如何一起批处理多个http调用？

Question

我有一份报告，我以此为基础执行大量 Http 调用以获取每一行的详细信息。

LoadReport ->
    ( model
    , Http.toTask (loadReport model.token model.page)
        |> Task.andThen
            (\report ->
                Task.map (addProductDetailsResultsToReport report) (Task.sequence (prepareRequests model.token report))
            )
        |> Task.map filterOnlyMissingBarcodes
        |> Task.attempt ProductData
    )

调用是按顺序一个接一个执行的，这非常慢，因为我需要连续执行 20 个调用。我想做一些类似于 JavaScript

的事情

 Promise.all(prepareRequests)

我曾经使用 Cmd.Batch 处理它们，但后来我找不到一种方法来知道整个批次何时完成加载，如果没有足够的行，我需要加载另一个批次屏幕。

Answer 1

随想：

鉴于每次调用都会收到响应，因此您可以在调用批处理之前创建预期响应的集合，然后在每次收到响应时从集合中删除适当的项目，从而跟踪收到的调用。

当该集合为空时，您已收到所有响应并可以启动下一批。

此模式有多种变体，应该可以达到预期的效果。（可能还有其他同样有效的模式。）

Answer 2

我相信 Murph 已经发布的解决方案是正确的。以下代码是演示该解决方案的示例，首先从 flickr 获取 collection 张照片，然后通过批处理一堆 http get 任务获取所有这些照片的标题。模型中维护了两个列表——无标题照片和有标题照片。当收到对 http 的响应时，适当的照片将添加到带有指定标题的带标题的照片中。

在这个例子中，当有标题列表的长度与无标题列表的长度相同时，代码可以告诉所有的获取都已被响应，但它可以通过从中删除照片来轻松完成无标题列表，直到它为空。

Here's a working demo

module Main exposing (..)

import Browser
import Html exposing (Html, div, text)
import Html.Attributes as HA
import Http
import Json.Decode as DC
import Svg
import Svg.Attributes as SA
import Task


type Msg
    = SetPhotos (Result Http.Error (List Photo))
    | SetDescription (Result Http.Error ( String, String ))


main : Program () Model Msg
main =
    Browser.element
        { init = init
        , view = view
        , update = update
        , subscriptions = \m -> Sub.none
        }


type alias Model =
    Result Http.Error
        { untitled : List Photo
        , titled : List Photo
        }


decodeUser : DC.Decoder String
decodeUser =
    DC.at [ "user", "id" ] DC.string


type alias Photo =
    { id : String
    , secret : String
    , server : String
    , farm : Int
    , description : Maybe String
    }



-- Create a Photo record from info retrieved from flickr api.
-- Get description later


initPhoto : String -> String -> String -> Int -> Photo
initPhoto id sec ser farm =
    Photo id sec ser farm Nothing


decodePhotoList : DC.Decoder (List Photo)
decodePhotoList =
    DC.list <|
        DC.map4 initPhoto
            (DC.at [ "id" ] DC.string)
            (DC.at [ "secret" ] DC.string)
            (DC.at [ "server" ] DC.string)
            (DC.at [ "farm" ] DC.int)



-- Decode photos from "flickr.people.getPublicPhotos" request.


decodePhotos : DC.Decoder (List Photo)
decodePhotos =
    DC.at [ "photos", "photo" ] decodePhotoList



-- Decode descripion of photo from "flickr.photos.getInfo" request.


decodePhotoDescription : DC.Decoder String
decodePhotoDescription =
    DC.at [ "photo", "description", "_content" ] DC.string



-- api key from flickr.  Anyone who clones this project should
-- get their own api key.


apiKey : String
apiKey =
    "e9d3fdd5c2e26f9ebd13f4983cf727db"


flickrRestServices : String
flickrRestServices =
    "https://api.flickr.com/services/rest/?"


noJsonCallback : String
noJsonCallback =
    "&format=json&nojsoncallback=1"


userUrl : String -> String
userUrl name =
    flickrRestServices
        ++ "&method=flickr.people.findByUserName"
        ++ "&api_key="
        ++ apiKey
        ++ "&username="
        ++ name
        ++ noJsonCallback


publicPhotosUrl : String -> String
publicPhotosUrl uid =
    flickrRestServices
        ++ "&method=flickr.people.getPublicPhotos"
        ++ "&api_key="
        ++ apiKey
        ++ "&user_id="
        ++ uid
        ++ noJsonCallback


photoInfoUrl : String -> String
photoInfoUrl photo =
    flickrRestServices
        ++ "&method=flickr.photos.getInfo"
        ++ "&api_key="
        ++ apiKey
        ++ "&photo_id="
        ++ photo
        ++ noJsonCallback



-- Cmd to get photo description from flickr.
-- Package results as SetDescription message.
-- Save the photo id with Task.map to apply the description to the right photo


setDescriptionCmd : Photo -> Cmd Msg
setDescriptionCmd dp =
    case dp.description of
        Nothing ->
            Task.attempt SetDescription (Task.map (\s -> ( dp.id, s )) <| Http.toTask <| Http.get (photoInfoUrl dp.id) decodePhotoDescription)

        Just des ->
            Cmd.none



-- Cmd to get users public photos from flickr.
-- Package results as SetPhotos message.


getPhotosCmd : String -> Cmd Msg
getPhotosCmd name =
    let
        req =
            Http.get (userUrl name) decodeUser

        userTask =
            Http.toTask req

        publicPhotosTask uid =
            Http.toTask (Http.get (publicPhotosUrl uid) decodePhotos)

        userPhotosTask =
            userTask |> Task.andThen publicPhotosTask
    in
    Task.attempt SetPhotos userPhotosTask


init : () -> ( Model, Cmd Msg )
init _ =
    ( Ok
        { untitled = []
        , titled = []
        }
    , getPhotosCmd "elmDemo" -- flickr user name
    )



-- UPDATE


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        SetPhotos (Ok photos) ->
            ( Ok
                { untitled = photos
                , titled = []
                }
            , Cmd.batch <| List.map setDescriptionCmd photos
            )

        SetPhotos (Err e) ->
            ( Err e
            , Cmd.none
            )

        -- Update description of the photo with matching id.
        SetDescription (Ok ( photoId, desc )) ->
            case model of
                Ok photos ->
                    let
                        justTitled =
                            photos.untitled
                                |> List.filter (\ph -> ph.id == photoId)
                                |> List.map (\ph -> { ph | description = Just desc })
                        newTitled = photos.titled ++ justTitled 
                        newPhotos = { photos | titled = newTitled }
                    in
                    ( Ok newPhotos
                    , if
                        List.length newPhotos.titled
                            == List.length newPhotos.untitled
                      then
                        Cmd.none -- Could do something else here.

                      else
                        Cmd.none
                    )

                Err e ->
                    ( Err e
                    , Cmd.none
                    )

        SetDescription (Err e) ->
            ( Err e
            , Cmd.none
            )



-- Compute a photo URL from a Photo record.
-- per: https://www.flickr.com/services/api/misc.urls.html


photoUrl : Photo -> String
photoUrl ps =
    "https://farm"
        ++ String.fromInt ps.farm
        ++ ".staticflickr.com/"
        ++ ps.server
        ++ "/"
        ++ ps.id
        ++ "_"
        ++ ps.secret
        ++ "_b.jpg"



-- show an image and description if available.


viewPhoto : Photo -> Html Msg
viewPhoto ps =
    div
        [ HA.style "height" "20%"
        , HA.style "width" "20%"
        , HA.style "margin" "0"
        ]
        [ div
            [ HA.style "height" "90%"
            , HA.style "width" "100%"
            , HA.style "margin" "0"
            ]
            [ Svg.svg
                [ SA.version "1.1"
                , SA.width "100%"
                , SA.height "100%"
                , SA.viewBox "-1 -0.6 2 1.2"
                , SA.preserveAspectRatio "none"
                ]
                [ Svg.image
                    [ SA.xlinkHref (photoUrl ps)
                    , SA.x "-1"
                    , SA.y "-0.6"
                    , SA.width "2"
                    , SA.height "1.2"
                    ]
                    []
                ]
            ]
        , div
            [ HA.style "height" "10%"
            , HA.style "width" "100%"
            , HA.style "margin" "0"
            ]
            [ div
                [ HA.style "text-align" "center" ]
                [ text <| Maybe.withDefault "" ps.description ]
            ]
        ]



-- Draw an image or display the reason the image is not available.


view : Model -> Html Msg
view model =
    case model of
        Err s ->
            text "Error: "

        Ok photos ->
            div []
                [ div [] [ text "UNTITLED" ]
                , div [] (List.map viewPhoto photos.untitled)
                , div [] [ text "TITLED" ]
                , div [] (List.map viewPhoto photos.titled)
                ]

如何一起批处理多个http调用？

How to batch multiple http calls together?

elm

elm-architecture