设置UICollectionView停止滚动是的偏移量，使其为与中心点
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
//计算最终显示的矩形框
CGRect rect;
rect.origin.x = proposedContentOffset.x;
rect.origin.y = 0;
rect.size = self.collectionView.frame.size;
//获取最终显示在矩形框中的元素的布局属性
NSArray *array = [super layoutAttributesForElementsInRect:rect];
//获取UICollectionView的中点，以contentView的左上角为原点
CGFloat centerX = proposedContentOffset.x + self.collectionView.frame.size.width * 0.5;
//获取所有元素到中点的最短距离
CGFloat minDelta = MAXFLOAT;
for (UICollectionViewLayoutAttributes *attribute in array)
{
CGFloat delta = attribute.center.x - centerX;
if (ABS(minDelta) > ABS(delta))
{
minDelta = delta;
}
}
//改变UICollectionView的偏移量
proposedContentOffset.x += minDelta;
return proposedContentOffset;
}
自定义圆形普通布局
定义成员属性，保存所有的布局属性
@property (nonatomic, strong) NSMutableArray *attrsArray;
懒加载，初始化attrsArray
- (NSMutableArray *)attrsArray
{
if (_attrsArray == nil)
{
_attrsArray = [NSMutableArray array];
}
return _attrsArray;
}
初始化布局
- (void)prepareLayout
{
[super prepareLayout];
//移除所有旧的布局属性
[self.attrsArray removeAllObjects];
//获取元素的个数
NSInteger count = [self.collectionView numberOfItemsInSection:0];
//布局所有的元素
for (NSInteger i = 0; i<count; i++)
{
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
//设置并获取indexPath位置元素的布局属性
UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];
//将indexPath位置元素的布局属性添加到所有布局属性数组中
[self.attrsArray addObject:attrs];
}
}
布局indexPath位置的元素
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(nonnull NSIndexPath *)indexPath
{
//获取元素的个数
NSInteger count = [self.collectionView numberOfItemsInSection:0];
/**设置圆心布局*/
//设置圆形的半径
CGFloat radius = 70;
//圆心的位置
CGFloat oX = self.collectionView.frame.size.width * 0.5;
CGFloat oY = self.collectionView.frame.size.height * 0.5;
//获取indexPath位置的元素的布局属性
UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
//设置尺寸
attrs.size = CGSizeMake(50, 50);
//设置位置
if (count == 1)
{
attrs.center = CGPointMake(oX, oY);
}
else
{
CGFloat angle = (2 * M_PI / count) * indexPath.item;
CGFloat centerX = oX + radius * sin(angle);
CGFloat centerY = oY + radius * cos(angle);
attrs.center = CGPointMake(centerX, centerY);
}
//返回indexPath位置元素的布局属性
return attrs;
}
