What does

tailrec

do here?

tailrec fun Context.activity(): Activity? = when {
  this is Activity -> this
  else -> (this as? ContextWrapper)?.baseContext?.activity()
}

It tells the compiler to generate bytecode that uses a loop instead of recursion. See the doc for more information: https://kotlinlang.org/docs/functions.html#tail-recursive-functions

imo this represents the intention better:

fun Context.activity(): Activity? =
    generateSequence(this) { (it as? ContextWrapper)?.baseContext }
        .filterIsInstance<Activity>()
        .firstOrNull()

the

generateSequence

version is dramatically less efficient than the

tailrec

version