The interface to a size picking service. This service provides other sub-components the ability to request that the user pick a size. The UI for the size picking is handled by the service, and is not within the calling sub-components view.
Clients of this code can think of it in the same way as a port: You make
a request by calling a function that returns a value which is returned
from its update function to the containing component. That request value
may contain a message function, so that a Msg is delivered to the client at
a later time.
Example:
import Command
import SizePicker
type alias Model = Maybe Int
type Msg =
= Start
| SetSize Int
update : Msg -> Model -> (Model, Command Msg)
update msg model =
case msg of
Start
-> (Nothing, Command.SizePicker <| SizePicker.askForSize SetSize)
SetSize size
-> (Just Size, Command.none)
Create a request for the size picker service to ask the user for a size.
When the user has chosen a size, it will be passed to the supplied function
to create a message object, which will be routed to your update function.
The supplied string is used to identify the request to the user.
Convert a Request based one message type to another. This is analogous
to Cmd.map. It lets parent components handle requests made by children
components. However, parent components don't usualy call this directly, but
use Command.map (which calls this internally).
Requests that the size picking service can accept.
Normally, a client uses a convience function, like
askForSize, to build a request, and the wraps the request to make it aCommand:update : Msg -> Model -> (Model, Command Msg) update msg model = ... let model_ = ... command_ = Command.SizePicker <| askForSize "Pikachu" SetSize in (model_, command_)Generally, clients are expected to use the request building functions (like
askForSize) rather than these constructors directly.The constructors are exposed so that the implementation,
SizePicker.Servicecan pattern match on them. There is no real issue with lack of encapsulation here because any information in these constructors has to have come from the client anyway.