fix(menu): detach lazily-rendered content when the menu is closed

Currently the lazily-rendered content is maintained in the background while a menu is open. These changes switch to detaching it once the user closes the menu.

Fixes #9915.

src/lib/chips/chip-list.ts

@@ -279,7 +279,7 @@ export class MatChipList extends _MatChipListMixinBase implements MatFormFieldCo
    */
   @Input()
   get selectable(): boolean { return this._selectable; }
-  set selectable(value: boolean) { 
+  set selectable(value: boolean) {
     this._selectable = coerceBooleanProperty(value);
     if (this.chips) {
       this.chips.forEach(chip => chip.chipListSelectable = this._selectable);

src/lib/menu/menu-content.ts

@@ -44,10 +44,10 @@ export class MatMenuContent implements OnDestroy {
   attach(context: any = {}) {
     if (!this._portal) {
       this._portal = new TemplatePortal(this._template, this._viewContainerRef);
-    } else if (this._portal.isAttached) {
-      this._portal.detach();
     }
 
+    this.detach();
+
     if (!this._outlet) {
       this._outlet = new DomPortalOutlet(this._document.createElement('div'),
           this._componentFactoryResolver, this._appRef, this._injector);
@@ -62,6 +62,16 @@ export class MatMenuContent implements OnDestroy {
     this._portal.attach(this._outlet, context);
   }
 
+  /**
+   * Detaches the content.
+   * @docs-private
+   */
+  detach() {
+    if (this._portal.isAttached) {
+      this._portal.detach();
+    }
+  }
+
   ngOnDestroy() {
     if (this._outlet) {
       this._outlet.dispose();

src/lib/menu/menu-directive.ts

@@ -33,6 +33,7 @@ import {
   NgZone,
 } from '@angular/core';
 import {Observable} from 'rxjs/Observable';
+import {Subject} from 'rxjs/Subject';
 import {merge} from 'rxjs/observable/merge';
 import {Subscription} from 'rxjs/Subscription';
 import {matMenuAnimations} from './menu-animations';
@@ -97,6 +98,9 @@ export class MatMenu implements AfterContentInit, MatMenuPanel, OnDestroy {
   /** Current state of the panel animation. */
   _panelAnimationState: 'void' | 'enter-start' | 'enter' = 'void';
 
+  /** Emits whenever an animation on the menu completes. */
+  _animationDone = new Subject<void>();
+
   /** Parent menu of the current menu panel. */
   parentMenu: MatMenuPanel | undefined;
 
@@ -306,5 +310,7 @@ export class MatMenu implements AfterContentInit, MatMenuPanel, OnDestroy {
     if (event.toState === 'enter-start') {
       this._panelAnimationState = 'enter';
     }
+
+    this._animationDone.next();
   }
 }

src/lib/menu/menu-trigger.ts

@@ -21,6 +21,7 @@ import {
 } from '@angular/cdk/overlay';
 import {TemplatePortal} from '@angular/cdk/portal';
 import {filter} from 'rxjs/operators/filter';
+import {take} from 'rxjs/operators/take';
 import {
   AfterContentInit,
   Directive,
@@ -238,14 +239,27 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
 
   /** Closes the menu and does the necessary cleanup. */
   private _destroyMenu() {
-    if (this._overlayRef && this.menuOpen) {
-      this._resetMenu();
-      this._closeSubscription.unsubscribe();
-      this._overlayRef.detach();
+    if (!this._overlayRef || !this.menuOpen) {
+      return;
+    }
+
+    const menu = this.menu;
+
+    this._resetMenu();
+    this._closeSubscription.unsubscribe();
+    this._overlayRef.detach();
+
+    if (menu instanceof MatMenu) {
+      menu._resetAnimation();
 
-      if (this.menu instanceof MatMenu) {
-        this.menu._resetAnimation();
+      if (menu.lazyContent) {
+        // Wait for the exit animation to finish before detaching the content.
+        menu._animationDone
+          .pipe(take(1))
+          .subscribe(() => menu.lazyContent!.detach());
       }
+    } else if (menu.lazyContent) {
+      menu.lazyContent.detach();
     }
   }
 

src/lib/menu/menu.spec.ts

@@ -327,8 +327,8 @@ describe('MatMenu', () => {
   describe('lazy rendering', () => {
     it('should be able to render the menu content lazily', fakeAsync(() => {
       const fixture = TestBed.createComponent(SimpleLazyMenu);
-      fixture.detectChanges();
 
+      fixture.detectChanges();
       fixture.componentInstance.triggerEl.nativeElement.click();
       fixture.detectChanges();
       tick(500);
@@ -340,6 +340,24 @@ describe('MatMenu', () => {
       expect(fixture.componentInstance.trigger.menuOpen).toBe(true, 'Expected menu to be open');
     }));
 
+    it('should detach the lazy content when the menu is closed', fakeAsync(() => {
+      const fixture = TestBed.createComponent(SimpleLazyMenu);
+
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openMenu();
+      fixture.detectChanges();
+      tick(500);
+
+      expect(fixture.componentInstance.items.length).toBeGreaterThan(0);
+
+      fixture.componentInstance.trigger.closeMenu();
+      fixture.detectChanges();
+      tick(500);
+      fixture.detectChanges();
+
+      expect(fixture.componentInstance.items.length).toBe(0);
+    }));
+
     it('should focus the first menu item when opening a lazy menu via keyboard', fakeAsync(() => {
       let zone: MockNgZone;
 
@@ -372,8 +390,8 @@ describe('MatMenu', () => {
 
     it('should be able to open the same menu with a different context', fakeAsync(() => {
       const fixture = TestBed.createComponent(LazyMenuWithContext);
-      fixture.detectChanges();
 
+      fixture.detectChanges();
       fixture.componentInstance.triggerOne.openMenu();
       fixture.detectChanges();
       tick(500);
@@ -1557,6 +1575,7 @@ class FakeIcon {}
 class SimpleLazyMenu {
   @ViewChild(MatMenuTrigger) trigger: MatMenuTrigger;
   @ViewChild('triggerEl') triggerEl: ElementRef;
+  @ViewChildren(MatMenuItem) items: QueryList<MatMenuItem>;
 }