A data state sealed class that treats Loading as a member of every state, rather than an exclusive state itself.

UiState.kt

/*
    This UiState model addresses the problems associated with treating Loading as its own state.

    Loading in general is not an exclusive state.
    You may be showing data, and fetching (Success + Loading).
    You might show an error, and try to fetch the data again (Error + Loading)
    The only time Loading is kind of its own thing is when the screen is first opened, and nothing is displayed yet.
    So that case, InitialBlankLoading should be a separate state.

    Here's what a flow in this would look like, fetching a list of messages:

    1. I open the screen:
        InitialBlankLoading(loading = true)

    2. I fetch a list of messages:
        still InitialBlankLoading(loading = true)

    3. I get the list of the messages:
        Success(loading = false)
        
    4. I fetch messages again, keeping the previously loaded messages onscreen until it's refreshed:
        Success(loading = true)

    5. I receive the updated message list:
        Success(loading = false)

    6. I try to fetch the message list again, displaying the old data until it's refreshed:
        Success(loading = true)

    7. I get an error, and display the error UI:
        Error(loading = false)

    8. I fetch again, keeping the error UI on screen until we get a response:
        Error(loading = true)

    9. I receive a correct response, but I've deleted all my messages, so it's just an empty list.
        Empty(loading = false)

    The copyLoading() method helps us change state without losing data.
    We don't want a flash of an empty screen every time we fetch it again.
*/
sealed interface UiState<out T: Any> {
    val loading: Boolean
    fun copyLoading(loading: Boolean): UiState<T>
}

/**
 * We've successfully gotten the data we need for this view.
 */
data class Success<out T: Any>(
    val data: T,
    override val loading: Boolean
) : UiState<T> {
    override fun copyLoading(loading: Boolean): UiState<T> {
        return this.copy(loading = loading)
    }
}

/**
 * There was a problem getting the data for this view.
 */
data class Error<out T: Any>(
    val exception: Exception,
    override val loading: Boolean
): UiState<T> {
    override fun copyLoading(loading: Boolean): UiState<T> {
        return this.copy(loading = loading)
    }
}

/**
 * We got a response, but there's nothing there, eg. an empty list.
 */
data class Empty<out T: Any>(
    override val loading: Boolean
): UiState<T> {
    override fun copyLoading(loading: Boolean): UiState<T> {
        return this.copy(loading = loading)
    }
}

/**
 * Only for opening the page for the first time, fetching data before any UI has previously loaded.
 */
data class InitialBlankLoading<out T: Any>(
    override val loading: Boolean = true
): UiState<T> {
    override fun copyLoading(loading: Boolean): UiState<T> {
        return this.copy(loading = loading)
    }
}