ios - UILabel can't mix strikethrough and non strikethrough styles? -

i want display string in uilabel, part of has strikethrough style. seems can't put strikethrough attribute on part of string. if it's on whole string, displays; if it's on part of string, not. bug? missing something?

here example code, , screen shot.

class viewcontroller: uiviewcontroller {      @iboutlet weak var label1: uilabel!     @iboutlet weak var label2: uilabel!      override func viewdidload() {         super.viewdidload()          let strikeattrs = [nsstrikethroughstyleattributename:                             nsunderlinestyle.stylesingle.rawvalue]          let str1 = nsmutableattributedstring()         str1.append(nsattributedstring(string: "aaa", attributes: strikeattrs))          let str2 = nsmutableattributedstring()         str2.append(nsattributedstring(string: "aaa", attributes: nil))         str2.append(nsattributedstring(string: "bbb", attributes: strikeattrs))          label1.attributedtext = str1         label2.attributedtext = str2     }  } 

the screen looks this: