WWDC 2013 Session笔记 - iOS7中弹簧式列表的制作

//ViewController.m

@interface ViewController ()<UICollectionViewDataSource, UICollectionViewDelegate>
@property (nonatomic, strong) VVSpringCollectionViewFlowLayout *layout;
@end

static NSString *reuseId = @"collectionViewCellReuseId";

@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
    
	self.layout = [[VVSpringCollectionViewFlowLayout alloc] init];
    self.layout.itemSize = CGSizeMake(self.view.frame.size.width, 44);
    UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:self.layout];
    
    collectionView.backgroundColor = [UIColor clearColor];
    
    [collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:reuseId];
    
    collectionView.dataSource = self;
    [self.view insertSubview:collectionView atIndex:0];
}

#pragma mark - UICollectionViewDataSource
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 50;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseId forIndexPath:indexPath];
    
    //Just give a random color to the cell. See https://gist.github.com/kylefox/1689973
    cell.contentView.backgroundColor = [UIColor randomColor];
    return cell;
}
@end

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

@implementation VVSpringCollectionViewFlowLayout
//...

-(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 = 0.5;
            spring.frequency = 0.8;
            
            [_animator addBehavior:spring];
        }
    }
}
@end

@interface UIDynamicAnimator (UICollectionViewAdditions)

// When you initialize a dynamic animator with this method, you should only associate collection view layout attributes with your behaviors.
// The animator will employ thecollection view layout’s content size coordinate system.
- (instancetype)initWithCollectionViewLayout:(UICollectionViewLayout*)layout;

// The three convenience methods returning layout attributes (if associated to behaviors in the animator) if the animator was configured with collection view layout
- (UICollectionViewLayoutAttributes*)layoutAttributesForCellAtIndexPath:(NSIndexPath*)indexPath;
- (UICollectionViewLayoutAttributes*)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath;
- (UICollectionViewLayoutAttributes*)layoutAttributesForDecorationViewOfKind:(NSString*)decorationViewKind atIndexPath:(NSIndexPath *)indexPath;

@end

//VVSpringCollectionViewFlowLayout.m
@implementation VVSpringCollectionViewFlowLayout

//...

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

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

//VVSpringCollectionViewFlowLayout.m
@implementation VVSpringCollectionViewFlowLayout

//...

-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    UIScrollView *scrollView = self.collectionView;
    CGFloat scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y;
    
    //Get the touch point
    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 / 500;
        
        UICollectionViewLayoutAttributes *item = [spring.items firstObject];
        CGPoint center = item.center;

		//In case the added value bigger than the scrollDelta, which leads an unreasonable effect
        center.y += (scrollDelta > 0) ? MIN(scrollDelta, scrollDelta * scrollResistance)
                                      : MAX(scrollDelta, scrollDelta * scrollResistance);
        item.center = center;
        
        [_animator updateItemUsingCurrentState:item];
    }
    return NO;
}

@end