = 0; numberOfViews < 5; numberOfViews++) {
//Create views
UIView* cell = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 320, 70)];
cell.backgroundColor = [UIColor grayColor];
[self addSubview:cell];
[viewsArray addObject:cell];
}
//3. Add Gravity
_gravity = [[UIGravityBehavior alloc] initWithItems:viewsArray];
[_animator addBehavior:_gravity];
//4. Add boundaries
_collision = [[UICollisionBehavior alloc] initWithItems:viewsArray];
_collision.translatesReferenceBoundsIntoBoundary = YES;
[_animator addBehavior:_collision];
//8. Add ItemProperties' Behaviors
UIDynamicItemBehavior* itemBehaviour = [[UIDynamicItemBehavior alloc] initWithItems:viewsArray];
itemBehaviour.elasticity = 0.6;
//9. itemBehaviour.action = ^{};
[_animator addBehavior:itemBehaviour];
NSLog(#"viewcount %d", [[self subviews] count]);
}
I only see one drop down to the bottom.

Answer 1

#HalR is correct—your views are being added to the animator; they're just all being added at the same time with the same starting attributes, so they will all behave in exactly the same way and appear as one item. And yes, by default, the collision behavior's mode is UICollisionBehaviorModeEverything, meaning its items should collide with each other and with the boundaries. Yet, by implying UICollisionBehaviorModeEverything and placing all of the items on top of each other, you've created an impossible scenario wherein you've explicitly told the edges of each item to overlap, but you've told the animator that they're not allowed to overlap. The animator's behavior at that point is undefined—I think it just ignores whatever physics rule you've manually broken, but it can get much weirder than that. In WWDC 2013 Session 206 (Getting Started with UIKit Dynamics), Apple at one point says "don't expect the impossible" saying that "you can create setups which don't have solutions" and showing overlapping items with collision behaviors set as an example. In other words, they saw this coming, and they just don't want you to do it. ;)
The way to make this work, of course, would just be to give each of the items a starting position that doesn't overlap with other items' starting positions, or to add them all to where you're adding them now, only a few seconds apart from each other so the last one can fall out of the way.