在 Elm 中取消订阅

Question

我正在按照 Elm 指南进行操作，现在正尝试为时钟示例实现一个暂停按钮。在指南中写道：

Add a button to pause the clock, turning the Time subscription off.

我所做的只是向模型添加一个 paused 属性并在更新函数中使用它。我应该如何取消订阅？

module Main exposing (..)

import Html exposing (Html)
import Html.App as App
import Html.Events exposing (onClick)
import Svg exposing (..)
import Svg.Attributes exposing (..)
import Time exposing (Time, second)


main =
    App.program
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
        }


type alias Model =
    { time : Time
    , paused : Bool
    }


init : ( Model, Cmd Msg )
init =
    ( Model 0 False, Cmd.none )


type Msg
    = Tick Time
    | Toggle


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Tick newTime ->
            if not model.paused then
                ( Model newTime False, Cmd.none )
            else
                ( model, Cmd.none )

        Toggle ->
            ( Model model.time (not model.paused), Cmd.none )


subscriptions : Model -> Sub Msg
subscriptions model =
    Time.every second Tick


clock : Time -> Html Msg
clock time =
    let
        sAngle =
            turns (Time.inMinutes time)

        sHandX =
            toString (50 + 40 * cos sAngle)

        sHandY =
            toString (50 + 40 * sin sAngle)
    in
        svg [ viewBox "0 0 100 100", width "300px" ]
            [ circle [ cx "50", cy "50", r "45", fill "#0B79CE" ] []
            , line [ x1 "50", y1 "50", x2 sHandX, y2 sHandY, stroke "#023963" ] []
            ]


view : Model -> Html Msg
view model =
    let
        btnText =
            if model.paused then
                "Start"
            else
                "Pause"
    in
        Html.div []
            [ clock model.time
           , Html.button [ onClick Toggle ] [ Html.text btnText ]
           ]

Answer 1

我想你可以简单地 return Sub.none 代替您的 subscription 功能暂停时。

如果你选择这样做，那么你可以恢复 Tick 您的 update 函数中的处理程序。

subscriptions : Model -> Sub Msg
subscriptions model =
    if model.paused
    then Sub.none
    else Time.every second Tick

在 Elm 中取消订阅

Cancel subscriptions in Elm

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