changes-while-dragging.md

Changes while dragging

For virtual list support see our virtual list pattern

❌ This behaviour is only supported in 11.x. We do plan on supporting this type of behaviour again in a future minor release. We needed to cut this existing behaviour order to get 12.x across the line. Going forward, tree behaviour will be supported on the latest version. We know this sucks, but we thought it better to move things forward.

@hello-pangea/dnd supports the addition and removal of <Draggable />s during a drag.

What behaviours does this unlock?

Lazy loading of list items

In this example we are adding more <Draggable />s to a list we scroll closer to the bottom of the list

Collapsing and expanding groups

We recommend you use the @atlaskit/tree component for this behaviour

Rules

We attempt to print helpful debug information to the console if you do not follow these rules in development builds

• You are allowed to add or remove Draggables during a drag
• You can only add or remove Draggables that are of the same type as the dragging item.
• Any changes must occur within a <Droppable /> that is a scroll container (has overflow: auto or overflow: scroll). This is prevent accidental shifts to other Droppables on the page
• The size of the internal content of the scroll container can change, but the outer bounds of the scroll container itself cannot change.
• You cannot modify the sizes of any existing <Draggable /> or <Droppable /> during a drag
• You cannot add or remove a <Droppable /> during a drag. We did this to avoid accidental shifting of other <Droppable />s
• When an item is removed or added it must be done instantly. You cannot animate the size of the item. You are welcome to animate a property when adding a <Draggable /> that does not impact the size of the item, such as opacity

<DragDropContext /> > onDragUpdate behavior

• onDragUpdate will be called if the DragUpdate > source > index of the dragging item has changed as the result of Draggables being added or removed before it.
• onDragUpdate will be called if the DragUpdate > destination of the dragging item has changed as a result of the addition or removal.

<DragDropContext /> > onDragEnd behavior

onDragEnd will be called with values that are adjusted for any additions or removals of Draggables during a drag. This can mean that the onDragStart: DragStart > source > index can be different from the onDragEnd: DropResult > source > index.

Sample onDragEnd flow

What is important to note is that the source property can change during a drag as a result of dynamic changes.

1. A drag starts.

onDragStart is called with:

{
  draggableId: 'item-1',,
  type: 'TYPE',
  source: {
    droppableId: 'droppable',
    index: 1,
  },
}

2. The first <Draggable /> in the list (item-0) is removed.

onDragUpdate is called with DragUpdate:

{
  draggableId: 'item-1',,
  type: 'TYPE',
  source: {
    droppableId: 'droppable',
+   // item-1 is now in index 0 as item-0 is gone
+    index: 0,
  },
  // adjusted destination
  destination: null,
}

3. The drag ends

onDragEnd is called with DropResult:

{
  draggableId: 'item-1',,
  type: 'TYPE',
  source: {
    droppableId: 'droppable',
+   // the source reflects the change
+    index: 0,
  },
  destination: null,
  reason: 'DROP',
}

Drag end while we are patching the virtual model

If a drag ends after a <Draggable /> has been added or removed, but we have not finished collecting and patching the virtual dimension model then we will delay the drop until the patch is finished. This is usually only a single frame. The onDropEnd callback will be called with a DropResult that is correct after the patch.

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