title: UICollectionView vertically Centred
body: <p>I have a <code>UICollectionView</code> object of size (320,500). </p>
  
  <p>I need a to create the <code>UICollectionView</code> in such a way that contents are vertically centred. Means if total size of the content is (100,100) then the cells should drawing itself in a rect (0,200):(320,300). that is 50 above and 50 below the vertical centre at 250. </p>
  
  <p>Problem is The number of cells and size of cells is decided only at runtime. For example if the text with in the cell can be 1 line is of size ~ (100,50) . If it is 4 lines size will be ~ (100,200). So it is only after I have run <code>collectionView:sizeForItemAtIndexPath:</code> for the last cell that I can determine what should be starting position for first cell. </p>
  
  <p>Any starting suggestion how I should approach this problem. Even if I subclass <code>Flowlayout</code> how will I know the position of the first cell before I draw the last one ?</p>