Building custom DOM event handlers in Elm

You’re just getting started with Elm 0.18. You’ve gotten used to the basics of the Elm architecture, and how to use the Html.Events package to listen to DOM events. Time to move on to building a more complex form.

You start by adding a select:

initialModel : List String
initialModel =
  [ "Dog"
  , "Cat"
  , "Hamster"
  ]

view : List String -> Html Msg
view animals =
  select [] (List.map animalOptions animals)

animalOptions : String -> Html a
animalOptions animal =
  option [] [ text animal ]

Looking good! Now all you need is to wire up the change event. Hmmm… it looks like Html.Events doesn’t provide an onChange event handler. What do you do now?

Event handlers in Elm

While Html.Events provides event handlers for the most common events, it does not provide then for all possible events (see this issue for a discussion on adding onChange). However, it does expose a few lower-level functions that you can use to implement your own custom event handler. If you look at the source of the pre-packaged event handlers, they are implemented in terms of these lower-level functions.

A custom event handler is composed of four parts:

1. The Html.Events.on function to create the handler
2. The name of a DOM event
3. A Msg constructor
4. A JSON decoder

Plain old DOM events

When a user selects an option from a dropdown, it broadcasts a change event along with some metadata. That metadata looks like:

{
  bubbles : true,
  cancelBubble : false,
  cancelable : false,
  currentTarget : null,
  defaultPrevented : false,
  target : {...},
  type : "change"
  // ... more
}

In most applications, what we’re interested in is the target key that points to the DOM node where the event originated (i.e. our select). Once we have that DOM node, we can access its current value.

Decoders

In order to pull a value out of a JavaScript object and into Elm, we’ll need to use a JSON decoder. You can make your own but Http.Events ships with a few convenient decoders to interact with the DOM event.

The most commonly used is Html.Events.targetValue which extracts the string from event.target.value.

Tagging

Great, we can get strings when a given DOM event fires. However, event handlers need to give us back a Msg type to be handled by our update function, not a string. We are going to want to wrap this string with our Msg (a practice often called “tagging”).

To transform the result of a JSON decoder, we can use the Json.Decode.map function:

type Msg = AnimalSelected String

selectedAnimalDecoder : Json.Decoder Msg
selectedAnimalDecoder =
  Json.Decode.map AnimalSelected Html.Events.targetValue

Putting it all together

Finally, you combine all these pieces together and allow a generic tagger to be passed in:

type Msg = AnimalSelected String

onChange : (String -> msg) -> Html.Attribute msg
onChange tagger =
  on "change" (Json.Decode.map tagger Html.Events.targetValue)

view : List String -> Html Msg
view animals =
  select [ onChange AnimalSelected ] (List.map animalOptions animals)

Because of the nice lower-level function provided, writing a custom onChange event handler is a one-liner!

Further reading

Read the source for Html.Events to get more examples of event handlers. They’re all one-liners too; no magic going on here.