There are various ways to listen to keyboard events in Elm. If you want to get all keyboard events, you can subscribe using functions from Browser.Events. And, if you want to get keyboard events for specific HTML elements, you can use Html.Events.on.
Each of those functions asks you to provide a Decoder msg to convert the
keyboard event into a message your application can handle. To help you along
the way, Html.Events has a handy
keyCode
decoder. You can use it to turn the keyboard event into a keycode -- which you
can then map into a message your app understands.
However, there is more information available in a keyboard event than just the keycode. So, we provide a decoder which gives you all the available information:
type alias KeyboardEvent =
{ altKey : Bool
, ctrlKey : Bool
, key : Maybe String
, keyCode : Key
, metaKey : Bool
, repeat : Bool
, shiftKey : Bool
}Even better, we:
normalize some browser-specific quirks, such as where to look for the keyCode (under "keyCode", "which" or "charCode")
turn the keyCode into a type-safe Key value.
But wait, there's more! We also have a version of the keyboard event decoder which allows you to filter events right in the decoder. That way, you can prevent some events from reaching your update function at all, which can be useful in some scenarios.
To listen for keyboard events on HTML elements, you can do something like this:
div
[ on "keydown" <|
Json.Decode.map HandleKeyboardEvent decodeKeyboardEvent
, tabindex 0
, id "id-for-auto-focus"
, style [ ( "outline", "none" ) ]
]
[]We use thetabIndex attribute to make the element focusable, since an HTML
element must be focusable in order to receive keyboard events. And, we provide
an id in case we want to programmatically focus on the element, via
Browser.Dom.focus.
For complete examples, see:
To listen for keyboard events globally, you can use functions from Browser.Events to subscribe to all keyboard events. For an example, see
Decodes a KeyboardEvent from a keyboard event.
You provide a function which, given a KeyboardEvent, turns it into a
message your update function can handle. You get back a Decoder for those
messages.
When your function returns Nothing, the decoder will fail. This means that
the event will simply be ignored -- that is, it will not reach your update
function at all.
Essentially, this allows you to filter keyboard events inside the decoder
itself, rather than in the update function. Whether this is a good idea or
not will depend on your scenario.
Some lower-level helpers that you might find useful.
A type alias for Int.
Decodes keyCode, which or charCode from a keyboard event
to get a numeric code for the key that was pressed.
Decodes the key field from a keyboard event.
Results in Nothing if the key field is not present, or blank.
A representation of a keyboard event.
The
keyfield may or may not be present, depending on the listener ("keydown" vs. "keypress" vs. "keyup"), browser, and key pressed (character key vs. special key). If not present, it will beNothinghere.The
keyCodeis normalized bydecodeKeyboardEventto use whichever ofwhich,keyCodeorcharCodeis provided, and made type-safe viaKeyboard.Key(see the excellent SwiftsNamesake/proper-keyboard for further manipulation of aKey).