feat(drag-drop): allow for dragging sequence to be delayed

Adds the `cdkDragStartDelay` input that allows for the dragging sequence to be delayed by X amount of milliseconds.

Fixes #13908.

Author: crisbeto

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

@@ -645,6 +645,36 @@ describe('CdkDrag', () => {
       }).toThrowError(/^cdkDrag must be attached to an element node/);
     }));
 
+    it('should allow for the dragging sequence to be delayed', fakeAsync(() => {
+      // We can't use Jasmine's `clock` because Zone.js interferes with it.
+      spyOn(Date, 'now').and.callFake(() => currentTime);
+      let currentTime = 0;
+
+      const fixture = createComponent(StandaloneDraggable);
+      fixture.componentInstance.dragStartDelay = 1000;
+      fixture.detectChanges();
+      const dragElement = fixture.componentInstance.dragElement.nativeElement;
+
+      expect(dragElement.style.transform).toBeFalsy('Expected element not to be moved by default.');
+
+      startDraggingViaMouse(fixture, dragElement);
+      currentTime += 750;
+      dispatchMouseEvent(document, 'mousemove', 50, 100);
+      fixture.detectChanges();
+
+      expect(dragElement.style.transform)
+          .toBeFalsy('Expected element not to be moved if the drag timeout has not passed.');
+
+      // The first `mousemove` here starts the sequence and the second one moves the element.
+      currentTime += 500;
+      dispatchMouseEvent(document, 'mousemove', 50, 100);
+      dispatchMouseEvent(document, 'mousemove', 50, 100);
+      fixture.detectChanges();
+
+      expect(dragElement.style.transform).toBe('translate3d(50px, 100px, 0px)',
+          'Expected element to be dragged after all the time has passed.');
+    }));
+
   });
 
   describe('draggable with a handle', () => {
@@ -2712,6 +2742,7 @@ describe('CdkDrag', () => {
       <div
         cdkDrag
         [cdkDragBoundary]="boundarySelector"
+        [cdkDragStartDelay]="dragStartDelay"
         (cdkDragStarted)="startedSpy($event)"
         (cdkDragReleased)="releasedSpy($event)"
         (cdkDragEnded)="endedSpy($event)"
@@ -2727,6 +2758,7 @@ class StandaloneDraggable {
   endedSpy = jasmine.createSpy('ended spy');
   releasedSpy = jasmine.createSpy('released spy');
   boundarySelector: string;
+  dragStartDelay: number;
 }
 
 @Component({

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

@@ -107,6 +107,12 @@ export class CdkDrag<T = any> implements AfterViewInit, OnChanges, OnDestroy {
    */
   @Input('cdkDragBoundary') boundaryElementSelector: string;
 
+  /**
+   * Amount of milliseconds to wait after the user has put their
+   * pointer down before starting to drag the element.
+   */
+  @Input('cdkDragStartDelay') dragStartDelay: number = 0;
+
   /** Whether starting to drag this element is disabled. */
   @Input('cdkDragDisabled')
   get disabled(): boolean {
@@ -277,6 +283,7 @@ export class CdkDrag<T = any> implements AfterViewInit, OnChanges, OnDestroy {
 
         ref.disabled = this.disabled;
         ref.lockAxis = this.lockAxis;
+        ref.dragStartDelay = this.dragStartDelay;
         ref.withBoundaryElement(this._getBoundaryElement());
         placeholder ? ref.withPlaceholderTemplate(placeholder.templateRef, placeholder.data) :
                       ref.withPlaceholderTemplate(null);

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

@@ -160,6 +160,9 @@ export class DragRef<T = any> {
    */
   private _lastTouchEventTime: number;
 
+  /** Time at which the last dragging sequence was started. */
+  private _dragStartTime: number;
+
   /** Cached reference to the boundary element. */
   private _boundaryElement: HTMLElement | null = null;
 
@@ -187,6 +190,12 @@ export class DragRef<T = any> {
   /** Axis along which dragging is locked. */
   lockAxis: 'x' | 'y';
 
+  /**
+   * Amount of milliseconds to wait after the user has put their
+   * pointer down before starting to drag the element.
+   */
+  dragStartDelay: number = 0;
+
   /** Whether starting to drag this element is disabled. */
   get disabled(): boolean {
     return this._disabled || !!(this.dropContainer && this.dropContainer.disabled);
@@ -454,12 +463,13 @@ export class DragRef<T = any> {
       const pointerPosition = this._getPointerPositionOnPage(event);
       const distanceX = Math.abs(pointerPosition.x - this._pickupPositionOnPage.x);
       const distanceY = Math.abs(pointerPosition.y - this._pickupPositionOnPage.y);
+      const isOverThreshold = distanceX + distanceY >= this._config.dragStartThreshold;
 
       // Only start dragging after the user has moved more than the minimum distance in either
       // direction. Note that this is preferrable over doing something like `skip(minimumDistance)`
       // in the `pointerMove` subscription, because we're not guaranteed to have one move event
       // per pixel of movement (e.g. if the user moves their pointer quickly).
-      if (distanceX + distanceY >= this._config.dragStartThreshold) {
+      if (isOverThreshold && (Date.now() >= this._dragStartTime + (this.dragStartDelay || 0))) {
         this._hasStartedDragging = true;
         this._ngZone.run(() => this._startDragSequence(event));
       }
@@ -653,6 +663,7 @@ export class DragRef<T = any> {
     const pointerPosition = this._pickupPositionOnPage = this._getPointerPositionOnPage(event);
     this._pointerDirectionDelta = {x: 0, y: 0};
     this._pointerPositionAtLastDirectionChange = {x: pointerPosition.x, y: pointerPosition.y};
+    this._dragStartTime = Date.now();
     this._dragDropRegistry.startDragging(this, event);
   }
 

tools/public_api_guard/cdk/drag-drop.d.ts

@@ -14,6 +14,7 @@ export declare class CdkDrag<T = any> implements AfterViewInit, OnChanges, OnDes
     boundaryElementSelector: string;
     data: T;
     disabled: boolean;
+    dragStartDelay: number;
     dropContainer: CdkDropList;
     dropped: EventEmitter<CdkDragDrop<any>>;
     element: ElementRef<HTMLElement>;