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，它通过首先从 flickr 获取一组照片，然后通过批处理一堆 http get 任务来获取所有这些照片的标题，从而演示了该解决方案。模型中维护了两个列表 - 未命名的照片和已命名的照片。随着对 http 的响应进入，相应的照片将添加到已分配标题的标题照片中。
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，代码可以告诉所有获取都已得到响应，但它可以通过从无标题列表中删除照片直到它是空的。
这是一个工作演示
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)
]