RGCardViewLayout.m

//
//  RGCardViewLayout.m
//  RGCardViewLayout
//
//  Created by ROBERA GELETA on 1/23/15.
//  Copyright (c) 2015 ROBERA GELETA. All rights reserved.
//

#import "RGCardViewLayout.h"

@implementation RGCardViewLayout
{
    CGFloat previousOffset;
    NSIndexPath *mainIndexPath;
    NSIndexPath *movingInIndexPath;
    CGFloat difference;
}

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

- (void)setupLayout
{
    CGFloat inset  = self.collectionView.bounds.size.width * (6/64.0f);
    inset = floor(inset);

    self.itemSize = CGSizeMake(self.collectionView.bounds.size.width - (2 *inset), self.collectionView.bounds.size.height * 3/4);
    self.sectionInset = UIEdgeInsetsMake(0,inset, 0,inset);
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes *attributes = [[super layoutAttributesForItemAtIndexPath:indexPath] copy];
    [self applyTransformToLayoutAttributes:attributes];
    
    return attributes;
}

// indicate that we want to redraw as we scroll
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    return YES;
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *attributesSuper = [super layoutAttributesForElementsInRect:rect];
    NSArray *attributes = [[NSArray alloc] initWithArray:attributesSuper copyItems:YES];
    
    NSArray *cellIndices = [self.collectionView indexPathsForVisibleItems];
    if(cellIndices.count == 0 )
    {
        return attributes;
    }
    else if (cellIndices.count == 1)
    {
        mainIndexPath = cellIndices.firstObject;
        movingInIndexPath = nil;
    }
    else if(cellIndices.count > 1)
    {
        NSIndexPath *firstIndexPath = cellIndices.firstObject;
        if(firstIndexPath == mainIndexPath)
        {
            movingInIndexPath = cellIndices[1];
        }
        else
        {
            movingInIndexPath = cellIndices.firstObject;
            mainIndexPath = cellIndices[1];
        }
        
    }
    
    difference =  self.collectionView.contentOffset.x - previousOffset;
    
    previousOffset = self.collectionView.contentOffset.x;
    
    for (UICollectionViewLayoutAttributes *attribute in attributes)
    {
        [self applyTransformToLayoutAttributes:attribute];
    }
    return  attributes;
}

- (void)applyTransformToLayoutAttributes:(UICollectionViewLayoutAttributes *)attribute
{
    if(attribute.indexPath.section == mainIndexPath.section)
    {
        UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:mainIndexPath];
        attribute.transform3D = [self transformFromView:cell];
        
    }
    else if (attribute.indexPath.section == movingInIndexPath.section)
    {
        UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:movingInIndexPath];
        attribute.transform3D = [self transformFromView:cell];
    }
}


#pragma mark - Logica
- (CGFloat)baseOffsetForView:(UIView *)view
{
    UICollectionViewCell *cell = (UICollectionViewCell *)view;
    CGFloat offset =  ([self.collectionView indexPathForCell:cell].section) * self.collectionView.bounds.size.width;
    
    return offset;
}

- (CGFloat)heightOffsetForView:(UIView *)view
{
    CGFloat height;
    CGFloat baseOffsetForCurrentView = [self baseOffsetForView:view ];
    CGFloat currentOffset = self.collectionView.contentOffset.x;
    CGFloat scrollViewWidth = self.collectionView.bounds.size.width;
    //TODO:make this constant a certain proportion of the collection view
    height = 120 * (currentOffset - baseOffsetForCurrentView)/scrollViewWidth;
    if(height < 0 )
    {
        height = - 1 * height;
    }
    return height;
}

- (CGFloat)angleForView:(UIView *)view
{
    CGFloat baseOffsetForCurrentView = [self baseOffsetForView:view ];
    CGFloat currentOffset = self.collectionView.contentOffset.x;
    CGFloat scrollViewWidth = self.collectionView.bounds.size.width;
    CGFloat angle = (currentOffset - baseOffsetForCurrentView)/scrollViewWidth;
    return angle;
}

- (BOOL)xAxisForView:(UIView *)view
{
    CGFloat baseOffsetForCurrentView = [self baseOffsetForView:view ];
    CGFloat currentOffset = self.collectionView.contentOffset.x;
    CGFloat offset = (currentOffset - baseOffsetForCurrentView);
    if(offset >= 0)
    {
        return YES;
    }
    return NO;    
}


#pragma mark - Transform Related Calculation

- (CATransform3D)transformFromView:(UIView *)view
{
    CGFloat angle = [self angleForView:view];
    CGFloat height = [self heightOffsetForView:view];
    BOOL xAxis = [self xAxisForView:view];
    return [self transformfromAngle:angle height:height xAxis:xAxis];
}

- (CATransform3D)transformfromAngle:(CGFloat)angle height:(CGFloat)height xAxis:(BOOL)axis
{
    CATransform3D t = CATransform3DIdentity;
    t.m34  = 1.0/-500;
    
    if (axis)
    {
        t = CATransform3DRotate(t,angle, 1, 1, 0);
    }
    else
    {
        t = CATransform3DRotate(t,angle, -1, 1, 0);
    }
    
    return t;
}

@end