elm-draggable

An easy way to make DOM elements draggable

Install

Have elm installed.

elm install zaboco/elm-draggable

Live examples

• Basic / Code
• Custom events / Code
• Constraints - restrict dragging to one axis at a time / Code
• Pan & Zoom - drag to pan & scroll to zoom / Code
• Multiple Targets / Code

Usage

This library is meant to be easy to use, by keeping its internal details hidden and only communicating to the parent application by emitting Event messages. So, each time the internals change and something relevant happens (such as "started dragging", "dragged at", etc.), a new message is sent as a Cmd and handled in the main update function. To better understand how this works, see the snippets below and also the working examples.

Basic

In order to make a DOM element draggable, you'll need to:

1. Import this library

import Draggable

2. Define your model

Include:

• The element's position.
• The internal Drag state. Note that, for simplicity, the model entry holding this state must be called drag, since the update function below follows this naming convention. A future update could allow using custom field names. Please note that for the sake of example, we are specifying String as the type to tag draggable elements with. If you have only one such element, () might be a better type.

type alias Model =
    { position : ( Int, Int )
    , drag : Draggable.State String
    }

3. Initialize the Drag state and the element's position

initModel : Model
initModel =
    { position = ( 0, 0 )
    , drag = Draggable.init
    }

4. Define the message types that will be handled by your application

• OnDragBy is for actually updating the position, taking a Draggable.Delta as an argument. Delta is just an alias for a tuple of (Float, Float) and it represents the distance between two consecutive drag points.
• DragMsg is for handling internal Drag state updates.

type Msg
    = OnDragBy Draggable.Delta
    | DragMsg (Draggable.Msg String)

5. Setup the config used when updating the Drag state

For the simplest case, you only have to provide a handler for onDragBy:

dragConfig : Draggable.Config String Msg
dragConfig =
    Draggable.basicConfig OnDragBy

6. Your update function must handle the messages declared above

• For OnDragBy, which will be emitted when the user drags the element, the new position will be computed using the Delta (dx, dy)
• DragMsg will be forwarded to Draggable.update which takes care of both updating the Drag state and sending the appropriate event commands. In order to do that, it receives the dragConfig. As mentioned above, this function assumes that the model has a drag field holding the internal Drag state.

update : Msg -> Model -> ( Model, Cmd Msg )
update msg ({ position } as model) =
    case msg of
        OnDragBy ( dx, dy ) ->
            let
                ( x, y ) =
                    position
            in
                ( { model | position = ( round (toFloat x + dx), round (toFloat y + dy) ) }, Cmd.none )

        DragMsg dragMsg ->
            Draggable.update dragConfig dragMsg model

7. In order to keep track of the mouse events, you must include the relevant subscriptions

subscriptions : Model -> Sub Msg
subscriptions { drag } =
    Draggable.subscriptions DragMsg drag

8. Triggering drag

Inside your view function, you must somehow make the element draggable. You do that by adding a trigger for the mousedown event. You must also specify a key for that element. This can be useful when there are multiple drag targets in the same view.

Of course, you'll also have to style your DOM element such that it reflects its moving position (with top: x; left: y or transform: translate)

view : Model -> Html Msg
view { position } =
    Html.div
        [ Draggable.mouseTrigger "my-element" DragMsg
        -- , Html.Attributes.style (someStyleThatSetsPosition position)
        ]
        [ Html.text "Drag me" ]

For working demos, see the basic example or the examples with multiple targets

9. Triggering on touch

If you want to trigger drags on touch events (i.e. on mobile platforms) as well as mouse events, you need to add touchTriggers to your elements. Building on the previous example, it looks like this.

view : Model -> Html Msg
view { position } =
    Html.div
        [ Draggable.mouseTrigger "my-element" DragMsg
        -- , Html.Attributes.style (someStyleThatSetsPosition position)
        ] ++ (Draggable.touchTriggers "my-element" DragMsg)
        [ Html.text "Drag me" ]

The basic example demonstrates this as well.

Advanced

Custom config

Besides tracking the mouse moves, this library can also track all the other associated events related to dragging. But, before enumerating these events, it's import to note that an element is not considered to be dragging if the mouse was simply clicked (without moving). That allows tracking both click and drag events:

• "mouse down" + "mouse up" = "click"
• "mouse down" + "mouse moves" + "mouse up" = "drag"

So, the mouse events are:

• onMouseDown - on mouse press.
• onDragStart - on the first mouse move after pressing.
• onDragBy - on every mouse move.
• onDragEnd - on releasing the mouse after dragging.
• onClick - on releasing the mouse without dragging.

All of these events are optional, and can be provided to Draggable.customConfig using an API similar to the one used by VirtualDom.node to specify the Attributes. For example, if we want to handle all the events, we define the config like:

import Draggable
import Draggable.Events exposing (onClick, onDragBy, onDragEnd, onDragStart, onMouseDown)

dragConfig : Draggable.Config String Msg
dragConfig =
    Draggable.customConfig
        [ onDragStart OnDragStart
        , onDragEnd OnDragEnd
        , onDragBy OnDragBy
        , onClick CountClick
        , onMouseDown (SetClicked True)
        ]

Note: If we need to handle mouseup after either a drag or a click, we can use the DOM event handler onMouseUp from Html.Events or Svg.Events.

See the full example

Custom Delta

By default, OnDragBy message will have a Draggable.Delta parameter, which, as we saw, is just an alias for (Float, Float). However, there are situations when we would like some other data type for representing our delta.

Luckily, that's pretty easy using function composition. For example, we can use a Vec2 type from the linear-algebra library, which provides handy function like translate, scale and negate.

import Math.Vector2 as Vector2 exposing (Vec2)

type Msg
    = OnDragBy Vec2
--  | ...

dragConfig : Draggable.Config Msg
dragConfig =
    Draggable.basicConfig (OnDragBy << (\( dx, dy ) -> Vector2.vec2 dx dy))

There is actually an example right for this use-case

Custom mouse trigger

There are cases when we need some additional information (e.g. mouse offset) about the mousedown event which triggers the drag. For these cases, there is an advanced customMouseTrigger which also takes a JSON Decoder for the MouseEvent.

import Json.Decode as Decode exposing (Decoder)

type Msg
    = CustomMouseDown (Draggable.Msg ()) (Float, Float)
--  | ...

update msg model =
    case msg of
        CustomMouseDown dragMsg startPoint ->
            { model | startPoint = startPoint }
                |> Draggable.update dragConfig dragMsg

view { scene } =
    Svg.svg
        [ Draggable.customMouseTrigger () mouseOffsetDecoder CustomMouseDown
--      , ...
        ]
        []

mouseOffsetDecoder : Decoder (Float, Float)
mouseOffsetDecoder =
    Decode.map2 (,)
        (Decode.field "offsetX" Decode.float)
        (Decode.field "offsetY" Decode.float)

Full example