perf(table): leaking reference through mostRecentCellOutlet (#12269)

Fixes the table leaking out a reference to a cell outlet via the `CdkCellOutlet.mostRecentCellOutlet` after all tables have been destroyed.

Fixes #12259.

Author: crisbeto

src/cdk/table/row.ts

@@ -14,6 +14,7 @@ import {
   IterableDiffer,
   IterableDiffers,
   OnChanges,
+  OnDestroy,
   SimpleChanges,
   TemplateRef,
   ViewContainerRef,
@@ -207,7 +208,7 @@ export interface CdkCellOutletMultiRowContext<T> {
  * @docs-private
  */
 @Directive({selector: '[cdkCellOutlet]'})
-export class CdkCellOutlet {
+export class CdkCellOutlet implements OnDestroy {
   /** The ordered list of cells to render within this outlet's view container */
   cells: CdkCellDef[];
 
@@ -226,6 +227,14 @@ export class CdkCellOutlet {
   constructor(public _viewContainer: ViewContainerRef) {
     CdkCellOutlet.mostRecentCellOutlet = this;
   }
+
+  ngOnDestroy() {
+    // If this was the last outlet being rendered in the view, remove the reference
+    // from the static property after it has been destroyed to avoid leaking memory.
+    if (CdkCellOutlet.mostRecentCellOutlet === this) {
+      CdkCellOutlet.mostRecentCellOutlet = null;
+    }
+  }
 }
 
 /** Header template container that contains the cell outlet. Adds the right class and role. */

src/cdk/table/table.spec.ts

@@ -14,7 +14,7 @@ import {BehaviorSubject, combineLatest, Observable, of as observableOf} from 'rx
 import {map} from 'rxjs/operators';
 import {CdkColumnDef} from './cell';
 import {CdkTableModule} from './index';
-import {CdkHeaderRowDef, CdkRowDef} from './row';
+import {CdkHeaderRowDef, CdkRowDef, CdkCellOutlet} from './row';
 import {CdkTable} from './table';
 import {
   getTableDuplicateColumnNameError,
@@ -137,6 +137,17 @@ describe('CdkTable', () => {
       });
     });
 
+    it('should clear the `mostRecentCellOutlet` on destroy', () => {
+      // Note: we cast the assertions here to booleans, because they may
+      // contain circular objects which will throw Jasmine into an infinite
+      // when its tries to stringify them to show a test failure.
+      expect(!!CdkCellOutlet.mostRecentCellOutlet).toBe(true);
+
+      fixture.destroy();
+
+      expect(!!CdkCellOutlet.mostRecentCellOutlet).toBe(false);
+    });
+
     describe('should correctly use the differ to add/remove/move rows', () => {
       function addInitialIndexAttribute() {
         // Each row receives an attribute 'initialIndex' the element's original place