#import <AsyncDisplayKit/ASPagerFlowLayout.h>

#import <AsyncDisplayKit/ASCellNode.h>

#import <AsyncDisplayKit/ASCollectionView.h>

@interface ASPagerFlowLayout () {

__weak ASCellNode *_currentCellNode;

}

@implementation ASPagerFlowLayout

- (ASCollectionView *)asCollectionView

{

// Dynamic cast is too slow and not worth it.

return (ASCollectionView *)self.collectionView;

}

- (void)prepareLayout

{

[super prepareLayout];

if (_currentCellNode == nil) {

[self _updateCurrentNode];

}

}

- (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 (!self.collectionView.decelerating && !self.collectionView.tracking) {

NSIndexPath *indexPath = [self.asCollectionView indexPathForNode:_currentCellNode];

if (indexPath) {

return [self _targetContentOffsetForItemAtIndexPath:indexPath proposedContentOffset:proposedContentOffset];

}

}

return [super targetContentOffsetForProposedContentOffset:proposedContentOffset];

}

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

{

if ([self _dataSourceIsEmpty]) {

return proposedContentOffset;

}

UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForItemAtIndexPath:indexPath];

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);

}

- (void)_updateCurrentNode

{

// Never change node during an animated bounds change (rotation)

// NOTE! Listening for -prepareForAnimatedBoundsChange and -finalizeAnimatedBoundsChange

// isn't sufficient here! It's broken!

NSArray *animKeys = self.collectionView.layer.animationKeys;

for (NSString *key in animKeys) {

if ([key hasPrefix:@"bounds"]) {

return;

}

}

CGRect bounds = self.collectionView.bounds;

CGRect rect = CGRectMake(CGRectGetMidX(bounds), CGRectGetMidY(bounds), 1, 1);

NSIndexPath *indexPath = [self layoutAttributesForElementsInRect:rect].firstObject.indexPath;

if (indexPath) {

ASCellNode *node = [self.asCollectionView nodeForItemAtIndexPath:indexPath];

if (node) {

_currentCellNode = node;

}

}

}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds

{

[self _updateCurrentNode];

return [super shouldInvalidateLayoutForBoundsChange:newBounds];

}

- (UICollectionViewLayoutInvalidationContext *)invalidationContextForBoundsChange:(CGRect)newBounds

{

UICollectionViewFlowLayoutInvalidationContext *ctx = (UICollectionViewFlowLayoutInvalidationContext *)[super invalidationContextForBoundsChange:newBounds];

ctx.invalidateFlowLayoutDelegateMetrics = YES;

ctx.invalidateFlowLayoutAttributes = YES;

return ctx;

}