#import "ASPagerFlowLayout.h"

@interface ASPagerFlowLayout () {

BOOL _didRotate;

CGRect _cachedCollectionViewBounds;

NSIndexPath *_currentIndexPath;

}

@implementation ASPagerFlowLayout

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity

{

NSInteger currentPage = ceil(proposedContentOffset.x / self.collectionView.bounds.size.width);

_currentIndexPath = [NSIndexPath indexPathForItem:currentPage inSection:0];

return [super targetContentOffsetForProposedContentOffset:proposedContentOffset withScrollingVelocity:velocity];

}

- (void)prepareForAnimatedBoundsChange:(CGRect)oldBounds

{

// Cache the current page if a rotation did happen. This happens before the rotation animation

// is occuring and the bounds changed so we use this as an opportunity to cache the current index path

if (_cachedCollectionViewBounds.size.width != self.collectionView.bounds.size.width) {

_cachedCollectionViewBounds = self.collectionView.bounds;

// Figurring out current page based on the old bounds visible space

CGRect visibleRect = oldBounds;

CGFloat visibleXCenter = CGRectGetMidX(visibleRect);

NSArray<UICollectionViewLayoutAttributes *> *layoutAttributes = [self layoutAttributesForElementsInRect:visibleRect];

for (UICollectionViewLayoutAttributes *attributes in layoutAttributes) {

if ([attributes representedElementCategory] == UICollectionElementCategoryCell && attributes.center.x == visibleXCenter) {

_currentIndexPath = attributes.indexPath;

break;

}

}

_didRotate = YES;

}

[super prepareForAnimatedBoundsChange:oldBounds];

}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset

{

// Don't mess around if the user is interacting with the page node. Although if just a rotation happened we should

// try to use the current index path to not end up setting the target content offset to something in between pages

if (_didRotate || (!self.collectionView.isDecelerating && !self.collectionView.isTracking)) {

_didRotate = NO;

if (_currentIndexPath) {

return [self _targetContentOffsetForItemAtIndexPath:_currentIndexPath proposedContentOffset:proposedContentOffset];

}

}

return [super targetContentOffsetForProposedContentOffset:proposedContentOffset];

}

- (CGPoint)_targetContentOffsetForItemAtIndexPath:(NSIndexPath *)indexPath proposedContentOffset:(CGPoint)proposedContentOffset

{

if ([self _dataSourceIsEmpty]) {

return proposedContentOffset;

}

UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForItemAtIndexPath:_currentIndexPath];

if (attributes == nil) {

return proposedContentOffset;

}

CGFloat xOffset = (CGRectGetWidth(self.collectionView.bounds) - CGRectGetWidth(attributes.frame)) / 2.0;

return CGPointMake(attributes.frame.origin.x - xOffset, proposedContentOffset.y);

}

- (BOOL)_dataSourceIsEmpty

{

return ([self.collectionView numberOfSections] == 0 ||

[self.collectionView numberOfItemsInSection:0] == 0);

}