其实是在swift与OC相对应的API，实在没找到，好不容易找到了留下做个标记，建议这个方法写到string的extension中去
/**
* 字符串 size计算
*/
func sizeWith(_ font : UIFont , _ maxSize : CGSize , _ lineMargin : CGFloat) ->CGSize {
let options = NSStringDrawingOptions.usesLineFragmentOrigin
let paragraphStyle : NSMutableParagraphStyle = NSMutableParagraphStyle()
paragraphStyle.lineSpacing = lineMargin
var attributes : [String : Any] = [:]
attributes[NSFontAttributeName] = font
attributes[NSParagraphStyleAttributeName] = paragraphStyle
let textBouds = self.boundingRect(with: maxSize,
options: options,
attributes: attributes,
context: nil)
return textBouds.size
}
