According to Dial degree to layout UICollectionViewCell, center point of cell is averge to dial degree.
override layout instance method:
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath;
FWCollectionViewDialLayout *layout = [[FWCollectionViewDialLayout alloc] initWithDegree:120 cellSize:CGSizeMake(50, 50)];
[self.collectionView setCollectionViewLayout:layout];
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