Implementation of XBImageGalleryLayout used in the blog post "À la (re)découverte de la UICollectionView"

XBImageGalleryLayout.m

//
//  XBImageGalleryLayout.m
//
//  Created by Simone Civetta on 4/24/13.
//  Copyright (c) 2013 Xebia IT Architects. All rights reserved.
//

#import "XBImageGalleryLayout.h"
#import <UICollectionView/UICollectionView.h>

static const CGFloat imageWitdh = 180.0f;
static const CGFloat imageHeight = 135.0f;
static const CGFloat imageLeftMargin = (320.0 - imageWitdh) / 2;

@implementation XBImageGalleryLayout

- (id)init
{
    self = [super init];
    
    if (self) {
        
        // Taille des cellules
        self.itemSize = CGSizeMake(imageWitdh, imageHeight);
        
        // Configuration de l'aspect
        self.minimumLineSpacing = -10; // Espace minimum entre les cellules
        self.sectionInset = UIEdgeInsetsMake(0, imageLeftMargin, 0, imageLeftMargin); // "Padding"
        
        // Direction de défilement
        self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    }

    return self;
}

- (NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *layoutAttributes = [super layoutAttributesForElementsInRect:rect];
    
    CGFloat horizontalCenter = (CGRectGetWidth(self.collectionView.bounds) / 2.0f);
    
    [layoutAttributes enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *layoutAttributes, NSUInteger idx, BOOL *stop) {
        
        CGPoint centerInCollectionView = layoutAttributes.center;
        CGPoint centerInMainView = [self.collectionView.superview convertPoint:centerInCollectionView
                                                                      fromView:self.collectionView];
        
        if (centerInMainView.x > -80 && centerInMainView.x < self.collectionView.frame.size.width + 80){
            
            CGFloat translateYBy = [self translateYByOffset:centerInMainView.x fromCenter:horizontalCenter];
            CGFloat scaleBy = [self scaleByOffset:centerInMainView.x fromCenter:horizontalCenter];
            CGFloat alpha = [self alphaByOffset:centerInMainView.x fromCenter:horizontalCenter];
            
            CATransform3D transform = CATransform3DIdentity;
            transform = CATransform3DScale(transform, scaleBy, scaleBy, 1.0);
            transform = CATransform3DTranslate(transform, 0.0, translateYBy, 0.0);
            layoutAttributes.transform3D = transform;
            layoutAttributes.alpha = alpha;
        }
    }];
    
    return layoutAttributes;
}


#pragma mark - Attribute calculation

- (CGFloat)scaleByOffset:(float)x fromCenter:(float)horizontalCenter
{
    CGFloat diff = x - horizontalCenter;
    CGFloat scaleBy = [self transformValue:diff fromMin:-240.0 fromMax:240.0 toMin:0.6 toMax:1.0];
    return scaleBy;
}

- (CGFloat)translateYByOffset:(float)x fromCenter:(float)horizontalCenter
{
    CGFloat diff = x - horizontalCenter;
    return [self transformValue:diff fromMin:-240.0 fromMax:240.0 toMin:50.0 toMax:0.0];
}

- (CGFloat)alphaByOffset:(float)x fromCenter:(float)horizontalCenter
{
    CGFloat diff = x - horizontalCenter;
    return [self transformValue:diff fromMin:-240.0 fromMax:240.0 toMin:0.1 toMax:1.0];
}


#pragma mark - Content offset

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
                                 withScrollingVelocity:(CGPoint)velocity
{
    
    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalOffset = proposedContentOffset.x + imageLeftMargin;
    
    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    
    NSArray *array = [super layoutAttributesForElementsInRect:targetRect];
    
    for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
        CGFloat itemOffset = layoutAttributes.frame.origin.x;
        if (fabsf(itemOffset - horizontalOffset) < fabsf(offsetAdjustment)) {
            offsetAdjustment = itemOffset - horizontalOffset;
        }
    }
    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

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

- (NSUInteger)imageGallery:(XBImageGallery *)imageGallery pageForContentOffset:(CGPoint)contentOffset
{
    return floor((contentOffset.x - imageWitdh / 2) / imageWitdh) + 1;
}


#pragma mark - Accessory Methods

- (CGFloat)transformValue:(CGFloat)x fromMin:(CGFloat)fromMin fromMax:(CGFloat)fromMax toMin:(CGFloat)toMin toMax:(CGFloat)toMax
{
    CGFloat middle = (fromMax - fromMin) / 2.0;
    return (middle - fabsf(x)) / (middle) * (toMax - toMin) + toMin;
}

@end