SVELTE DND ACTION 

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This is a feature-complete implementation of drag and drop for Svelte using a custom action. It supports almost every imaginable drag and drop use-case, any input device and is fully accessible.

It requires very minimal configuration, while offering a rich set of primitives that allow overriding basically any of its default behaviours (using the handler functions).


See full features list below.

!dnd_demo2

Play with this example in the REPL.

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The library is production ready, and I am in the process of integrating it into several production systems that will be used at scale.
It is being actively maintained.
I am doing my best to avoid breaking-changes and keep the API stable.

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- Awesome drag and drop with minimal fuss
- Supports horizontal, vertical or any other type of container (it doesn't care much about the shape)
- Supports nested dnd-zones (draggable containers with other draggable elements inside, think Trello)
- Rich animations (can be opted out of)
- Touch support
- Define what can be dropped where (dnd-zones optionally have a "type")
- Scroll dnd-zones (of the relevant "type"), parent containers that contains them and/or the window horizontally or vertically by placing the dragged element next to the edge
- Supports advanced use-cases such as various flavours of copy-on-drag and custom drag handles (see examples below)
- Performant and small footprint (no external dependencies, no fluff code)
- Fully accessible (beta) - keyboard support, aria attributes and assistive instructions for screen readers

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A custom action allows for a much more elegant API (no slot props thanks god) as well as more control.

If you prefer a generic dnd list component that accepts different child components as your abstraction, you can very easily wrap this library with one (see here).

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Pre-requisites: svelte-3 (>=3.23.0)

``bash
yarn add -D svelte-dnd-action
`

or

`bash
npm install --save-dev svelte-dnd-action
`

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`html


`html


An options-object with the following attributes:
| Name | Type | Required? | Default Value | Description |
| ------------------------- | -------------- | ------------------------------------------------------------ | ------------------------------------------------- | ------------------------------------------------------------ |
| items | Array<Object> | Yes. Each object in the array has to have an id property (key name can be overridden globally) with a unique value (within all dnd-zones of the same type) | N/A | The data array that is used to produce the list with the draggable items (the same thing you run your #each block on). The dndzone should not have children that don't originate in items |
| flipDurationMs | Number | No | 0 | The same value you give the flip animation on the items (to make them animated as they "make space" for the dragged item). Set to zero if you dont want animations, if unset it defaults to 100ms |
| type | String | No | Internal | dnd-zones that share the same type can have elements from one dragged into another. By default, all dnd-zones have the same type |
| dragDisabled | Boolean | No | false | Setting it to true will make it impossible to drag elements out of the dnd-zone. You can change it at any time, and the zone will adjust on the fly |
| morphDisabled | Boolean | No | false | By default, when dragging over a zone, the dragged element is morphed to look like it would if dropped. You can prevent it by setting this option. |
| dropFromOthersDisabled | Boolean | No | false | Setting it to true will make it impossible to drop elements from other dnd-zones of the same type. Can be useful if you want to limit the max number of items for example. You can change it at any time, and the zone will adjust on the fly |
| zoneTabIndex | Number | No | 0 | Allow user to set custom tabindex to the list container when not dragging. Can be useful if you want to make the screen reader to skip the list container. You can change it at any time. |
| zoneItemTabIndex | Number | No | 0 | Allow user to set custom tabindex to the list container items when not dragging. Can be useful if you use Drag handles. You can change it at any time. |
| dropTargetStyle | Object<String> | No | {outline: 'rgba(255, 255, 102, 0.7) solid 2px'} | An object of styles to apply to the dnd-zone when items can be dragged into it. Note: the styles override any inline styles applied to the dnd-zone. When the styles are removed, any original inline styles will be lost |
| dropTargetClasses| Array<String> | No | [] | A list of classes to apply to the dnd-zone when items can be dragged into it. Note: make sure the classes you use are global. |
| transformDraggedElement | Function | No | () => {} | A function that is invoked when the draggable element enters the dnd-zone or hover overs a new index in the current dnd-zone.
Signature:
function(element, data, index) {}
element: The dragged element.
data: The data of the item from the items array.
index: The index the dragged element will become in the new dnd-zone.

This allows you to override properties on the dragged element, such as innerHTML to change how it displays. If what you are after is altering styles, do it to the children, not to the dragged element itself |
| autoAriaDisabled | Boolean | No | false | Setting it to true will disable all the automatically added aria attributes and aria alerts (for example when the user starts/ stops dragging using the keyboard).
Use it only if you intend to implement your own custom instructions, roles and alerts. In such a case, you might find the exported function alertToScreenReader(string) useful. |
| centreDraggedOnCursor | Boolean | No | false | Setting it to true will cause elements from this dnd-zone to position their center on the cursor on drag start, effectively turning the cursor to the focal point that triggers all the dnd events (ex: entering another zone). Useful for dnd-zones with large items that can be dragged over small items. |
| useCursorForDetection | Boolean | No | false | Setting it to true will use the cursor position instead of the dragged element's center for drop zone detection. This improves accuracy when dragging large elements over small drop targets. Unlike centreDraggedOnCursor, this option does not reposition the dragged element. |
| dropAnimationDisabled | Boolean | No | false | Setting it to true will disable the animation of the dropped element to its final place. |
| delayTouchStart | Boolean \| Number | No | false | Prevents accidental drags on touch devices that should have been scrolls.
• true – enable with sensible default (80 ms).
• number – custom delay in milliseconds.
• false / omitted – legacy behaviour (drag starts immediately). |

##### Output:

The action dispatches two custom events:

- consider - dispatched whenever the dragged element needs to make room for itself in a new position in the items list and when it leaves. The host (your component) is expected to update the items list (you can keep a copy of the original list if you need to)
- finalize - dispatched on the target and origin dnd-zones when the dragged element is dropped into position. This is the event you want to use to save the items to the server for example.

The expectation is the same for both event handlers - update the list of items.
In both cases the payload (within e.detail) is the same: an object with two attributes: items and info.

- items: contains the updated items list.
- info: This one can be used to achieve very advanced custom behaviours (ex: copy on drag). In most cases, don't worry about it. It is an object with the following properties:
- trigger: will be one of the exported list of TRIGGERS (Please import if you plan to use): [DRAG_STARTED, DRAGGED_ENTERED, DRAGGED_ENTERED_ANOTHER, DRAGGED_OVER_INDEX, DRAGGED_LEFT, DRAGGED_LEFT_ALL, DROPPED_INTO_ZONE, DROPPED_INTO_ANOTHER, DROPPED_OUTSIDE_OF_ANY, DRAG_STOPPED]. Most triggers apply to both pointer and keyboard, but some are only relevant for pointer (dragged_entered, dragged_over_index and dragged_left), and some only for keyboard (drag_stopped).
- id: the item id of the dragged element
- source: will be one of the exported list of SOURCES (Please import if you plan to use): [POINTER, KEYBOARD]

You have to listen for both events and update the list of items in order for this library to work correctly.

For advanced use-cases (ex: custom styling for the placeholder element) you might also need to import SHADOW_ITEM_MARKER_PROPERTY_NAME, which marks the placeholder element that is temporarily added to the list the dragged element hovers over.
For use cases that have recursively nested zones (ex: crazy nesting), you might want to import SHADOW_PLACEHOLDER_ITEM_ID in order to filter the placeholder out when passing the items in to the nested component.
If you need to manipulate the dragged element either dynamically (and don't want to use the transformDraggedElement option), or statically targeting it or its children with CSS, you can import and use DRAGGED_ELEMENT_ID;

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If you want screen-readers to tell the user which item is being dragged and which container it interacts with, please add aria-label on the container and on every draggable item. The library will take care of the rest.
For example:

`html


{listName}

If you don't provide the aria-labels everything will still work, but the messages to the user will be less informative.
_Note_: in general you probably want to use semantic-html (ex: ol and li elements rather than section and div) but the library is screen readers friendly regardless (or at least that's the goal :)).
If you want to implement your own custom screen-reader alerts, roles and instructions, you can use the autoAriaDisabled options and wire everything up yourself using markup and the consider and finalize handlers (for example: unsortable list).

##### Keyboard support

- Tab into a dnd container to get a description and instructions
- Tab into an item and press the _Space_/_Enter_ key to enter dragging-mode. The reader will tell the user a drag has started.
- Use the _arrow keys_ while in dragging-mode to change the item's position in the list (down and right are the same, up and left are the same). The reader will tell the user about position changes.
- Tab to another dnd container while in dragging-mode in order to move the item to it (the item will be moved to it when it gets focus). The reader will tell the user that item was added to the new list.
- Press _Space_/_Enter_ key while focused on an item, or the _Escape_ key anywhere to exit dragging mode. The reader will tell the user that they are no longer dragging.
- Clicking on another item while in drag mode will make it the new drag target. Clicking outside of any draggable will exit dragging-mode (and tell the user)
- Mouse drag and drop can be preformed independently of keyboard dragging (as in an item can be dragged with the mouse while in or out of keyboard initiated dragging-mode)
- Keyboard drag uses the same consider (only on drag start) and finalize (every time the item is moved) events but share only some of the TRIGGERS. The same handlers should work fine for both.

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Due to popular demand, starting in version 0.9.46 the library exports a wrapper action that greatly improves the ergonomics around using drag handles.
Notes:

- A draggable item within a dragHandleZone would not be draggable unless it has an element that uses the dragHandle action inside (doesn't have to be a direct child but has to be inside the bounding rect of the item).
- Don't forget an aria-label on the handle
Usage:

`html


Drag Handles

- Only one element can be dragged in any given time
- The data that represents items within dnd-zones of the same type is expected to have the same shape (as in a data object that represents an item in one container can be added to another without conversion).
- Item ids (#each keys) are unique in all dnd containers of the same type. EVERY DRAGGABLE ITEM (passed in through items) MUST HAVE AN ID PROPERTY CALLED id. You can override it globally if you'd like to use a different key (see below)
- Item ids are provided as the key for the #each block (no keyless each blocks please)
- If you need to make a copy an item, you allocate a new id for the copy upon creation.
- The items in the list that is passed-in are in the same order as the children of the container (i.e the items are rendered in an #each block), and the container has no extra (and no fewer) children.
- Any data that should "survive" when the items are dragged around and dropped should be included in the items array that is passed in.
- The host component must refresh the items that are passed in to the custom-action when receiving consider and finalize events (do not omit any handler).
- FYI, the library assumes it is okay to add a temporary item to the items list in any of the dnd-zones while an element is dragged around.
- Svelte's built-in transitions might not play nice with this library. Luckily, it is an easy issue to work around. There are examples above.

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Sometimes it is useful to use a different key for your items instead of id, for example when working with PouchDB which expects _id. It can save some annoying conversions back and forth.
In such cases you can import and call overrideItemIdKeyNameBeforeInitialisingDndZones. This function accepts one parameter of type string which is the new id key name.
For example:

`javascript
import {overrideItemIdKeyNameBeforeInitialisingDndZones} from "svelte-dnd-action";
overrideItemIdKeyNameBeforeInitialisingDndZones("_id");
`

It applies globally (as in, all of your items everywhere are expected to have a unique identifier with this name). It can only be called when there are no rendered dndzones (I recommend calling it within the top-level


#### Custom types with DndEvent

You can use generics to set the type of items you are expecting in DndEvent. Simply add a type to it like so: DndEvent. For example:

`html

`

##### Svelte 5:

Svelte 5 prefers onconsider and onfinalize (over on:consider and on:finalize) but works both ways as long as it's consistent within a file.

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This is an experimental feature added in version 0.9.29. If you have multiple levels of nesting, the lib might do unnecessary work when dragging an element that has nested zones inside.
Specifically, it allows nested zones within the shadow element (the placeholder under the dragged element) to register and destroy.
This is because Svelte calls nested actions before the parent action (opposite to the rendering order).
You can use a data attribute on the items to help the lib prevent this: data-is-dnd-shadow-item-hint={item[SHADOW_ITEM_MARKER_PROPERTY_NAME]}
Starting with version 0.9.42. if you use the hint make sure to include it in the key you provide in your each block e.g:

`sveltehtml
{#each columnItems as column (${column._id}${column[SHADOW_ITEM_MARKER_PROPERTY_NAME] ? "_" + column[SHADOW_ITEM_MARKER_PROPERTY_NAME] : ""})}
...
{/each}
`

#### Simplified Example (just shows where to place the attribute):

`html



If your page is scrollable and you found that quick swipes on touch devices sometimes start a drag instead of scrolling, enable the new delayTouchStart option:

`sveltehtml


Setting it to true waits 80 ms before hijacking the gesture; passing a number lets you tune the threshold.

If you want screen-readers to tell the user which item is being dragged and which container it interacts with, please add aria-label` on the container and on every draggable item. The library will take care of the rest.