private fun fibonacci() = GlobalScope.launch(<Dispatchers.IO>) { val result = fib(25) ...

# coroutines

[pteale]

private fun fibonacci() = GlobalScope.launch(Dispatchers.IO) {
val result = fib(25)
withContext(Dispatchers.Main) {
textViewCalculating.text = result.toString()
}
}

[louiscad]

Do not use GlobalScope, it's dangerous (leaks…). Use AndroidX lifecycle runtime KTX and use lifecycleScope instead.