refactor(drag-drop): expose method for resetting the drag position

Exposes an API that allows people to reset the position of a `cdkDrag`.

Fixes #13661.

Author: crisbeto

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

@@ -388,6 +388,39 @@ describe('CdkDrag', () => {
       expect(dragElement.style.transform).toBeFalsy();
     }));
 
+    it('should be able to reset a freely-dragged item to its initial position', fakeAsync(() => {
+      const fixture = createComponent(StandaloneDraggable);
+      fixture.detectChanges();
+      const dragElement = fixture.componentInstance.dragElement.nativeElement;
+
+      expect(dragElement.style.transform).toBeFalsy();
+
+      dragElementViaMouse(fixture, dragElement, 50, 100);
+      expect(dragElement.style.transform).toBe('translate3d(50px, 100px, 0px)');
+
+      fixture.componentInstance.dragInstance.reset();
+
+      expect(dragElement.style.transform).toBeFalsy();
+    }));
+
+    it('should start dragging an item from its initial position after a reset', fakeAsync(() => {
+      const fixture = createComponent(StandaloneDraggable);
+      fixture.detectChanges();
+      const dragElement = fixture.componentInstance.dragElement.nativeElement;
+
+      expect(dragElement.style.transform).toBeFalsy();
+
+      dragElementViaMouse(fixture, dragElement, 50, 100);
+      expect(dragElement.style.transform).toBe('translate3d(50px, 100px, 0px)');
+
+      fixture.componentInstance.dragInstance.reset();
+
+      expect(dragElement.style.transform).toBeFalsy();
+
+      dragElementViaMouse(fixture, dragElement, 25, 50);
+      expect(dragElement.style.transform).toBe('translate3d(25px, 50px, 0px)');
+    }));
+
   });
 
   describe('draggable with a handle', () => {

src/cdk/drag-drop/drag.ts

@@ -248,6 +248,13 @@ export class CdkDrag<T = any> implements AfterViewInit, OnDestroy {
     return this._rootElement;
   }
 
+  /** Resets a standalone drag item to its initial position. */
+  reset(): void {
+    this._rootElement.style.transform = '';
+    this._activeTransform = {x: 0, y: 0};
+    this._passiveTransform = {x: 0, y: 0};
+  }
+
   ngAfterViewInit() {
     // We need to wait for the zone to stabilize, in order for the reference
     // element to be in the proper place in the DOM. This is mostly relevant