swift 高度自适应方法

   func ZuoHeightWithConstrainedWidth ( width : CGFloat , font :  UIFont,  str :NSString )  ->  CGFloat  {

    let constraintRect = CGSize ( width : width , height : CGFloat . max )

    

    let boundingBox =  str.boundingRectWithSize ( constraintRect , options : NSStringDrawingOptions . UsesLineFragmentOrigin , attributes :  [NSFontAttributeName : font ], context : nil )

    

   //返回boundingBox的。高度

        return boundingBox.height

    }