Reactive Sample with Live Data

InputConfirmViewModel.kt

class InputConfirmViewModel : ViewModel() {
    // inputs
    val inputStream = MutableLiveData<String>()
    val inputClickBtn = MutableLiveData<Unit?>().apply { observeForever {
            errorEventStream.value =
                    if(inputStream.value.isIntNum())  null else "not num"
    } }
    // outputs
    val btnEnableStream = MediatorLiveData<Boolean>().apply { value = false }.also { enable ->
        enable.addSource(inputStream) { ins: String ->
            enable.value = ins.isLength(3)
        }
    }
    val errorEventStream = MediatorLiveData<String?>().apply { value = null }.also { inState ->
        inState.addSource(btnEnableStream) { enable: Boolean ->
            inState.value = if (enable) null else "error"
        }
    }

    fun checkInput() {
        inputClickBtn.value = null
    }

    fun String.isLength(len: Int) = this.length == len

    fun String?.isIntNum(): Boolean {
        return this != null && this.toIntOrNull()?.let { true } ?: false
    }
}