Making (Android) Spannable great again with Kotlin

Demo.kt

val world = "World"
val spannedText = SpannableString("Hello $world!")
spannedText
    .spanWith(world) {
      what = BackgroundColorSpan(Color.RED)
      flags = Spanned.SPAN_EXCLUSIVE_EXCLUSIVE
    }
    .spanWith(world) {
      what = StyleSpan(Typeface.BOLD)
      flags = Spanned.SPAN_EXCLUSIVE_EXCLUSIVE
    }

SpannableExtensions.kt

fun SpannableString.spanWith(
  target: String,
  apply: SpannableBuilder.() -> Unit
): SpannableString {
  val builder = SpannableBuilder()
  apply(builder)

  val start = this.indexOf(target)
  val end = start + target.length

  setSpan(builder.what, start, end, builder.flags)
  return this
}

class SpannableBuilder {
  lateinit var what: Any
  var flags: Int = 0
}