I am a beginner in Elm and to learn it I have started ts-converter, a tiny browser extension to convert timestamps in a readable format. This is one of my daily needs.
I want to use what I have learnt from it, sharing some notes on how to manage event listeners in a Elm application.
I'll use onEnter
event in the examples.
Scenario 1: Listen for all application events
To capture a key pressed in the whole application we need to introduce a subscription
What's a subscription ?
A subscription
is an operation allowing us to register to an external event.
Creating a subscription we say to Elm runtime that we want to do an action when a particular event is spawned. When the runtime captures an event, produces an update message.
subscriptions _ =
onKeyPress keyDecoder
A subscription is created on event onKeyPress
and when this event will be intercepted by the Elm runtime the function keyCoder
will be invoked.
keyDecoder : Decode.Decoder Msg
keyDecoder =
Decode.map toKey (Decode.field "key" Decode.string)
The event object is serialized as a JSON object and its field key
contains the key pressed value, Decode.field "key" Decode.string
extracts this number and puts it into a valid Elm value.
The extracted value is then passed to toKey
function.
toKey : String -> Msg
toKey keyValue =
case String.uncons keyValue of
Just ( char, "" ) ->
CharacterKey char
_ ->
ControlKey keyValue
String.uncons
takes a string as a characters array and splits it into its head and tail.
Depending on whether the leading character is an alphanumeric value
or not, produces the related update
message.
CharacterKey k ->
YOUR CODE
ControlKey k ->
YOUR CODE
The two messages will be managed into the update
function.
Scenario 2: Listen for a event bound to a input field
At the moment (v0.19) Elm doesn't support natively onEnter
event, but it offers a function on
that permits to create a custom event listener.
onEnter: Msg -> Attribute Msg
onEnter msg =
let
isEnter code =
if code == 13 then
Decode.succeed msg
else
Decode.fail "not ENTER"
in
on "keydown" (Decode.andThen isEnter keyCode)
With on "keydown" (Decode.andThen isEnter keyCode)
we'll create a listener on keydown
event and the Elm runtime will execute (Decode.andThen isEnter keyCode)
on its capture.
keyCode
is a bultin function of Html.Events
module and it returns the value of pressed key as an integer.
isEnter
checks if key enter
has been pressed (enter value is 13) producing respectively a message for the update
function or a failure.
onEnter
is used into the input field we will listen for events.
input [ size 20, value (tsToString model.ts), onInput SetTs, onEnter Convert, autofocus True] []
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