从文档可以看出 这个方法有可能返回为空
Return Value
The cell object at the corresponding index path or nil if the cell is not visible or indexPath is out of range.
让cell可见解决办法 ：（实现选中某个cell的功能）
self.collectionView.reloadData()
let indexPath = NSIndexPath(forItem: 0,inSection: 0)
var cell = collectionView.cellForItemAtIndexPath(indexPath)
if cell == nil {
// self.collectionView.reloadData()
self.collectionView.layoutIfNeeded()
cell = collectionView.cellForItemAtIndexPath(indexPath)
cell!.selected = true
collectionView.selectItemAtIndexPath(indexPath, animated: true, scrollPosition: .Top)
}
http://stackoverflow.com/questions/21469459/cellforitematindexpath-returns-nil-after-force-scrolling-to-make-it-visible/21480786#21480786
