另类布局另类布局

http://blog.it985.com/5121.html

http://blog.jobbole.com/61101/

func viewWillLayoutSubviews() {

super.viewWillLayoutSubviews()
if tableView.tableViewHeaderView == nil {
let header: MyHeaderView = MyHeaderView.createHeaderView()
header.setNeedsUpdateConstraints()
header.updateConstraintsIfNeeded()
header.frame = CGRectMake(0, 0, CGRectGetWidth(tableView.bounds), CGFloat.max)
var newFrame = header.frame
header.setNeedsLayout()
header.layoutIfNeeded()
let newSize = header.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize)
newFrame.size.height = newSize.height
header.frame = newFrame
self.tableView.tableHeaderView = header
}