CollectionViewDelegateFlowLayout's functions are all optional and are involved in the collection view's design. 

  • collectionView:layout:minimumInteritemSpacingForSection: Returns the desired spacing between successive items in the rows or columns of a section.
  • collectionView:minimumLineSpacingForSectionAtIndex: Returns the desired spacing between successive rows or columns of a section.
  • collectionView:layout:referenceSizeForFooterInSection: Returns the desired size of the footer view in the specified section.
  • collectionView:layout:referenceSizeForHeaderInSection: Returns the desired size of the header view in the specified section.
  • collectionView:layout:sizeForItemAtRow:andSection: Return the size of the specified item’s cell.
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