ngx-virtual-dnd

Angular drag-and-drop library optimized for virtual scrolling. Handles thousands of items efficiently by only rendering visible elements.

Inspired by react-virtualized-dnd.

Features

• Virtual Scrolling - Renders only visible items plus overscan buffer
• Dynamic Item Heights - Auto-measured via ResizeObserver with O(log N) lookups
• Smooth Drag & Drop - 60fps with RAF throttling
• Cross-List Support - Drag between multiple lists with group filtering
• Auto-Scroll - Configurable edge scrolling with speed/threshold control
• Drag Handles - Restrict drag initiation to specific elements
• Container Constraints - Constrain drag preview to container boundaries
• Axis Locking - Lock dragging to horizontal or vertical axis
• Custom Previews - Template-based drag preview and placeholder customization
• Keyboard Accessible - Space to grab, arrows to move, Escape to cancel
• Touch Support - Works with mouse and touch, with configurable delay/threshold
• Angular 21+ - Signals, standalone components, modern patterns

Installation

npm install ngx-virtual-dnd

Quick Start

import {
  VirtualSortableListComponent,
  DroppableGroupDirective,
  DraggableDirective,
  DragPreviewComponent,
  DropEvent,
  moveItem,
} from 'ngx-virtual-dnd';

@Component({
  imports: [
    VirtualSortableListComponent,
    DroppableGroupDirective,
    DraggableDirective,
    DragPreviewComponent,
  ],
  template: `
    <!-- Item template -->
    <ng-template #itemTpl let-item>
      <div class="item" [vdndDraggable]="item.id" [vdndDraggableData]="item">
        {{ item.name }}
      </div>
    </ng-template>

    <!-- Lists wrapped in a group -->
    <div vdndGroup="my-group">
      <vdnd-sortable-list
        droppableId="list-1"
        group="my-group"
        [items]="list1()"
        [itemHeight]="50"
        [containerHeight]="400"
        [itemIdFn]="getItemId"
        [itemTemplate]="itemTpl"
        (drop)="onDrop($event)"
      />

      <vdnd-sortable-list
        droppableId="list-2"
        group="my-group"
        [items]="list2()"
        [itemHeight]="50"
        [containerHeight]="400"
        [itemIdFn]="getItemId"
        [itemTemplate]="itemTpl"
        (drop)="onDrop($event)"
      />
    </div>

    <!-- Required: renders the dragged item preview -->
    <vdnd-drag-preview />
  `,
})
export class MyComponent {
  list1 = signal<Item[]>([...]);
  list2 = signal<Item[]>([...]);

  getItemId = (item: Item) => item.id;

  onDrop(event: DropEvent): void {
    moveItem(event, {
      'list-1': this.list1,
      'list-2': this.list2,
    });
  }
}

That's it! VirtualSortableListComponent handles placeholder positioning, sticky items during drag, and virtual scroll integration automatically.

API Overview

The library exports these main pieces (use IDE completion for full details):

Components:

• VirtualSortableListComponent - High-level component combining droppable, virtual scroll, and placeholder
• VirtualScrollContainerComponent - Low-level virtual scroll container
• VirtualViewportComponent - Self-contained virtual scroll viewport
• VirtualContentComponent - Virtual content for external scroll containers (page-level scroll)
• DragPreviewComponent - Renders the dragged item preview (required, supports custom templates)
• PlaceholderComponent - Drop position indicator

Directives:

• DraggableDirective (vdndDraggable) - Makes an element draggable (supports drag handles, axis locking, threshold/delay)
• DroppableDirective (vdndDroppable) - Marks a drop target (supports container constraints, auto-scroll config)
• DroppableGroupDirective (vdndGroup) - Provides group context to children
• ScrollableDirective (vdndScrollable) - Marks external scroll container
• VirtualForDirective (*vdndVirtualFor) - Structural directive for virtual lists
• ContentHeaderDirective (vdndContentHeader) - Marks a projected header inside VirtualContentComponent (auto-measured via ResizeObserver)

Services:

• DragStateService - Access drag state (isDragging, draggedItem, placeholderIndex, etc.)
• AutoScrollService - Controls edge auto-scrolling
• PositionCalculatorService - Calculates placeholder positions

Utilities:

• moveItem() - Move between signal-based lists
• reorderItems() - Reorder within a single list
• applyMove() - Immutable version (returns new arrays)
• isNoOpDrop() - Check if drop would be a no-op
• insertAt() / removeAt() - Low-level array helpers

Strategies:

• VirtualScrollStrategy - Interface for custom virtual scroll strategies
• FixedHeightStrategy - Fixed index * itemHeight math (zero overhead)
• DynamicHeightStrategy - Variable heights with auto-measurement and binary search

Advanced Usage

Dynamic Item Heights

When items have variable heights, enable dynamicItemHeight. Items are auto-measured via ResizeObserver — no manual height tracking needed. The itemHeight value serves as the initial estimate for unmeasured items.

With VirtualSortableListComponent:

<vdnd-sortable-list
  droppableId="list-1"
  group="my-group"
  [items]="list()"
  [itemHeight]="80"
  [dynamicItemHeight]="true"
  [itemIdFn]="getItemId"
  [itemTemplate]="itemTpl"
  (drop)="onDrop($event)"
/>

With VirtualScrollContainerComponent:

<vdnd-virtual-scroll
  [items]="items()"
  [itemHeight]="80"
  [dynamicItemHeight]="true"
  [itemIdFn]="getItemId"
  [trackByFn]="trackById"
  [itemTemplate]="itemTpl"
/>

With VirtualForDirective:

<ng-container
  *vdndVirtualFor="
    let item of items();
    itemHeight: 80;
    dynamicItemHeight: true;
    trackBy: trackById;
    droppableId: 'list-1'
  "
>
  <div class="item">{{ item.description }}</div>
</ng-container>

Notes:

• FixedHeightStrategy is used by default when dynamicItemHeight is not set
• Setting dynamicItemHeight switches to DynamicHeightStrategy with automatic height measurement
• Heights are tracked by trackBy key, so they survive reordering
• The itemHeight value is used as the initial estimate for items not yet measured
• Inside a viewport component (vdnd-virtual-viewport or vdnd-virtual-content), itemHeight, dynamicItemHeight, and droppableId are inherited automatically — only trackBy is needed on the directive

Drag Handles

Use dragHandle with a CSS selector to restrict where users can initiate a drag:

<div [vdndDraggable]="item.id" dragHandle=".handle">
  <span class="handle">⠿</span>
  <span>{{ item.name }}</span>
</div>

Only clicks on elements matching the selector will start a drag. The rest of the element remains interactive.

Container Constraints

Constrain the drag preview and placeholder to stay within the droppable container:

<vdnd-sortable-list
  droppableId="list-1"
  group="my-group"
  [items]="items()"
  [itemHeight]="50"
  [constrainToContainer]="true"
  [itemIdFn]="getItemId"
  [itemTemplate]="itemTpl"
  (drop)="onDrop($event)"
/>

Or on the directive directly:

<div vdndDroppable="list-1" [constrainToContainer]="true">...</div>

The preview cannot leave the droppable area, and the placeholder snaps to the edges.

Axis Locking

Lock dragging to a single axis:

<div [vdndDraggable]="item.id" lockAxis="y">{{ item.name }}</div>

Values: 'x' (horizontal only), 'y' (vertical only), or omit for free movement.

Drag Threshold & Delay

<div [vdndDraggable]="item.id" [dragThreshold]="10" [dragDelay]="200">{{ item.name }}</div>

• dragThreshold — minimum distance (px) before drag starts (default: 5). Prevents accidental drags on click.
• dragDelay — delay (ms) after pointer down before drag activates (default: 0). Useful on touch devices to distinguish scrolling from dragging. Use the vdnd-drag-pending CSS class to show visual feedback when the delay passes.

Custom Drag Preview

Provide a custom template for the drag preview:

<ng-template #preview let-data let-draggableId="draggableId">
  <div class="custom-preview">Dragging: {{ data.name }}</div>
</ng-template>

<vdnd-drag-preview [previewTemplate]="preview" [cursorOffset]="{ x: 16, y: 16 }" />

• previewTemplate — custom template for the preview. Context provides $implicit (the draggable's data), draggableId, and droppableId.
• cursorOffset — offset from cursor in pixels (default: { x: 8, y: 8 }).

Without a custom template, the library clones the dragged element as the preview.

Auto-Scroll Configuration

Configure auto-scroll behavior when dragging near container edges:

<vdnd-sortable-list
  droppableId="list-1"
  group="my-group"
  [items]="items()"
  [itemHeight]="50"
  [autoScrollConfig]="{ threshold: 80, maxSpeed: 20 }"
  [itemIdFn]="getItemId"
  [itemTemplate]="itemTpl"
  (drop)="onDrop($event)"
/>

Option	Default	Description
threshold	50	Distance from edge (px) to trigger
maxSpeed	15	Maximum scroll speed (px/frame)
accelerate	true	Speed up based on distance from edge

Set [autoScrollEnabled]="false" to disable auto-scroll entirely. These options are available on VirtualSortableListComponent, DroppableDirective, and ScrollableDirective.

Disabling Drag & Drop

Use the disabled input to conditionally disable draggables, droppables, or entire lists:

<!-- Disable a single item -->
<div [vdndDraggable]="item.id" [disabled]="!item.canDrag">{{ item.name }}</div>

<!-- Disable an entire list -->
<vdnd-sortable-list
  droppableId="list-1"
  group="my-group"
  [items]="items()"
  [itemHeight]="50"
  [disabled]="isReadOnly()"
  [itemIdFn]="getItemId"
  [itemTemplate]="itemTpl"
  (drop)="onDrop($event)"
/>

Disabled draggables get the vdnd-draggable-disabled CSS class. Disabled droppables get vdnd-droppable-disabled.

Low-Level API

For maximum control, use individual components instead of VirtualSortableListComponent:

@Component({
  imports: [
    VirtualScrollContainerComponent,
    DroppableGroupDirective,
    DroppableDirective,
    DraggableDirective,
    DragPreviewComponent,
  ],
  template: `
    <ng-template #itemTpl let-item>
      <div class="item" [vdndDraggable]="item.id" [vdndDraggableData]="item">
        {{ item.name }}
      </div>
    </ng-template>

    <div vdndGroup="demo">
      <div vdndDroppable="list-1" (drop)="onDrop($event)">
        <vdnd-virtual-scroll
          droppableId="list-1"
          [items]="items()"
          [itemHeight]="50"
          [itemIdFn]="getItemId"
          [trackByFn]="trackById"
          [itemTemplate]="itemTpl"
        />
      </div>
    </div>
    <vdnd-drag-preview />
  `,
})
export class ListComponent {
  items = signal<Item[]>([]);
}

Page-Level Scroll

Use VirtualContentComponent with vdndScrollable for page-level scrolling with headers/footers:

@Component({
  imports: [
    ScrollableDirective,
    VirtualContentComponent,
    VirtualForDirective,
    DraggableDirective,
    DroppableDirective,
    DroppableGroupDirective,
    DragPreviewComponent,
    ContentHeaderDirective,
  ],
  template: `
    <ion-content [scrollY]="false">
      <div class="scroll-container ion-content-scroll-host" vdndScrollable>
        <div vdndGroup="tasks">
          <vdnd-virtual-content
            [itemHeight]="72"
            vdndDroppable="list-1"
            (drop)="onDrop($event)"
          >
            <!-- Header — auto-measured via ResizeObserver, scrolls with content -->
            <div class="header" vdndContentHeader>Welcome!</div>

            <ng-container
              *vdndVirtualFor="
                let item of items();
                trackBy: trackById
              "
            >
              <div class="item" [vdndDraggable]="item.id">{{ item.name }}</div>
            </ng-container>

          </vdnd-virtual-content>
        </div>

        <!-- Footer — normal sibling in document flow -->
        <div class="footer">Load more</div>
      </div>
    </ion-content>

    <vdnd-drag-preview />
  `,
})
export class PageComponent {
  items = signal<Item[]>([...]);
}

Key points:

• vdndScrollable marks the scroll container
• VirtualContentComponent provides wrapper-based positioning and derives total items from the child *vdndVirtualFor automatically
• vdndContentHeader marks a projected header — its height is auto-measured via ResizeObserver and used as the content offset (no manual measurement needed)
• contentOffset input is available as an escape hatch when the header lives outside the component
• *vdndVirtualFor inherits itemHeight, dynamicItemHeight, and droppableId from the parent viewport/droppable — only trackBy is required

Screen Reader Announcements

The library emits events with position data. Implement announcements in your app:

@Component({
  template: `
    <div
      vdndDraggable="item-1"
      (dragStart)="announce('Grabbed ' + item.name)"
      (dragEnd)="announceEnd($event)"
    >
      {{ item.name }}
    </div>
    <div aria-live="assertive" class="sr-only">{{ announcement() }}</div>
  `,
})
export class MyComponent {
  announcement = signal('');

  announce(msg: string) {
    this.announcement.set(msg);
  }

  announceEnd(e: DragEndEvent) {
    this.announce(
      e.cancelled
        ? `Cancelled. Returned to position ${e.sourceIndex + 1}`
        : `Dropped at position ${e.destinationIndex! + 1}`,
    );
  }
}

Keyboard Navigation

Key	Action
Tab	Navigate to draggable items
Space	Start/end drag
Arrow ↑/↓	Move item up/down
Arrow ←/→	Move to adjacent list
Escape	Cancel drag

ARIA attributes (aria-grabbed, aria-dropeffect, tabindex) are managed automatically.

CSS Classes

Class	Applied When
vdnd-draggable	Always on draggable
vdnd-draggable-dragging	While being dragged
vdnd-draggable-disabled	When disabled
vdnd-drag-pending	After delay, ready to drag
vdnd-droppable	Always on droppable
vdnd-droppable-active	When a draggable is over it
vdnd-droppable-disabled	When disabled

Events

All event types are importable from ngx-virtual-dnd.

Output	Event Type	Emitted By
(dragStart)	DragStartEvent	DraggableDirective
(dragEnd)	DragEndEvent	DraggableDirective
(drop)	DropEvent	DroppableDirective, VirtualSortableListComponent

DragEndEvent includes a cancelled boolean to distinguish drops from cancellations.

How It Works

Traditional drag-and-drop libraries query sibling DOM elements via getBoundingClientRect(). This fails with virtual scrolling because items outside the viewport aren't rendered.

This library uses element-under-point detection: temporarily hide the dragged element, use document.elementFromPoint() to find what's at the cursor, walk up to find the droppable, and calculate placeholder position mathematically.

Browser Support

• Chrome/Edge (latest)
• Firefox (latest)
• Safari (latest)

Development

npm start                    # Dev server (localhost:4200)
ng build ngx-virtual-dnd     # Build library (required after lib edits)
npm test                     # Unit tests
npm run e2e                  # E2E tests

AI Agent Skills

Install a skill to teach AI coding assistants how to integrate this library:

npx skills add gultyayev/angular-vdnd

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License

MIT