Add an initial state to your book.

type alias SharedState =
    { value : String }




book : Book SharedState
book "MyApp"
    |> withStatefulOptions [
        initialState { value = "" }
    ]

Add subscriptions to your book.

import ElmBook.Actions exposing (updateState)


type alias SharedState =
    { value : String
    , timestamp : Posix
    }




updateAnimationState : Posix -> SharedState -> SharedState
updateAnimationState posix state =
    { state | timestamp = posix }




book : Book SharedState
book "MyApp"
    |> withStatefulOptions [
        subscriptions [
            Browser.Events.onAnimationFrame
                (updateStateWith updateAnimationState)
        ]
    ]

Change your book's state based on the themes current mode.

This can be useful for showcasing your own dark themed components when using elm-book's dark mode.

type alias SharedState =
    { darkMode : Bool
    }


initialState : SharedState
initialState =
    { darkMode = False
    }


book : Book SharedState
book "MyApp"
    |> withStatefulOptions
        [ ElmBook.StatefulOptions.initialState
            initialState
        , ElmBook.StatefulOptions.onDarkModeChange
            (\darkMode state ->
                { state | darkMode = darkMode }
            )
        ]