/*

     File: APLStackLayout.m

 Abstract: Custon collection view layout to stack collection view cells.

  Version: 1.1

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#import "APLStackLayout.h"

@interface APLStackLayout ()

@property (nonatomic, readwrite) NSInteger stackCount;

@property (nonatomic, readwrite) CGSize itemSize;

@property (nonatomic, readwrite) NSMutableArray *angles;

@property (nonatomic, readwrite) NSMutableArray *attributesArray;

@end

#pragma mark -

@implementation APLStackLayout

- (id)init

{

    self = [super init];

    if (self != nil)

    {

        _stackCount = 5;

        _itemSize = CGSizeMake(150.0, 200.0);

        _angles = [[NSMutableArray alloc] initWithCapacity:self.stackCount * 10];

    }

    return self;

}

- (void)prepareLayout

{

    // Compute the angles for each photo in the stack layout

    //

    // Keep in mind we only display one section in this layout.

    //

    // We use rand() to generate the varying angles, but with always the same seed value

    // so we have consistent angles when calling this method.

    //

    srand(42);

    CGSize size = self.collectionView.bounds.size;

    CGPoint center = CGPointMake(size.width / 2.0, size.height / 2.0);

    // we only display one section in this layout

    NSInteger itemCount = [self.collectionView numberOfItemsInSection:0];

    // remove all the old attributes

    [self.angles removeAllObjects];

    CGFloat maxAngle = M_1_PI / 3.0;

    CGFloat minAngle = - M_1_PI / 3.0;

    CGFloat diff = maxAngle - minAngle;

    // compute and add the necessary angles for each photo

    [_angles addObject:@0.0];

    for (NSInteger i = 1; i < self.stackCount * 10; i++)

    {

        CGFloat currentAngle = ((((CGFloat)rand()) / RAND_MAX) * diff) + minAngle;

        [self.angles addObject:[NSNumber numberWithFloat:currentAngle]];

    }

    if (self.attributesArray == nil)

    {

        _attributesArray = [[NSMutableArray alloc] initWithCapacity:itemCount];

    }

    // generate the new attributes array for each photo in the stack

    for (NSInteger i = 0; i < itemCount; i++)

    {

        NSInteger angleIndex = i % (self.stackCount * 10);

        NSNumber *angleNumber = self.angles[angleIndex];

        CGFloat angle = angleNumber.floatValue;

        UICollectionViewLayoutAttributes *attributes =

            [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:[NSIndexPath indexPathForItem:i inSection:0]];

        attributes.size = self.itemSize;

        attributes.center = center;

        attributes.transform = CGAffineTransformMakeRotation(angle);

        if (i > self.stackCount)

        {

            attributes.alpha = 0.0;

        }

        else

        {

            attributes.alpha = 1.0;

        }

        attributes.zIndex = (itemCount - i);

        [self.attributesArray addObject:attributes];

    }

}

- (void)invalidateLayout

{

    [super invalidateLayout];

    _attributesArray = nil;

}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds

{

    CGRect bounds = self.collectionView.bounds;

    return ((CGRectGetWidth(newBounds) != CGRectGetWidth(bounds) ||

            (CGRectGetHeight(newBounds) != CGRectGetHeight(bounds))));

}

- (CGSize)collectionViewContentSize

{

    return self.collectionView.bounds.size;

}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath

{

    return self.attributesArray[indexPath.item];

}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect

{

    return self.attributesArray;

}

@end