弹性collectionViewLayout

import "VVSpringCollectionViewFlowLayout.h"

@interface VVSpringCollectionViewFlowLayout()
@property (nonatomic, strong) UIDynamicAnimator *animator;
@end

@implementation VVSpringCollectionViewFlowLayout

    if  ([super init])  {
        _springDamping = 0.5;
        _springFrequency = 0.8;
        _resistanceFactor = 500;
    }
    return self;
}```

```- (void)setSpringDamping:(CGFloat)springDamping {
    if (springDamping >= 0 && _springDamping != springDamping) {
        _springDamping = springDamping;
        for (UIAttachmentBehavior *spring in _animator.behaviors) {
            spring.damping = _springDamping;
        }
    }
}```

```- (void)setSpringFrequency:(CGFloat)springFrequency {
    if (springFrequency >= 0 && _springFrequency != springFrequency) {
        _springFrequency = springFrequency;
        for (UIAttachmentBehavior *spring in _animator.behaviors) {
            spring.frequency = _springFrequency;
        }
    }
}```

```- (void)prepareLayout {
    [super prepareLayout];
    if (!_animator) {
        _animator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
        CGSize contentSize = [self collectionViewContentSize];
        NSArray *items = [super layoutAttributesForElementsInRect:CGRectMake(0, 0, contentSize.width, contentSize.height)];```
        
        for (UICollectionViewLayoutAttributes *item in items) {
            UIAttachmentBehavior *spring = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:item.center];
            
            spring.length = 0;
            spring.damping = self.springDamping;
            spring.frequency = self.springFrequency;
            
            [_animator addBehavior:spring];
        }
    }
}

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

```- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    return [_animator layoutAttributesForCellAtIndexPath:indexPath];
}```

```- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    UIScrollView *scrollView = self.collectionView;
    CGFloat scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y;
    CGPoint touchLocation = [scrollView.panGestureRecognizer locationInView:scrollView];```
    
   ``` for (UIAttachmentBehavior *spring in _animator.behaviors) {
        CGPoint anchorPoint = spring.anchorPoint;
        CGFloat distanceFromTouch = fabsf(touchLocation.y - anchorPoint.y);
        CGFloat scrollResistance = distanceFromTouch / self.resistanceFactor;```
        
        ```UICollectionViewLayoutAttributes *item = [spring.items firstObject];
        CGPoint center = item.center;
        center.y += (scrollDelta > 0) ? MIN(scrollDelta, scrollDelta * scrollResistance)
        : MAX(scrollDelta, scrollDelta * scrollResistance);
        item.center = center;
        [_animator updateItemUsingCurrentState:item];
    }
    return NO;
}```

@end