CircleLayout

https://developer.apple.com/library/ios/samplecode/CircleLayout/Introduction/Intro.html#//apple_ref/doc/uid/DTS40012315

//1.

[self.collectionView performBatchUpdates:^{

    [self.collectionView reloadItemsAtIndexPaths:@[indexPath]];

} completion:^(BOOL finished) {

}];

//这个方法会在第一个 block 中处理 insert/delete/reload/move 等操作，等操作完成之后会执行第二个block。

//2.自定义UICollectionViewLayout 类 CircleLayout

_viewController = [[ViewController alloc] initWithCollectionViewLayout:[[CircleLayout alloc] init]];

//(1)继承父类方法

-(void)prepareLayout

{

    [super prepareLayout];

    //在此方法中初始化此类的属性

    CGSize size = self.collectionView.frame.size;

   _cellCount = [[self collectionView] numberOfItemsInSection:0];

   _center = CGPointMake(size.width / 2.0, size.height / 2.0);

   _radius = MIN(size.width, size.height) / 2.5;

}

//(2)返回CollectionView 内容页的大小

-(CGSize)collectionViewContentSize

{

    return [self collectionView].frame.size;

}

//(3)在rect区域内 返回每个cell的LayoutAttributes

-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect

{

    NSMutableArray* attributes = [NSMutableArray array];

    for (NSInteger i=0 ; i < self.cellCount; i++) {

        NSIndexPath* indexPath = [NSIndexPath indexPathForItem:i inSection:0];

        [attributes addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];

    }

    return attributes;

}

//(4) 确定 attributes 的size 和 center

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path

{

    UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];

    attributes.size = CGSizeMake(ITEM_SIZE, ITEM_SIZE);

    attributes.center = CGPointMake(_center.x + _radius * cosf(2 * path.item * M_PI / _cellCount),

                                    _center.y + _radius * sinf(2 * path.item * M_PI / _cellCount));

    return attributes;

}