elm动画-为div的高度设置动画，使其适合内容

import Animation exposing (..)
import Animation.Attributes exposing (..)
import Animation.Easing exposing (..)

type Msg = Animate

type alias Model =
    { height : Float
    , animation : Animation Msg
    }

init : Model
init =
    { height = 0
    , animation = Animation.none
    }

update : Msg -> Model -> Model
update msg model =
    case msg of
        Animate ->
            { model | animation = animateTo 500 (to 300) model.height }

subscriptions : Model -> Sub Msg
subscriptions model =
    Time.every 1000 (\_ -> Animate)

view : Model -> Html Msg
view model =
    div []
        [ button [ onClick Animate ] [ text "Start Animation" ]
        , div [ style [ ("height", px model.height) ] ] []
        ]

main : Program Never Model Msg
main =
    Html.program
        { init = init
        , update = update
        , subscriptions = subscriptions
        , view = view
        }