elm-pages maintains a single Maybe Navigation state which is accessible from your Route modules through app.navigation.

You can use it to show a loading indicator while a page is loading:

import Pages.Navigation as Navigation


pageLoadingIndicator app =
    case app.navigation of
        Just (Navigation.Loading path _) ->
            Spinner.view


        Nothing ->
            emptyView


emptyView : Html msg
emptyView =
    Html.text ""

You can also use it to derive Pending UI or Optimistic UI from a pending form submission:

import Form
import Form.Handler
import Pages.Navigation as Navigation


view app =
    let
        optimisticProduct : Maybe Product
        optimisticProduct =
            case app.navigation of
                Just (Navigation.Submitting formData) ->
                    formHandler
                        |> Form.Handler.run formData
                        |> Form.toResult
                        |> Result.toMaybe


                Just (Navigation.LoadAfterSubmit formData path _) ->
                    formHandler
                        |> Form.Handler.run formData
                        |> Form.toResult
                        |> Result.toMaybe


                Nothing ->
                    Nothing
    in
    -- our `productsView` function could show a loading spinner (Pending UI),
    -- or it could assume the product will be created successfully (Optimistic UI) and
    -- display it as a regular product in the list
    productsView optimisticProduct app.data.products


allForms : Form.Handler.Handler String Product
allForms =
    Form.Handler.init identity productForm


editItemForm : Form.HtmlForm String Product input msg
editItemForm =
    Debug.todo "Form definition here"