set tableHeaderView height dynamicly

  let targetWidth = self.view.bounds.size.width;
        let targetSize = CGSize(width: targetWidth, height: UIView.layoutFittingCompressedSize.height)
        //如果headView中有通过aspect ratio决定高度的constraint,这将导致systemLayoutSizeFittingSize计算错误，所以在代码中把这种constraint转为固定高度的constraint
        //see : https://stackoverflow.com/questions/44801492/systemlayoutsizefitting-ignoring-layout-constraints-and-using-image-view-intrins
        let imageView = self.empyHeadView.subviews.last!
        imageView.constraintWithIdentifier("heightConstraint")!.constant = (targetWidth - 40) * CGFloat(0.3);
        //宽度固定，高度可变
        let resultSize =  self.empyHeadView.systemLayoutSizeFitting(targetSize, withHorizontalFittingPriority: UILayoutPriority.defaultHigh, verticalFittingPriority:UILayoutPriority.fittingSizeLevel );
        
        //set
        self.empyHeadView.bounds = CGRect(origin: CGPoint.zero, size: resultSize)
        self.tableView.tableHeaderView = self.empyHeadView;