tableView.rowHeight = UITableViewAUtomaticDimension
UITableViewAutomaticDimension is the default value for rowHeight,
it is useful for understanding what is going on. Setting the
estimatedRowHeight property on the table view can improve performance. Instead
of asking each cell for its height when the table view loads, setting this property allows
some of that performance cost to be deferred until the user starts scrolling.
原文：http://www.cnblogs.com/davidgu/p/5340163.html