Shows a way to use function types to handle errors.

ErrorHandling.kt

// Create a custom error class that holds both the error that occured,
// and a function to invoke if it retries. 
data class MyCustomError(
    val error: Throwable? = null,
    val retry: (() -> Unit)? = null
)

// Create a state class that represents the current state of your screen.
// This is where you'd wanna put data you have to display, like a list of items
// or maybe an error, and a function to call if you want to retry something
data class MyState(
    val data: List<Item>? = null,
    val error: MyCustomError? = null
)

class MyPresenter : MyContract.Presenter {
    private var view: MyContract.View = //...

    private var currentState = MyState()

    fun loadData() {
        myFlowable
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(
                { result ->
                    val newState = MyState(data = result.data)
                    handleState(newState)
                },
                { error ->
                    val retry = { this.loadData() } // This is how you define a function type in Kotlin
                    val myError = MyCustomError(error, retry)
                    val newState = MyState(error = myError)
                    handleState(newState)
                }
            )
    }

    // Somewhere in the view, after you show the error, you'll probably have a button click 
    // that would call this method
    fun retryButtonClicked() {
        currentState.error?.retry?.invoke()
    }

    private fun handleState(newState: MyState) {
        currentState = newState

        if (currentState.data != null) {
            myView.showItems(currentState.data)
        }

        if (currentState.error != null) {
            myView.showError(currentState.error)
        }
    }
}