@atlaskit/drag-and-drop

Drag and Drop

The goal of this library is to create a beautiful drag and drop experience for lists. It provides highly performant physics based mouse and keyboard dragging without creating any wrapping DOM nodes.

DragDropContext

In order to use drag and drop, you need to have the part of your react tree that you want to be able to use drag and drop in wrapped in a DragDropContext. It is advised to just wrap your entire application in a DragDropContext. Having nested DragDropContext's is not supported. You will be able to achieve your desired conditional dragging and dropping using the props of Droppable and Draggable. You can think of DragDropContext as having a similar purpose to the react-redux Provider component

Prop type information

type Hooks = {|
  onDragStart?: (id: DraggableId, location: DraggableLocation) => void,
  onDragEnd: (result: DropResult) => void,
|}

type Props = Hooks & {|
  children?: ReactElement,
|}

Basic usage

class App extends React.Component {
  onDragStart = () => {...}
  onDragEnd = () => {...}

  render() {
    return (
      <DragDropContext
        onDragStart={this.onDragStart}
        onDragEnd={this.onDragEnd}
      >
        <div>Hello world</div>
      </DragDropContext>
    )
  }
}

Hooks

These are top level application events that you can use to perform your own state updates.

onDragStart (optional)

This function will get notified when a drag starts. You are provided with the following details:

• id: the id of the Draggable that is now dragging
• location: the location (droppableId and index) of where the dragging item has started within a Droppable.

This function is optional and therefore does not need to be provided. It is highly recommended that you use this function to block updates to all Draggable and Droppable components during a drag. (See Best hooks practices)

Type information

onDragStart?: (id: DraggableId, location: DraggableLocation) => void

// supporting types
type Id = string;
type DroppableId: Id;
type DraggableId: Id;
type DraggableLocation = {|
  droppableId: DroppableId,
  // the position of the draggable within a droppable
  index: number
|};

onDragEnd (required)

This function is extremely important and has an important role to play in the application lifecycle. This function must result in the synchronous reordering of a list of Draggables

It is provided with all the information about a drag:

result: DragResult

• result.draggableId: the id of the Draggable was dragging.
• result.source: the location that the Draggable started in.
• result.destination: the location that the Draggable finished in. The destination will be null if the user dropped into no position (such as outside any list) or if they dropped the Draggable back into the same position that it started in.

Synchronous reordering

Because this library does not control your state, it is up to you to synchronously reorder your lists based on the result.

Here is what you need to do:

• if the destination is null: all done!
• if source.droppableId equals destination.droppableId you need to remove the item from your list and insert it at the correct position.
• if source.droppableId does not equal destination.droppable you need to the Draggable from the source.droppableId list and add it into the correct position of the destination.droppableId list.

(links assume you store your ids in arrays)

Type information

onDragEnd: (result: DropResult) => void

// supporting types
type DropResult = {|
  draggableId: DraggableId,
  source: DraggableLocation,
  // may not have any destination (drag to nowhere)
  destination: ?DraggableLocation
|}

type Id = string;
type DroppableId: Id;
type DraggableId: Id;
type DraggableLocation = {|
  droppableId: DroppableId,
  // the position of the droppable within a droppable
  index: number
|};

Best practices for hooks

Block updates during a drag

It is highly recommended that while a user is dragging that you block any state updates that might impact the amount of Draggables and Droppables, or their dimensions. Please listen to onDragStart and block updates to the Draggables and Droppables until you receive at onDragEnd.

When the user starts dragging we take a snapshot of all of the dimensions of the applicable Draggable and Droppable nodes. If these change during a drag we will not know about it.

Here are a few poor user experiences that can occur if you change things during a drag:

• If you increase the amount of nodes the library will not know about them and they will not be moved when the user would expect them to be.
• If you decrease the amount of nodes then there might be gaps and unexpected movements in your lists.
• If you change the dimensions of any node, it can cause the changed node as well as others to move at incorrect times.
• If you remove the node that the user is dragging the drag will instantly end
• If you change the dimension of the dragging node then other things will not move out of the way at the correct time.

onDragStart and onDragEnd pairing

We try very hard to ensure that each onDragStart event is paired with a single onDragEnd event. However, there maybe a rouge situation where this is not the case. If that occurs - it is a bug. Currently there is no mechanism to tell the library to cancel a current drag externally.

Dynamic hooks Your hook functions will only be captured once at start up. Please do not change the function after that. If there is a valid use case for this then dynamic hooks could be supported. However, at this time it is not.

Droppable

Droppable components can be dropped on by a Draggable. They also contain Draggables. A Draggable must be contained within a Droppable.

<Droppable
  droppableId="droppable-1"
  type="PERSON"
>
  {(provided, snapshot) => (
    <div
      ref={provided.innerRef}
      style={{backgroundColor: snapshot.isDraggingOver ? 'blue' : 'grey'}}
    >
      I am a droppable!
    </div>
  )}
</Droppable>

Props

• droppableId: A required DroppableId(string) that uniquely identifies the droppable for the application. Please do not change this prop - especially during a drag.
• type: An optional TypeId(string) that can be used to simply accept a class of Draggable. For example, if you use the type PERSON then it will only allow Draggables of type PERSON to be dropped on itself. Draggables of type TASK would not be able to be dropped on a Droppable with type PERSON. If no type is provided, it will be set to 'DEFAULT'. Currently the type of the Draggables within a Droppable must be the same. This restriction might be loosened in the future if there is a valid use case.
• isDropDisabled: An optional flag to control whether or not dropping is currently allowed on the Droppable. You can use this to implement your own conditional dropping logic. It will default to false.

Children function

The React children of a Droppable must be a function that returns a ReactElement.

<Droppable droppableId="droppable-1">
  {(provided, snapshot) => (
    // ...
  )}
</Droppable>

The function is provided with two arguments:

1. provided: (Provided)

type Provided = {|
  innerRef: (HTMLElement) => mixed,
|}

In order for the droppable to function correctly, you must bind the provided.innerRef to the highest possible DOM node in the ReactElement. We do this in order to avoid needing to use ReactDOM to look up your DOM node.

<Droppable droppableId="droppable-1">
  {(provided, snapshot) => (
    <div ref={provided.innerRef}>
      Good to go
    </div>
  )}
</Droppable>

2. snapshot: (StateSnapshot)

type StateSnapshot = {|
  isDraggingOver: boolean,
|}

The children function is also provided with a small about of state relating to the current drag state. This can be optionally used to enhance your component. A common use case is changing the appearance of a Droppable while it is being dragged over.

<Droppable droppableId="droppable-1">
  {(provided, snapshot) => (
    <div
      ref={provided.innerRef}
      style={{backgroundColor: snapshot.isDraggingOver ? 'blue' : 'grey'}}
    >
      I am a droppable!
    </div>
  )}
</Droppable>

Conditionally dropping

Keep in mind that this is not supported at this time. In this current initial version we only support reordering within a single list.

• Droppables can only be dropped on by Draggables who share the same type. This is a simple way of allowing conditional dropping. If you do not provide a type for the Droppable then it will only accept Draggables which also have the default type. Draggables and Droppables both will have their types set to 'DEFAULT' when none is provided. There is currently no way to set multiple types, or a type wildcard that will accept Draggables of multiple any types. This could be added if there is a valid use case.
• Using the isDropDisabled prop you can conditionally allow dropping. This allows you to do arbitrarily complex conditional transitions. This will only be considered if the type of the Droppable matches the type of the currently dragging Draggable.
• You can disable dropping on a Droppable altogether by always setting isDropDisabled to false. You can do this to create a list that is never able to be dropped on, but contains Draggables.
• Technically you do not need to use type and do all of your conditional drop logic with the isDropDisabled function. The type parameter is a convenient shortcut for a common use case.

Draggable

Draggable components can be dragged around and dropped onto Droppables. A Draggable must always be contained within a Droppable. It is possible to reorder a Draggable within its home Droppable or move to another Droppable. It is possible because a Droppable is free to control what it allows to be dropped on it.

Note: moving between Droppable is currently not supported in the initial version.

<Draggable
  draggableId="draggable-1"
  type="PERSON"
>
  {(provided, snapshot) => (
    <div
      ref={draggableProvided.innerRef}
      style={draggableProvided.draggableStyle}
      {...draggableProvided.dragHandleProps}
    >
      <h4>My draggable</h4>
    </div>
  )}
</Draggable>

Props

• draggableId: A required DraggableId(string) that uniquely identifies the Draggable for the application. Please do not change this prop - especially during a drag.
• type: An optional type (TypeId(string)) of the Draggable. This is used to control what Droppables the Draggable is permitted to drop on. Draggables can only drop on Droppables that share the same type. If no type is provided, it will be set to 'DEFAULT'. Currently the type of a Draggable must be the same as its container Droppable. This restriction might be loosened in the future if there is a valid use case.
• isDragDisabled: An optional flag to control whether or not dropping is currently allowed on the Droppable. You can use this to implement your own conditional dropping logic. It will default to false.

Children function

The React children of a Draggable must be a function that returns a ReactElement.

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div>
      <div
        ref={provided.innerRef}
        style={provided.draggableStyle}
        {...provided.dragHandleProps}
      >
        Drag me!
      </div>
      {provided.placeholder}
    </div>
  )}
</Draggable>

The function is provided with two arguments:

1. provided: (Provided)

type Provided = {|
  innerRef: (HTMLElement) => void,
  draggableStyle: ?DraggableStyle,
  dragHandleProps: ?DragHandleProvided,
  placeholder: ?ReactElement,
|}

Everything within the provided object must be applied for the Draggable to function correctly.

• provided.innerRef (innerRef: (HTMLElement) => void): In order for the Droppable to function correctly, you must bind the innerRef function to the ReactElement that you want to be considered the Draggable node. We do this in order to avoid needing to use ReactDOM to look up your DOM node.

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div ref={provided.innerRef}>
      Drag me!
    </div>
  )}
</Draggable>

Type information

innerRef: (HTMLElement) => void

• provided.draggableStyle (?DraggableStyle): This is an Object or null that contains an a number of styles that needs to be applied to the Draggable. This needs to be applied to the same node that you apply provided.innerRef to. The controls the movement of the draggable when it is dragging and not dragging. You are welcome to add your own styles to this object - but please do not remove or replace any of the properties.

Be careful with position:absolute

LTDR: do not apply any top, left, bottom, right styles to the dragging element.

One of the styles of provided.draggableStyle is position:absolute. This intended position of the element is the original position. No top, left, bottom, right values are applied to enable this. Be careful that you are not applying one of these styles (top, left, bottom, right) to the element or its positioning will be broken. This is especially true if one of your parents has position:relative on it.

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div>
      <div
        ref={provided.innerRef}
        style={provided.draggableStyle}
      >
        Drag me!
      </div>
    </div>
  )}
</Draggable>

Extending with your own styles

<Draggable draggable="draggable-1">
  {(provided, snapshot) => {
    const style = {
      ...provided.draggableStyle,
      backgroundColor: snapshot.isDragging : 'blue' : 'white',
      fontSize: 18,
    }
    return (
      <div>
        <div
          ref={provided.innerRef}
          style={style}
        >
          Drag me!
        </div>
      </div>
    );
  }}
</Draggable>

Type information

type DraggableStyle = MovementStyle | (PlacementStyle & MovementStyle);

type PlacementStyle = {|
  position: 'absolute',
  boxSizing: 'border-box',
  zIndex: ZIndex,
  width: number,
  height: number,
|}

type MovementStyle = {|
  transform: string,
|}

• provided.placeholder (?ReactElement) The Draggable element has position:absolute applied to it while it is dragging. The role of the placeholder is to sit in the place that the Draggable was during a drag. It is needed to stop the Droppable list from collapsing when you drag. It is advised to render it as a sibling to the Draggable node.

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div>
      <div
        ref={provided.innerRef}
        style={provided.draggableStyle}
      >
        Drag me!
      </div>
      {/* Always render me - I will be null if not required */}
      {provided.placeholder}
    </div>
  )}
</Draggable>

• provided.dragHandleProps (?DragHandleProps) every Draggable has a drag handle. This is what is used to drag the whole Draggable. Often this will be the same as the node as the Draggable, but sometimes it can be a child of the Draggable. DragHandleProps need to be applied to the node that you want to be the drag handle. This is a number of props that need to be applied to the Draggable node. The simpliest approach is to spread the props onto the draggable node ({...provided.dragHandleProps}). However, you are also welcome to monkey patch these props if you also need to respond to them. DragHandleProps will be null when isDragDisabled is set to true.

Type information

type DragHandleProps = {|
  onMouseDown: (event: MouseEvent) => void,
  onKeyDown: (event: KeyboardEvent) => void,
  onClick: (event: MouseEvent) => void,
  tabIndex: number,
  'aria-grabbed': boolean,
  draggable: boolean,
  onDragStart: () => void,
  onDrop: () => void
|}

Standard example

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div>
      <div
        ref={provided.innerRef}
        style={provided.draggableStyle}
        {...provided.dragHandleProps}
      >
        Drag me!
      </div>
      {provided.placeholder}
    </div>
  )}
</Draggable>

Custom drag handle

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => (
    <div>
      <div
        ref={provided.innerRef}
        style={provided.draggableStyle}
      >
        <h2>Hello there</h2>
        <div {...provided.dragHandleProps}>
          Drag handle
        </div>
      </div>
      {provided.placeholder}
    </div>
  )}
</Draggable>

Monkey patching

If you want to also use one of the props in DragHandleProps

const myOnClick = (event) => console.log('clicked on', event.target);

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => {
    const onClick = (() => {
      // dragHandleProps might be null
      if(!provided.dragHandleProps) {
        return myOnClick;
      }

      // creating a new onClick function that calls my onClick
      // event as well as the provided one.
      return (event) => {
        provided.dragHandleProps.onClick(event);
        // You may want to check if event.defaultPrevented
        // is true and optionally fire your handler
        myOnClick(event);
      }
    })();

    return (
      <div>
        <div
          ref={provided.innerRef}
          style={provided.draggableStyle}
          {...provided.dragHandleProps}
          onClick={onClick}
        >
          Drag me!
        </div>
        {provided.placeholder}
      </div>
    );
  }}
</Draggable>

2. snapshot: (StateSnapshot)

type StateSnapshot = {|
  isDragging: boolean,
|}

The children function is also provided with a small about of state relating to the current drag state. This can be optionally used to enhance your component. A common use case is changing the appearance of a Draggable while it is being dragged.

<Draggable draggableId="draggable-1">
  {(provided, snapshot) => {
    const style = {
      ...provided.draggableStyle,
      backgroundColor: snapshot.isDragging ? 'blue' : 'grey',
    };

    return (
      <div>
        <div
          ref={provided.innerRef}
          style={style}
          {...provided.dragHandleProps}
        >
          Drag me!
        </div>
        {provided.placeholder}
      </div>
    );
  }}
</Draggable>

Sloppy clicks and click blocking

A drag will not start until a user has dragged their mouse past a small threshold. If this threshold is not exceeded then the library will not impact the mouse click and will release the event to the browser.

When a user presses the mouse down on an element, we cannot determine if the user was clicking or dragging. If the sloppy click threshold was not exceeded then the event will be treated as if it where a click and the click event will bubble up unmodified. If the user has started dragging by moving the mouse beyond the sloppy click threshold then the click event will be prevented. This behavior allows you to wrap an element that has click behavior such as an anchor and have it work just like a standard anchor while also allowing it to be dragged.

Focus management

This library does not create any wrapper elements. This means that it will not impact the usage tab flow of a document. For example, if you are wrapping an anchor tag then the user will tab to the anchor directly and not an element surrounding the anchor. Whatever element you wrap will be given a tab-index to ensure that users can tab to the element to perform keyboard dragging.

Engineering health

Typed

This codebase is typed with flowtype to promote greater internal consistency and more resilient code.

Tested

This code base employs a number of different testing strategies including unit, performance and integration tests. Testing various aspects of the system helps to promote its quality and stability.

While code coverage is not a guarantee of code health, it is a good indicator. This code base currently sits at ~95% coverage.

Performance

This codebase is designed to be extremely performant. It builds on prior investigations into React performance that you can read about here and here. It is designed to perform the minimum number of renders required for each task.

Keyboard handling

Currently the keyboard handling is hard coded. This could be changed in the future to become a prop of Draggable. Here is the existing keyboard mapping:

• tab ↹ - standard browser tabbing will navigate through the Droppable's. The library does not do anything fancy with tab while users are selecting. Once a drag has started, tab is blocked for the duration of the drag.
• spacebar - lift a focused Draggable. Also, drop a dragging Draggable where the drag was started with a spacebar.
• Up arrow ↑ - move a Draggable that is dragging up on a vertical list
• Down arrow ↓ - move a Draggable that is dragging down on a vertical list
• Escape esc - cancel an existing drag - regardless of whether the user is dragging with the keyboard or mouse.

Short term backlog

• Correct scroll handling while dragging with keyboard or mouse
• Moving items between vertical lists (until this lands conditional dropping will not be available)

Medium term backlog

• Dragging within a horizontal list
• Moving items between horizontal lists
• Moving a Draggable from a vertical list to a horizontal list
• Nesting

Long term backlog

• Automatically disabling physics for a drag when the frame rate drops below a threshold. This can be because of low system resources, or when the are 100's of impacted items during a drag
• A mechanism to perform dragging without user input
• Dragging multiple items at a time
• And lots more!

Author

Alex Reardon - [e-mail]

* Please note that this module could have dependencies that are governed by the Atlassian Design Guidelines license which will be automatically included on install. Each dependency has a license file that indicates whether the Atlassian Design Guidelines license applies.

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