title: System font for both iOS 8 and iOS 9
body: <p>I want to support both iOS 8 and iOS 9 systems for my app. And maybe iOS 7. As we know, system font for iOS 7 and 8 is Helvetica Neue. But in iOS 9 system font is San-Francisco. And if you don't set Helvetica font explicitly via <code>[UIFont fontWithName:@"HelveticaNeue" size:15];</code>, but use <code>[UIFont systemFontOfSize:15];</code>, you'll get Helvetica for iOS 7 and 8 and San-Francisco for iOS 9 automatically. And it's great!
  For interface builder's labels and buttons you can set thin, ultra thin, medium etc. system fonts. It is great too. But how can I set these thin, ultra, medium system fonts in code, programmatically?
  Do I need to create a category with a fork for iOS 9 and previous iOS?</p>