making blocks collide with screen boundary

Question

I'm following along with the Stanford ios7 course, chapter 8, where the instructor builds a simplified Tetris game, with colored blocks dropping from above, where you have to fill in rows. After adding gravity behavior to make the blocks fall, he adds a collision behavior (note the property below), lazily instantiates it and, while doing that, he sets the bounds like this

 _collider.translatesReferenceBoundsIntoBoundary = YES;

which makes the blocks collide with the bottom of the screen (rather than falling through) so they can stack on top of each other. He then adds the collision behavior to the animator property, and, as a final step, in the drop method, he adds the dropView (which are the blocks) to the collision behavior. When he runs, the blocks hit the bottom and stack ontop of each other. When I run, using the code below, the blocks continue to fall through the bottom of the screen (on the simulator). In other words, there is no stacking.

Can you see why the collision behavior might not be working.

ViewController

@property (strong,nonatomic) UIDynamicAnimator *animator;
@property (strong, nonatomic) UIGravityBehavior *gravity;
@property (strong, nonatomic) UICollisionBehavior *collider;

@end

@implementation DropItViewController

static const CGSize DROP_SIZE = { 40, 40 };
- (IBAction)tap:(UITapGestureRecognizer *)sender {

    [self drop];
}

- (UICollisionBehavior *)collider
{
    if (!_collider){
        _collider = [[UICollisionBehavior alloc] init];
        _collider.translatesReferenceBoundsIntoBoundary = YES;
        [self.animator addBehavior:_collider];
    }
    return _collider;
}

- (UIDynamicAnimator *)animator
{
    if (!_animator) {
        _animator = [[UIDynamicAnimator alloc] init];
    }
    return _animator;
}

-(UIGravityBehavior *)gravity
{
    if (!_gravity) {
        _gravity = [[UIGravityBehavior alloc] init];
        [self.animator addBehavior:_gravity];

    }
    return _gravity;
}

Add the dropView to the collider in the drop method

-(void)drop
{
    CGRect frame;
    frame.origin = CGPointZero;
    frame.size = DROP_SIZE;
    int x = (arc4random() % (int)self.gameView.bounds.size.width) / DROP_SIZE.width;

    frame.origin.x = x * DROP_SIZE.width;
    UIView *dropView = [[UIView alloc] initWithFrame:frame];
    dropView.backgroundColor = self.randomColor;

    [self.gameView addSubview:dropView];

    [self.gravity addItem:dropView];
    [self.collider addItem:dropView];
}

Answer 1

When you instantiate your UIDynamicAnimator, use initWithReferenceView instead of init. Then when you use translatesReferenceBoundsIntoBoundary, it will know what reference bounds to use.

Answer 2

I'm in the same course and thought I was seeing the same issue. However, try running in the 4.0 inch simulator. Your squares are probably collecting just offscreen (outside the bounds of a 3.5 inch screen).