AttributesString.swift

import Foundation
#if os(iOS)
import UIKit
#endif

#if os(watchOS)
import WatchKit
#endif

extension NSMutableAttributedString {
  public var fontAttribute: StringAttributer<UIFont> {
    get {
      return StringAttributer(string:self, attribute: NSFontAttributeName)
    }
    set {
      // please the compiler
    }
  }
}

public struct StringAttributer<T> {
  
  let string: NSMutableAttributedString
  let attribute: String
  
  public subscript(from start: Int, to end: Int) -> T? {
    get {
      return string.attributesAtIndex(start, effectiveRange: nil)[attribute] as? T
    }
    set {
      guard let value = newValue as? AnyObject else { return }
      string.addAttribute(attribute, value: value, range: NSMakeRange(start, end - start))
    }
  }
  
}

let plainString = "test"
let attrString = NSMutableAttributedString(string:plainString)
attrString.fontAttribute[from: 0, to:1] = UIFont.systemFontOfSize(25, weight: UIFontWeightSemibold)
attrString.fontAttribute[from: 2, to:3] = UIFont.systemFontOfSize(20, weight: UIFontWeightRegular)