fix(drag-drop): only add top-level drop lists to drop group (#14130)

Currently all drop lists will add themselves to the closest group that is resolved through DI. This can lead to some weird results where a parent list and a list inside it will be grouped automatically. These changes rework the way the group is provided in order to prevent it from bleeding into child lists.

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

@@ -26,6 +26,7 @@ import {CdkDragDrop} from './drag-events';
 import {moveItemInArray} from './drag-utils';
 import {CdkDropList} from './drop-list';
 import {CdkDragHandle} from './drag-handle';
+import {CdkDropListGroup} from './drop-list-group';
 
 const ITEM_HEIGHT = 25;
 const ITEM_WIDTH = 75;
@@ -2218,6 +2219,14 @@ describe('CdkDrag', () => {
         expect(fixture.componentInstance.droppedSpy).not.toHaveBeenCalled();
     }));
 
+    it('should not add child drop lists to the same group as their parents', fakeAsync(() => {
+      const fixture = createComponent(NestedDropListGroups);
+      const component = fixture.componentInstance;
+      fixture.detectChanges();
+
+      expect(Array.from(component.group._items)).toEqual([component.listOne, component.listTwo]);
+    }));
+
   });
 
 });
@@ -2603,6 +2612,25 @@ class ConnectedDropZonesWithSingleItems {
   droppedSpy = jasmine.createSpy('dropped spy');
 }
 
+@Component({
+  template: `
+    <div cdkDropListGroup #group="cdkDropListGroup">
+      <div cdkDropList #listOne="cdkDropList">
+        <div cdkDropList #listThree="cdkDropList"></div>
+        <div cdkDropList #listFour="cdkDropList"></div>
+      </div>
+
+      <div cdkDropList #listTwo="cdkDropList"></div>
+    </div>
+  `
+})
+class NestedDropListGroups {
+  @ViewChild('group') group: CdkDropListGroup<CdkDropList>;
+  @ViewChild('listOne') listOne: CdkDropList;
+  @ViewChild('listTwo') listTwo: CdkDropList;
+}
+
+
 /**
  * Component that passes through whatever content is projected into it.
  * Used to test having drag elements being projected into a component.

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

@@ -15,7 +15,8 @@ import {Directive, OnDestroy} from '@angular/core';
  * from `cdkDropList`.
  */
 @Directive({
-  selector: '[cdkDropListGroup]'
+  selector: '[cdkDropListGroup]',
+  exportAs: 'cdkDropListGroup',
 })
 export class CdkDropListGroup<T> implements OnDestroy {
   /** Drop lists registered inside the group. */

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

@@ -20,6 +20,7 @@ import {
   Optional,
   Directive,
   ChangeDetectorRef,
+  SkipSelf,
 } from '@angular/core';
 import {Directionality} from '@angular/cdk/bidi';
 import {CdkDrag} from './drag';
@@ -81,6 +82,8 @@ interface ListPositionCacheEntry {
   selector: '[cdkDropList], cdk-drop-list',
   exportAs: 'cdkDropList',
   providers: [
+    // Prevent child drop lists from picking up the same group as their parent.
+    {provide: CdkDropListGroup, useValue: undefined},
     {provide: CDK_DROP_LIST_CONTAINER, useExisting: CdkDropList},
   ],
   host: {
@@ -157,7 +160,7 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
     private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>,
     private _changeDetectorRef: ChangeDetectorRef,
     @Optional() private _dir?: Directionality,
-    @Optional() private _group?: CdkDropListGroup<CdkDropList>) {}
+    @Optional() @SkipSelf() private _group?: CdkDropListGroup<CdkDropList>) {}
 
   ngOnInit() {
     this._dragDropRegistry.registerDropContainer(this);