Making (Android) Spannable great again with Kotlin

Demo.kt

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

SpannableExtensions.kt

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

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

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

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