我有一份报告，以此为基础来执行许多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
)
这些呼叫被排序并一个接一个地执行，这非常慢，因为我需要连续执行20个呼叫。我想做一些类似于JavaScript的
Promise.all(prepareRequests)
我曾经使用Cmd.Batch对它们进行处理，但是后来我找不到一种方法来知道整个批次何时完成加载，如果屏幕上没有足够的行，则需要加载另一个批次。 / p>
答案 0 :(得分：1)
随机思想：
鉴于您将获得每个呼叫的响应，可以通过在调用批处理之前创建预期响应的集合来跟踪所收到的呼叫，然后在每次接收到响应时从集合中删除相应的项目。
在该收藏集为空时，您已经收到所有回复，可以解雇下一批。
此模式上的任何数量的变化都应达到期望的结果。 （可能还有其他同样有效的模式。）
答案 1 :(得分：1)
我相信Murph已经发布的解决方案是正确的。下面的代码是一个示例，通过首先从flickr获取照片集，然后通过批量处理一堆http get任务来获取所有照片的标题，来演示该解决方案。模型中保留两个列表-未命名的照片和已命名的照片。随着对http的响应进入，会将适当的照片添加到已分配标题的带标题的照片中。
在此示例中，当标题列表的长度与无标题列表的长度相同时，代码可以告诉所有获取都已响应，但是通过从无标题列表中删除照片也可以轻松完成此操作列出直到它为空。
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)
]