refactor(drag-drop): add sorted event

Adds a new `cdkDropListSorted` event that emits the item's previous index and current index as it's being sorted in a list.

Fixes #13863.

Author: crisbeto

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

@@ -71,3 +71,15 @@ export interface CdkDragMove<T = any> {
    */
   delta: {x: -1 | 0 | 1, y: -1 | 0 | 1};
 }
+
+/** Event emitted when the user swaps the position of two drag items. */
+export interface CdkDragSortEvent<T = any, I = T> {
+  /** Index from which the item was sorted previously. */
+  previousIndex: number;
+  /** Index that the item is currently in. */
+  currentIndex: number;
+  /** Container that the item belongs to. */
+  container: CdkDropListContainer<T>;
+  /** Item that is being sorted. */
+  item: CdkDrag<I>;
+}

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

@@ -633,6 +633,36 @@ describe('CdkDrag', () => {
           .toEqual(['One', 'Two', 'Zero', 'Three']);
     }));
 
+    it('should dispatch the `sorted` event as an item is being sorted', fakeAsync(() => {
+      const fixture = createComponent(DraggableInDropZone);
+      fixture.detectChanges();
+
+      const items = fixture.componentInstance.dragItems.map(item => item.element.nativeElement);
+      const draggedItem = items[0];
+      const {top, left} = draggedItem.getBoundingClientRect();
+
+      startDraggingViaMouse(fixture, draggedItem, left, top);
+
+      // Drag over each item one-by-one going downwards.
+      for (let i = 1; i < items.length; i++) {
+        const elementRect = items[i].getBoundingClientRect();
+
+        dispatchMouseEvent(document, 'mousemove', elementRect.left, elementRect.top + 5);
+        fixture.detectChanges();
+
+        expect(fixture.componentInstance.sortedSpy.calls.mostRecent().args[0]).toEqual({
+          previousIndex: i - 1,
+          currentIndex: i,
+          item: fixture.componentInstance.dragItems.first,
+          container: fixture.componentInstance.dropInstance
+        });
+      }
+
+      dispatchMouseEvent(document, 'mouseup');
+      fixture.detectChanges();
+      flush();
+    }));
+
     it('should not move items in a vertical list if the pointer is too far away', fakeAsync(() => {
       const fixture = createComponent(DraggableInDropZone);
       fixture.detectChanges();
@@ -2075,6 +2105,7 @@ const DROP_ZONE_FIXTURE_TEMPLATE = `
     style="width: 100px; background: pink;"
     [id]="dropZoneId"
     [cdkDropListData]="items"
+    (cdkDropListSorted)="sortedSpy($event)"
     (cdkDropListDropped)="droppedSpy($event)">
     <div
       *ngFor="let item of items"
@@ -2097,6 +2128,7 @@ class DraggableInDropZone {
     {value: 'Three', height: ITEM_HEIGHT, margin: 0}
   ];
   dropZoneId = 'items';
+  sortedSpy = jasmine.createSpy('sorted spy');
   droppedSpy = jasmine.createSpy('dropped spy').and.callFake((event: CdkDragDrop<string[]>) => {
     moveItemInArray(this.items, event.previousIndex, event.currentIndex);
   });

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

@@ -24,7 +24,7 @@ import {
 import {Directionality} from '@angular/cdk/bidi';
 import {CdkDrag} from './drag';
 import {DragDropRegistry} from './drag-drop-registry';
-import {CdkDragDrop, CdkDragEnter, CdkDragExit} from './drag-events';
+import {CdkDragDrop, CdkDragEnter, CdkDragExit, CdkDragSortEvent} from './drag-events';
 import {moveItemInArray} from './drag-utils';
 import {CDK_DROP_LIST_CONTAINER} from './drop-list-container';
 
@@ -139,6 +139,10 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
   @Output('cdkDropListExited')
   exited: EventEmitter<CdkDragExit<T>> = new EventEmitter<CdkDragExit<T>>();
 
+  /** Emits as the user is swapping items while actively dragging. */
+  @Output('cdkDropListSorted')
+  sorted: EventEmitter<CdkDragSortEvent<T>> = new EventEmitter<CdkDragSortEvent<T>>();
+
   constructor(
     public element: ElementRef<HTMLElement>,
     private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>,
@@ -311,6 +315,13 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
     // Shuffle the array in place.
     moveItemInArray(siblings, currentIndex, newIndex);
 
+    this.sorted.emit({
+      previousIndex: currentIndex,
+      currentIndex: newIndex,
+      container: this,
+      item
+    });
+
     siblings.forEach((sibling, index) => {
       // Don't do anything if the position hasn't changed.
       if (oldOrder[index] === sibling) {