HCElasticFlowLayout - Elastic Flow layout for UICollectionView

HCElasticFlowLayout.m

#import <UIKit/UIKit.h>

#ifndef __IPHONE_7_0
#error Because of Dynamic Kit, this custom flowLayout requires APIs only available in iOS SDK 7.0 and later
#endif

/**
 *  A UICollectionViewFlowLayout subclass that, when implemented,
 *  creates a dynamic / elastic scroll effect for UICollectionViews
 */
@interface HCElasticFlowLayout : UICollectionViewFlowLayout


/**
 *  Call this method to instanciate a custom bouncing flow layout for UICollectionView
 *
 *  @param scrollResistance the scroll resistance, ie elasticity between cells (by default @ 900)
 *
 *  @return the instanciated CWElasticFlowLayout object
 */
- (instancetype)initWithScrollResistance:(CGFloat)scrollResistance;


@end



/******************************************/


#import "HCElasticFlowLayout.h"

const CGFloat DefaultScrollResistanceFactor = 900.0f;

const CGFloat DefaultSpringLength = 1.0f;
const CGFloat DefaultSpringDamping = 0.8f;
const CGFloat DefaultSpringFrequency = 1.0f;


@implementation HCElasticFlowLayout {

    CGFloat scrollResistanceFactor;

    NSMutableSet<NSIndexPath *> *visibleIndexPathsSet;
    NSMutableSet *visibleHeaderAndFooterSet;
    CGFloat latestDelta;

    /// The dynamic animator used to animate the collection's bounce
    UIDynamicAnimator *dynamicAnimator;
}

#pragma mark - LifeCycle

- (instancetype)initWithScrollResistance:(CGFloat)scrollResistance
{
    self = [super init];
    if (self)
    {
        scrollResistanceFactor = scrollResistance;
        [self setup];
    }
    return self;
}

- (instancetype)init
{
    self = [super init];
    if (self)
    {
        [self setup];
    }
    return self;
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder
{
    self = [super initWithCoder:aDecoder];
    if (self)
    {
        [self setup];
    }
    return self;
}

#pragma mark - Custom stuff

- (void)setup
{
    dynamicAnimator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
    visibleIndexPathsSet = [[NSMutableSet<NSIndexPath*> alloc] init];
    visibleHeaderAndFooterSet = [[NSMutableSet alloc] init];
}

#pragma mark - Flow Layout override

- (void)prepareLayout
{
    [super prepareLayout];

    // Need to overflow our actual visible rect slightly to avoid flickering.
    CGRect visibleRect = CGRectInset((CGRect){.origin = self.collectionView.bounds.origin, .size = self.collectionView.frame.size}, -100, -100);
    NSArray *itemsInVisibleRectArray = [super layoutAttributesForElementsInRect:visibleRect];
    NSSet *itemsIndexPathsInVisibleRectSet = [NSSet setWithArray:[itemsInVisibleRectArray valueForKey:@"indexPath"]];


    // Remove any behaviours that are no longer visible.
    NSArray<__kindof UIDynamicBehavior *> *noLongerVisibleBehaviours = [dynamicAnimator.behaviors filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL (UIAttachmentBehavior *behaviour, NSDictionary *bindings) {
        return [itemsIndexPathsInVisibleRectSet containsObject:[(UICollectionViewLayoutAttributes *)[[behaviour items] firstObject] indexPath]] == NO;
    }]];

    [noLongerVisibleBehaviours enumerateObjectsUsingBlock:^(id obj, NSUInteger index, BOOL *stop) {
        [dynamicAnimator removeBehavior:obj];
        [visibleIndexPathsSet removeObject:[(UICollectionViewLayoutAttributes *)[[obj items] firstObject] indexPath]];
        [visibleHeaderAndFooterSet removeObject:[(UICollectionViewLayoutAttributes *)[[obj items] firstObject] indexPath]];

    }];

    // Add any newly visible behaviours.
    // A "newly visible" item is one that is in the itemsInVisibleRect(Set|Array) but not in the visibleIndexPathsSet
    NSArray *newlyVisibleItems = [itemsInVisibleRectArray filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL (UICollectionViewLayoutAttributes *item, NSDictionary *bindings) {
        return (item.representedElementCategory == UICollectionElementCategoryCell ?
                [visibleIndexPathsSet containsObject:item.indexPath]:[visibleHeaderAndFooterSet containsObject:item.indexPath]) == NO;
    }]];

    CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];

    [newlyVisibleItems enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *item, NSUInteger idx, BOOL *stop) {
        CGPoint center = item.center;
        UIAttachmentBehavior *springBehaviour = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:center];

        springBehaviour.length = DefaultSpringLength;
        springBehaviour.damping = DefaultSpringDamping;
        springBehaviour.frequency = DefaultSpringFrequency;

        // If our touchLocation is not (0,0), we'll need to adjust our item's center "in flight"
        if (!CGPointEqualToPoint(CGPointZero, touchLocation))
        {
            CGFloat scrollResistance;
            CGFloat distanceFromTouch;

            if (self.scrollDirection == UICollectionViewScrollDirectionVertical)
            {
                distanceFromTouch = fabs(touchLocation.y - springBehaviour.anchorPoint.y);


                if (scrollResistanceFactor)
                {
                    scrollResistance = distanceFromTouch / scrollResistanceFactor;
                } else
                {
                    scrollResistance = distanceFromTouch / DefaultScrollResistanceFactor;
                }

                if (latestDelta < 0)
                {
                    center.y += MAX(latestDelta, latestDelta*scrollResistance);
                } else
                {
                    center.y += MIN(latestDelta, latestDelta*scrollResistance);
                }

                item.center = center;

            } else
            {
                distanceFromTouch = fabs(touchLocation.x - springBehaviour.anchorPoint.x);

                if (scrollResistanceFactor)
                {
                    scrollResistance = distanceFromTouch / scrollResistanceFactor;
                } else
                {
                    scrollResistance = distanceFromTouch / DefaultScrollResistanceFactor;
                }

                if (latestDelta < 0)
                {
                    center.x += MAX(latestDelta, latestDelta*scrollResistance);
                } else
                {
                    center.x += MIN(latestDelta, latestDelta*scrollResistance);
                }

            }
            item.center = center;
        }

        [dynamicAnimator addBehavior:springBehaviour];

        if (item.representedElementCategory == UICollectionElementCategoryCell)
        {
            [visibleIndexPathsSet addObject:item.indexPath];
        } else
        {
            [visibleHeaderAndFooterSet addObject:item.indexPath];
        }

    }];
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    return [dynamicAnimator itemsInRect:rect];
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes *dynamicLayoutAttributes = [dynamicAnimator layoutAttributesForCellAtIndexPath:indexPath];

    return (dynamicLayoutAttributes) ? dynamicLayoutAttributes:[super layoutAttributesForItemAtIndexPath:indexPath];
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{

    UIScrollView *scrollView = self.collectionView;

    CGFloat delta;
    if (self.scrollDirection == UICollectionViewScrollDirectionVertical)
    {
        delta = newBounds.origin.y - scrollView.bounds.origin.y;
    } else
    {
        delta = newBounds.origin.x - scrollView.bounds.origin.x;
    }

    latestDelta = delta;
    CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];

    [dynamicAnimator.behaviors enumerateObjectsUsingBlock:^(UIAttachmentBehavior *springBehaviour, NSUInteger idx, BOOL *stop) {

        UICollectionViewLayoutAttributes *item = (UICollectionViewLayoutAttributes *)[springBehaviour.items firstObject];
        CGPoint center = item.center;
        CGFloat scrollResistance;
        CGFloat distanceFromTouch;
        if (self.scrollDirection == UICollectionViewScrollDirectionVertical)
        {
            distanceFromTouch = fabs(touchLocation.y - springBehaviour.anchorPoint.y);


            if (scrollResistanceFactor)
            {
                scrollResistance = distanceFromTouch / scrollResistanceFactor;
            } else
            {
                scrollResistance = distanceFromTouch / DefaultScrollResistanceFactor;
            }


            if (delta < 0)
                center.y += MAX(delta, delta*scrollResistance);
            else
                center.y += MIN(delta, delta*scrollResistance);


        } else
        {

            distanceFromTouch = fabs(touchLocation.x - springBehaviour.anchorPoint.x);

            if (scrollResistanceFactor)
            {
                scrollResistance = distanceFromTouch / scrollResistanceFactor;
            } else
            {
                scrollResistance = distanceFromTouch / DefaultScrollResistanceFactor;
            }

            if (delta < 0)
                center.x += MAX(delta, delta*scrollResistance);
            else
                center.x += MIN(delta, delta*scrollResistance);

        }

        item.center = center;
        [dynamicAnimator updateItemUsingCurrentState:item];

    }];

    return NO;
}

@end