feat(drag-drop): add directive to connect drop lists automatically

Currently the only way to connect drop lists is via the `cdkDropListConnectedTo` input which works if the consumer knows the amount of drop lists that they're going to have. For dynamic lists they can pass a list of ids, however that can be a little boilerplate-y. These changes introduce the `cdkDropListGroup` directive which can be used to connect a dynamic list of drop containers automatically.

Fixes #13750.

Author: crisbeto

src/cdk/drag-drop/drag-drop-module.ts

@@ -8,6 +8,7 @@
 
 import {NgModule} from '@angular/core';
 import {CdkDropList} from './drop-list';
+import {CdkDropListGroup} from './drop-list-group';
 import {CdkDrag} from './drag';
 import {CdkDragHandle} from './drag-handle';
 import {CdkDragPreview} from './drag-preview';
@@ -16,13 +17,15 @@ import {CdkDragPlaceholder} from './drag-placeholder';
 @NgModule({
   declarations: [
     CdkDropList,
+    CdkDropListGroup,
     CdkDrag,
     CdkDragHandle,
     CdkDragPreview,
     CdkDragPlaceholder,
   ],
   exports: [
     CdkDropList,
+    CdkDropListGroup,
     CdkDrag,
     CdkDragHandle,
     CdkDragPreview,

src/cdk/drag-drop/drag-drop.md

@@ -22,12 +22,12 @@ update the data model once the user finishes dragging.
 ### Transferring items between lists
 The `cdkDropList` directive supports transferring dragged items between connected drop zones.
 You can connect one or more `cdkDropList` instances together by setting the `cdkDropListConnectedTo`
-property.
+property or by wrapping the elements in an element with the `cdkDropListGroup` attribute.
 
 <!-- example(cdk-drag-drop-connected-sorting) -->
 
-Note that `cdkDropListConnectedTo` works both with a direct reference to another `cdkDropList`, or by
-referencing the `id` of another drop container:
+Note that `cdkDropListConnectedTo` works both with a direct reference to another `cdkDropList`, or
+by referencing the `id` of another drop container:
 
 ```html
 <!-- This is valid -->
@@ -39,6 +39,19 @@ referencing the `id` of another drop container:
 <div cdkDropList id="list-two" [cdkDropListConnectedTo]="['list-one']"></div>
 ```
 
+If you have an unknown amount of connected drop lists, you can use the `cdkDropListGroup` directive
+to set up the connection automatically. Note that any new `cdkDropList` that is added under a group
+will be connected to all other automatically.
+
+```html
+<div cdkDropListGroup>
+  <!-- All lists in here will be connected. -->
+  <div cdkDropList *ngFor="let list of lists"></div>
+</div>
+```
+
+<!-- example(cdk-drag-drop-connected-sorting-group) -->
+
 ### Attaching data
 You can associate some arbitrary data with both `cdkDrag` and `cdkDropList` by setting `cdkDragData`
 or `cdkDropListData`, respectively. Events fired from both directives include this data, allowing

src/cdk/drag-drop/drag.spec.ts

@@ -1600,6 +1600,31 @@ describe('CdkDrag', () => {
       });
     }));
 
+    it('should be able to connect two drop zones using the drop list group', fakeAsync(() => {
+      const fixture = createComponent(ConnectedDropZonesViaGroupDirective);
+      fixture.detectChanges();
+
+      const dropInstances = fixture.componentInstance.dropInstances.toArray();
+      const groups = fixture.componentInstance.groupedDragItems;
+      const element = groups[0][1].element.nativeElement;
+      const targetRect = groups[1][2].element.nativeElement.getBoundingClientRect();
+
+      dragElementViaMouse(fixture, element, targetRect.left + 1, targetRect.top + 1);
+      flush();
+      fixture.detectChanges();
+
+      const event = fixture.componentInstance.droppedSpy.calls.mostRecent().args[0];
+
+      expect(event).toBeTruthy();
+      expect(event).toEqual({
+        previousIndex: 1,
+        currentIndex: 3,
+        item: groups[0][1],
+        container: dropInstances[1],
+        previousContainer: dropInstances[0]
+      });
+    }));
+
     it('should be able to pass a single id to `connectedTo`', fakeAsync(() => {
       const fixture = createComponent(ConnectedDropZones);
       fixture.detectChanges();
@@ -1963,6 +1988,42 @@ class ConnectedDropZones implements AfterViewInit {
   }
 }
 
+@Component({
+  encapsulation: ViewEncapsulation.None,
+  styles: [`
+    .cdk-drop-list {
+      display: block;
+      width: 100px;
+      min-height: ${ITEM_HEIGHT}px;
+      background: hotpink;
+    }
+
+    .cdk-drag {
+      display: block;
+      height: ${ITEM_HEIGHT}px;
+      background: red;
+    }
+  `],
+  template: `
+    <div cdkDropListGroup>
+      <div
+        cdkDropList
+        [cdkDropListData]="todo"
+        (cdkDropListDropped)="droppedSpy($event)">
+        <div [cdkDragData]="item" *ngFor="let item of todo" cdkDrag>{{item}}</div>
+      </div>
+
+      <div
+        cdkDropList
+        [cdkDropListData]="done"
+        (cdkDropListDropped)="droppedSpy($event)">
+        <div [cdkDragData]="item" *ngFor="let item of done" cdkDrag>{{item}}</div>
+      </div>
+    </div>
+  `
+})
+class ConnectedDropZonesViaGroupDirective extends ConnectedDropZones {}
+
 
 @Component({
   template: `

src/cdk/drag-drop/drop-list-group.ts

@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Directive, OnDestroy} from '@angular/core';
+
+/**
+ * Declaratively connects sibling `cdkDropList` instances together. All of the `cdkDropList`
+ * elements that are placed inside a `cdkDropListGroup` will be connected to each other
+ * automatically. Can be used as an alternative to the `cdkDropListConnectedTo` input
+ * from `cdkDropList`.
+ */
+@Directive({
+  selector: '[cdkDropListGroup]'
+})
+export class CdkDropListGroup<T> implements OnDestroy {
+  /** Drop lists registered inside the group. */
+  readonly _items = new Set<T>();
+
+  ngOnDestroy() {
+    this._items.clear();
+  }
+}

src/cdk/drag-drop/drop-list.ts

@@ -26,6 +26,7 @@ import {DragDropRegistry} from './drag-drop-registry';
 import {CdkDragDrop, CdkDragEnter, CdkDragExit} from './drag-events';
 import {moveItemInArray} from './drag-utils';
 import {CDK_DROP_LIST_CONTAINER} from './drop-list-container';
+import {CdkDropListGroup} from './drop-list-group';
 
 
 /** Counter used to generate unique ids for drop zones. */
@@ -104,21 +105,30 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
   constructor(
     public element: ElementRef<HTMLElement>,
     private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>,
-    @Optional() private _dir?: Directionality) {}
+    @Optional() private _dir?: Directionality,
+    @Optional() private _group?: CdkDropListGroup<CdkDropList>) {}
 
   ngOnInit() {
     this._dragDropRegistry.registerDropContainer(this);
+
+    if (this._group) {
+      this._group._items.add(this);
+    }
   }
 
   ngOnDestroy() {
     this._dragDropRegistry.removeDropContainer(this);
+
+    if (this._group) {
+      this._group._items.delete(this);
+    }
   }
 
   /** Whether an item in the container is being dragged. */
   _dragging = false;
 
   /** Cache of the dimensions of all the items and the sibling containers. */
-  private _positionCache = {
+  private readonly _positionCache = {
     items: [] as {drag: CdkDrag, clientRect: ClientRect, offset: number}[],
     siblings: [] as {drop: CdkDropList, clientRect: ClientRect}[],
     self: {} as ClientRect
@@ -333,6 +343,8 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
   /** Refreshes the position cache of the items and sibling containers. */
   private _cachePositions() {
     const isHorizontal = this.orientation === 'horizontal';
+
+    this._positionCache.self = this.element.nativeElement.getBoundingClientRect();
     this._positionCache.items = this._activeDraggables
       .map(drag => {
         const elementToMeasure = this._dragDropRegistry.isDragging(drag) ?
@@ -364,12 +376,10 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
                               a.clientRect.top - b.clientRect.top;
       });
 
-    this._positionCache.siblings = coerceArray(this.connectedTo)
-      .map(drop => typeof drop === 'string' ? this._dragDropRegistry.getDropContainer(drop)! : drop)
-      .filter(drop => drop && drop !== this)
-      .map(drop => ({drop, clientRect: drop.element.nativeElement.getBoundingClientRect()}));
-
-    this._positionCache.self = this.element.nativeElement.getBoundingClientRect();
+    this._positionCache.siblings = this._getConnectedLists().map(drop => ({
+      drop,
+      clientRect: drop.element.nativeElement.getBoundingClientRect()
+    }));
   }
 
   /** Resets the container to its initial state. */
@@ -449,6 +459,23 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
     return pointerY > top - yThreshold && pointerY < bottom + yThreshold &&
            pointerX > left - xThreshold && pointerX < right + xThreshold;
   }
+
+  /** Gets an array of unique drop lists that the current list is connected to. */
+  private _getConnectedLists(): CdkDropList[] {
+    const siblings = coerceArray(this.connectedTo).map(drop => {
+      return typeof drop === 'string' ? this._dragDropRegistry.getDropContainer(drop)! : drop;
+    });
+
+    if (this._group) {
+      this._group._items.forEach(drop => {
+        if (siblings.indexOf(drop) === -1) {
+          siblings.push(drop);
+        }
+      });
+    }
+
+    return siblings.filter(drop => drop && drop !== this);
+  }
 }
 
 

src/cdk/drag-drop/public-api.ts

@@ -7,6 +7,7 @@
  */
 
 export * from './drop-list';
+export * from './drop-list-group';
 export * from './drop-list-container';
 export * from './drag';
 export * from './drag-handle';

src/demo-app/drag-drop/drag-drop-demo.html

@@ -1,13 +1,11 @@
-<div>
+<div cdkDropListGroup>
   <div class="demo-list">
     <h2>To do</h2>
     <div
       cdkDropList
-      #one="cdkDropList"
       (cdkDropListDropped)="drop($event)"
       [cdkDropListLockAxis]="axisLock"
-      [cdkDropListData]="todo"
-      [cdkDropListConnectedTo]="[two]">
+      [cdkDropListData]="todo">
       <div *ngFor="let item of todo" cdkDrag>
         {{item}}
         <mat-icon cdkDragHandle svgIcon="dnd-move"></mat-icon>
@@ -19,11 +17,9 @@ <h2>To do</h2>
     <h2>Done</h2>
     <div
       cdkDropList
-      #two="cdkDropList"
       (cdkDropListDropped)="drop($event)"
       [cdkDropListLockAxis]="axisLock"
-      [cdkDropListData]="done"
-      [cdkDropListConnectedTo]="[one]">
+      [cdkDropListData]="done">
       <div *ngFor="let item of done" cdkDrag>
         {{item}}
         <mat-icon cdkDragHandle svgIcon="dnd-move"></mat-icon>