fix(drag-drop): remove circular dependencies

Currently we've got the following circular dependencies in master, after the `CdkDragDropRegistry` got introduced:

```
drop.ts -> drag.ts -> drag-drop-registry.ts -> drop.ts
drag.ts -> drag-drop-registry.ts -> drag.ts
drop.ts -> drag-drop-registry.ts -> drop.ts
```

These changes rework the registry to avoid importing `CdkDrag` and `CdkDrop`, in order to break the circular references.

Author: crisbeto

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

@@ -14,7 +14,7 @@ import {CdkDrop} from './drop';
 describe('DragDropRegistry', () => {
   let fixture: ComponentFixture<SimpleDropZone>;
   let testComponent: SimpleDropZone;
-  let registry: CdkDragDropRegistry;
+  let registry: CdkDragDropRegistry<CdkDrag, CdkDrop>;
 
   beforeEach(fakeAsync(() => {
     TestBed.configureTestingModule({
@@ -26,7 +26,7 @@ describe('DragDropRegistry', () => {
     testComponent = fixture.componentInstance;
     fixture.detectChanges();
 
-    inject([CdkDragDropRegistry], (c: CdkDragDropRegistry) => {
+    inject([CdkDragDropRegistry], (c: CdkDragDropRegistry<CdkDrag, CdkDrop>) => {
       registry = c;
     })();
   }));
@@ -59,7 +59,7 @@ describe('DragDropRegistry', () => {
     registry.startDragging(firstItem, createMouseEvent('mousedown'));
     expect(registry.isDragging(firstItem)).toBe(true);
 
-    registry.remove(firstItem);
+    registry.removeDragItem(firstItem);
     expect(registry.isDragging(firstItem)).toBe(false);
   });
 
@@ -130,7 +130,7 @@ describe('DragDropRegistry', () => {
   });
 
   it('should not throw when trying to register the same container again', () => {
-    expect(() => registry.register(testComponent.dropInstances.first)).not.toThrow();
+    expect(() => registry.registerDropContainer(testComponent.dropInstances.first)).not.toThrow();
   });
 
   it('should throw when trying to register a different container with the same id', () => {

src/cdk-experimental/drag-drop/drag-drop-registry.ts

@@ -10,8 +10,6 @@ import {Injectable, NgZone, OnDestroy, Inject} from '@angular/core';
 import {DOCUMENT} from '@angular/common';
 import {supportsPassiveEventListeners} from '@angular/cdk/platform';
 import {Subject} from 'rxjs';
-import {CdkDrop} from './drop';
-import {CdkDrag} from './drag';
 
 /** Event options that can be used to bind an active event. */
 const activeEventOptions = supportsPassiveEventListeners() ? {passive: false} : false;
@@ -20,35 +18,35 @@ const activeEventOptions = supportsPassiveEventListeners() ? {passive: false} :
 type PointerEventHandler = (event: TouchEvent | MouseEvent) => void;
 
 /**
- * Service that keeps track of all the `CdkDrag` and `CdkDrop` instances, and
- * manages global event listeners on the `document`.
+ * Service that keeps track of all the drag item and drop container
+ * instances, and manages global event listeners on the `document`.
  * @docs-private
  */
 @Injectable({providedIn: 'root'})
-export class CdkDragDropRegistry implements OnDestroy {
+export class CdkDragDropRegistry<I, C extends {id: string}> implements OnDestroy {
   private _document: Document;
 
-  /** Registered `CdkDrop` instances. */
-  private _dropInstances = new Set<CdkDrop>();
+  /** Registered drop container instances. */
+  private _dropInstances = new Set<C>();
 
-  /** Registered `CdkDrag` instances. */
-  private _dragInstances = new Set<CdkDrag>();
+  /** Registered drag item instances. */
+  private _dragInstances = new Set<I>();
 
-  /** `CdkDrag` instances that are currently being dragged. */
-  private _activeDragInstances = new Set<CdkDrag>();
+  /** Drag item instances that are currently being dragged. */
+  private _activeDragInstances = new Set<I>();
 
   /** Keeps track of the event listeners that we've bound to the `document`. */
   private _globalListeners = new Map<string, {handler: PointerEventHandler, options?: any}>();
 
   /**
    * Emits the `touchmove` or `mousemove` events that are dispatched
-   * while the user is dragging a `CdkDrag` instance.
+   * while the user is dragging a drag item instance.
    */
   readonly pointerMove: Subject<TouchEvent | MouseEvent> = new Subject<TouchEvent | MouseEvent>();
 
   /**
    * Emits the `touchend` or `mouseup` events that are dispatched
-   * while the user is dragging a `CdkDrag` instance.
+   * while the user is dragging a drag item instance.
    */
   readonly pointerUp: Subject<TouchEvent | MouseEvent> = new Subject<TouchEvent | MouseEvent>();
 
@@ -58,52 +56,44 @@ export class CdkDragDropRegistry implements OnDestroy {
     this._document = _document;
   }
 
-  /** Adds a `CdkDrop` instance to the registry. */
-  register(drop: CdkDrop);
-
-  /** Adds a `CdkDrag` instance to the registry. */
-  register(drag: CdkDrag);
-
-  register(instance: CdkDrop | CdkDrag) {
-    if (instance instanceof CdkDrop) {
-      if (!this._dropInstances.has(instance)) {
-        if (this.getDropContainer(instance.id)) {
-          throw Error(`Drop instance with id "${instance.id}" has already been registered.`);
-        }
-
-        this._dropInstances.add(instance);
-      }
-    } else {
-      this._dragInstances.add(instance);
-
-      if (this._dragInstances.size === 1) {
-        this._ngZone.runOutsideAngular(() => {
-          // The event handler has to be explicitly active, because
-          // newer browsers make it passive by default.
-          this._document.addEventListener('touchmove', this._preventScrollListener,
-              activeEventOptions);
-        });
+  /** Adds a drop container to the registry. */
+  registerDropContainer(drop: C) {
+    if (!this._dropInstances.has(drop)) {
+      if (this.getDropContainer(drop.id)) {
+        throw Error(`Drop instance with id "${drop.id}" has already been registered.`);
       }
+
+      this._dropInstances.add(drop);
     }
   }
 
-  /** Removes a `CdkDrop` instance from the registry. */
-  remove(drop: CdkDrop);
+  /** Adds a drag item instance to the registry. */
+  registerDragItem(drag: I) {
+    this._dragInstances.add(drag);
+
+    if (this._dragInstances.size === 1) {
+      this._ngZone.runOutsideAngular(() => {
+        // The event handler has to be explicitly active, because
+        // newer browsers make it passive by default.
+        this._document.addEventListener('touchmove', this._preventScrollListener,
+            activeEventOptions);
+      });
+    }
+  }
 
-  /** Removes a `CdkDrag` instance from the registry. */
-  remove(drag: CdkDrag);
+  /** Removes a drop container from the registry. */
+  removeDropContainer(drop: C) {
+    this._dropInstances.delete(drop);
+  }
 
-  remove(instance: CdkDrop | CdkDrag) {
-    if (instance instanceof CdkDrop) {
-      this._dropInstances.delete(instance);
-    } else {
-      this._dragInstances.delete(instance);
-      this.stopDragging(instance);
+  /** Removes a drag item instance from the registry. */
+  removeDragItem(drag: I) {
+    this._dragInstances.delete(drag);
+    this.stopDragging(drag);
 
-      if (this._dragInstances.size === 0) {
-        this._document.removeEventListener('touchmove', this._preventScrollListener,
-            activeEventOptions as any);
-      }
+    if (this._dragInstances.size === 0) {
+      this._document.removeEventListener('touchmove', this._preventScrollListener,
+          activeEventOptions as any);
     }
   }
 
@@ -112,7 +102,7 @@ export class CdkDragDropRegistry implements OnDestroy {
    * @param drag Drag instance which is being dragged.
    * @param event Event that initiated the dragging.
    */
-  startDragging(drag: CdkDrag, event: TouchEvent | MouseEvent) {
+  startDragging(drag: I, event: TouchEvent | MouseEvent) {
     this._activeDragInstances.add(drag);
 
     if (this._activeDragInstances.size === 1) {
@@ -134,28 +124,28 @@ export class CdkDragDropRegistry implements OnDestroy {
     }
   }
 
-  /** Stops dragging a `CdkDrag` instance. */
-  stopDragging(drag: CdkDrag) {
+  /** Stops dragging a drag item instance. */
+  stopDragging(drag: I) {
     this._activeDragInstances.delete(drag);
 
     if (this._activeDragInstances.size === 0) {
       this._clearGlobalListeners();
     }
   }
 
-  /** Gets whether a `CdkDrag` instance is currently being dragged. */
-  isDragging(drag: CdkDrag) {
+  /** Gets whether a drag item instance is currently being dragged. */
+  isDragging(drag: I) {
     return this._activeDragInstances.has(drag);
   }
 
-  /** Gets a `CdkDrop` instance by its id. */
-  getDropContainer<T = any>(id: string): CdkDrop<T> | undefined {
+  /** Gets a drop container by its id. */
+  getDropContainer(id: string): C | undefined {
     return Array.from(this._dropInstances).find(instance => instance.id === id);
   }
 
   ngOnDestroy() {
-    this._dragInstances.forEach(instance => this.remove(instance));
-    this._dropInstances.forEach(instance => this.remove(instance));
+    this._dragInstances.forEach(instance => this.removeDragItem(instance));
+    this._dropInstances.forEach(instance => this.removeDropContainer(instance));
     this._clearGlobalListeners();
     this.pointerMove.complete();
     this.pointerUp.complete();

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

@@ -138,10 +138,10 @@ export class CdkDrag<T = any> implements OnDestroy {
     private _ngZone: NgZone,
     private _viewContainerRef: ViewContainerRef,
     private _viewportRuler: ViewportRuler,
-    private _dragDropRegistry: CdkDragDropRegistry,
+    private _dragDropRegistry: CdkDragDropRegistry<CdkDrag<T>, CdkDropContainer>,
     @Optional() private _dir: Directionality) {
       this._document = document;
-      _dragDropRegistry.register(this);
+      _dragDropRegistry.registerDragItem(this);
     }
 
   /**
@@ -165,7 +165,7 @@ export class CdkDrag<T = any> implements OnDestroy {
     }
 
     this._nextSibling = null;
-    this._dragDropRegistry.remove(this);
+    this._dragDropRegistry.removeDragItem(this);
     this._destroyed.next();
     this._destroyed.complete();
   }

src/cdk-experimental/drag-drop/drop-container.ts

@@ -13,6 +13,9 @@ export interface CdkDropContainer<T = any> {
   /** Arbitrary data to attach to all events emitted by this container. */
   data: T;
 
+  /** Unique ID for the drop zone. */
+  id: string;
+
   /** Direction in which the list is oriented. */
   orientation: 'horizontal' | 'vertical';
 

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

@@ -85,14 +85,14 @@ export class CdkDrop<T = any> implements OnInit, OnDestroy {
 
   constructor(
     public element: ElementRef<HTMLElement>,
-    private _dragDropRegistry: CdkDragDropRegistry) {}
+    private _dragDropRegistry: CdkDragDropRegistry<CdkDrag, CdkDrop<T>>) {}
 
   ngOnInit() {
-    this._dragDropRegistry.register(this);
+    this._dragDropRegistry.registerDropContainer(this);
   }
 
   ngOnDestroy() {
-    this._dragDropRegistry.remove(this);
+    this._dragDropRegistry.removeDropContainer(this);
   }
 
   /** Whether an item in the container is being dragged. */