UICollectionViewFlowLayout sub-class for layout attributes of all the items in a circle.

CollectionViewCircleFlowLayout.m

//
//  CollectionViewCarouselFlowLayout.m
//  iP2
//
//  Created by Lee Probert on 07/03/2013.
//
//

#import "CollectionViewCircleFlowLayout.h"

#define ITEM_SIZE 70.0f
#define ACTIVE_DISTANCE 200
#define ZOOM_FACTOR 0.2
#define PADDING 20
#define MAX_ANGLE 80
#define PERSPECTIVE 1.0/-400

@interface CollectionViewCircleFlowLayout (){
    
    UIInterfaceOrientation _orientation;
    CGPoint _center;
    CGFloat _radius;
    int _cellCount;
}

-(void)initLayoutProperties;
-(UICollectionViewLayoutAttributes*)attributesForCenterPosition:(NSIndexPath *)itemIndexPath;

@end

@implementation CollectionViewCircleFlowLayout

-(void)prepareLayout {
    
    [super prepareLayout];
    [self initLayoutProperties];
}

-(void)initLayoutProperties {
    
    _orientation = [[UIApplication sharedApplication] statusBarOrientation];
    self.itemSize = CGSizeMake(ITEM_SIZE, ITEM_SIZE);
    
    self.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
    
    CGSize size = self.collectionView.frame.size;
    
    _center = CGPointMake(size.width/2, size.height/2);
    _radius = MIN(size.width,size.height)/2.5;
    
    _cellCount = [[self collectionView] numberOfItemsInSection:0];
}

-(CGSize)collectionViewContentSize {
    
    return [self collectionView].frame.size;
}

/*
    Layout all items in a circle
 */

-(UICollectionViewLayoutAttributes*)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    
    UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    
    attributes.size = CGSizeMake(ITEM_SIZE, ITEM_SIZE);
    attributes.center = CGPointMake(_center.x + _radius * cosf(2 * indexPath.item * M_PI / _cellCount),
                                    _center.y + _radius * sinf(2 * indexPath.item * M_PI / _cellCount));
    
    attributes.zIndex = indexPath.row;
    
    return attributes;
}

-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect {
    
    NSMutableArray*attributes = [NSMutableArray array];
    for(NSInteger i=0; i < _cellCount; i++) {
        
        NSIndexPath* indexPath = [NSIndexPath indexPathForItem:i inSection:0];
        [attributes addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];
    }
    
    return attributes;
}

/*
    Methods for inserting and deleting items
 */

- (UICollectionViewLayoutAttributes*)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath {
    
    UICollectionViewLayoutAttributes* attributes = [self attributesForCenterPosition:itemIndexPath];
    return attributes;
}

- (UICollectionViewLayoutAttributes*)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath {
    
    UICollectionViewLayoutAttributes* attributes = [self attributesForCenterPosition:itemIndexPath];
    return attributes;
}

-(UICollectionViewLayoutAttributes*)attributesForCenterPosition:(NSIndexPath *)itemIndexPath {
    
    UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
    attributes.alpha = 0.0f;
    attributes.center = CGPointMake(_center.x, _center.y);
    return attributes;
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    
    return YES;
}
@end