feat(menu): support lazy rendering and passing in context data

* Introduces the `matMenuContent` directive that allows for menu content to be rendered lazily.
* Adds the `matMenuTriggerData` input to the `MatMenuTrigger` that allows for contextual data to be passed in to the lazily-rendered menu panel. This allows for the menu instance to be re-used between triggers.

Fixes #9251.

src/cdk/portal/dom-portal-outlet.ts

@@ -22,7 +22,8 @@ import {BasePortalOutlet, ComponentPortal, TemplatePortal} from './portal';
  */
 export class DomPortalOutlet extends BasePortalOutlet {
   constructor(
-      private _hostDomElement: Element,
+      /** Element into which the content is projected. */
+      public hostDomElement: Element,
       private _componentFactoryResolver: ComponentFactoryResolver,
       private _appRef: ApplicationRef,
       private _defaultInjector: Injector) {
@@ -59,7 +60,7 @@ export class DomPortalOutlet extends BasePortalOutlet {
     }
     // At this point the component has been instantiated, so we move it to the location in the DOM
     // where we want it to be rendered.
-    this._hostDomElement.appendChild(this._getComponentRootNode(componentRef));
+    this.hostDomElement.appendChild(this._getComponentRootNode(componentRef));
 
     return componentRef;
   }
@@ -78,7 +79,7 @@ export class DomPortalOutlet extends BasePortalOutlet {
     // But for the DomPortalOutlet the view can be added everywhere in the DOM
     // (e.g Overlay Container) To move the view to the specified host element. We just
     // re-append the existing root nodes.
-    viewRef.rootNodes.forEach(rootNode => this._hostDomElement.appendChild(rootNode));
+    viewRef.rootNodes.forEach(rootNode => this.hostDomElement.appendChild(rootNode));
 
     this.setDisposeFn((() => {
       let index = viewContainer.indexOf(viewRef);
@@ -96,8 +97,8 @@ export class DomPortalOutlet extends BasePortalOutlet {
    */
   dispose(): void {
     super.dispose();
-    if (this._hostDomElement.parentNode != null) {
-      this._hostDomElement.parentNode.removeChild(this._hostDomElement);
+    if (this.hostDomElement.parentNode != null) {
+      this.hostDomElement.parentNode.removeChild(this.hostDomElement);
     }
   }
 

src/lib/menu/menu-content.ts

@@ -0,0 +1,68 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {
+  Directive,
+  TemplateRef,
+  ComponentFactoryResolver,
+  ApplicationRef,
+  Injector,
+  ViewContainerRef,
+  Inject,
+  OnDestroy,
+} from '@angular/core';
+import {TemplatePortal, DomPortalOutlet} from '@angular/cdk/portal';
+import {DOCUMENT} from '@angular/common';
+
+/**
+ * Menu content that will be rendered lazily once the menu is opened.
+ */
+@Directive({
+  selector: 'ng-template[matMenuContent]'
+})
+export class MatMenuContent implements OnDestroy {
+  private _portal: TemplatePortal<any>;
+  private _outlet: DomPortalOutlet;
+
+  constructor(
+    private _template: TemplateRef<any>,
+    private _componentFactoryResolver: ComponentFactoryResolver,
+    private _appRef: ApplicationRef,
+    private _injector: Injector,
+    private _viewContainerRef: ViewContainerRef,
+    @Inject(DOCUMENT) private _document: any) {}
+
+  /**
+   * Attaches the content with a particular context.
+   * @docs-private
+   */
+  attach(context: any = {}) {
+    if (!this._portal) {
+      this._portal = new TemplatePortal(this._template, this._viewContainerRef);
+    } else if (this._portal.isAttached) {
+      this._portal.detach();
+    }
+
+    if (!this._outlet) {
+      this._outlet = new DomPortalOutlet(this._document.createElement('div'),
+          this._componentFactoryResolver, this._appRef, this._injector);
+    }
+
+    const element: HTMLElement = this._template.elementRef.nativeElement;
+
+    // Since the menu might be attached to a different DOM node, we have to re-insert it every time.
+    element.parentNode!.insertBefore(this._outlet.hostDomElement, element);
+    this._portal.attach(this._outlet, context);
+  }
+
+  ngOnDestroy() {
+    if (this._outlet) {
+      this._outlet.dispose();
+    }
+  }
+}

src/lib/menu/menu-directive.ts

@@ -17,6 +17,7 @@ import {
   AfterContentInit,
   ChangeDetectionStrategy,
   Component,
+  ContentChild,
   ContentChildren,
   ElementRef,
   EventEmitter,
@@ -38,6 +39,7 @@ import {matMenuAnimations} from './menu-animations';
 import {throwMatMenuInvalidPositionX, throwMatMenuInvalidPositionY} from './menu-errors';
 import {MatMenuItem} from './menu-item';
 import {MatMenuPanel} from './menu-panel';
+import {MatMenuContent} from './menu-content';
 import {MenuPositionX, MenuPositionY} from './menu-positions';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 
@@ -128,6 +130,12 @@ export class MatMenu implements AfterContentInit, MatMenuPanel, OnDestroy {
   /** List of the items inside of a menu. */
   @ContentChildren(MatMenuItem) items: QueryList<MatMenuItem>;
 
+  /**
+   * Menu content that will be rendered lazily.
+   * @docs-private
+   */
+  @ContentChild(MatMenuContent) lazyContent: MatMenuContent;
+
   /** Whether the menu should overlap its trigger. */
   @Input()
   set overlapTrigger(value: boolean) {
@@ -232,7 +240,14 @@ export class MatMenu implements AfterContentInit, MatMenuPanel, OnDestroy {
    * to focus the first item when the menu is opened by the ENTER key.
    */
   focusFirstItem() {
-    this._keyManager.setFirstItemActive();
+    // When the content is rendered lazily, it takes a bit before the items are inside the DOM.
+    if (this.lazyContent) {
+      this._ngZone.onStable.asObservable()
+        .pipe(take(1))
+        .subscribe(() => this._keyManager.setFirstItemActive());
+    } else {
+      this._keyManager.setFirstItemActive();
+    }
   }
 
   /**

src/lib/menu/menu-module.ts

@@ -10,21 +10,24 @@ import {NgModule} from '@angular/core';
 import {CommonModule} from '@angular/common';
 import {MatCommonModule} from '@angular/material/core';
 import {OverlayModule} from '@angular/cdk/overlay';
+import {PortalModule} from '@angular/cdk/portal';
 import {MatMenu, MAT_MENU_DEFAULT_OPTIONS} from './menu-directive';
 import {MatMenuItem} from './menu-item';
 import {MatMenuTrigger, MAT_MENU_SCROLL_STRATEGY_PROVIDER} from './menu-trigger';
 import {MatRippleModule} from '@angular/material/core';
+import {MatMenuContent} from './menu-content';
 
 
 @NgModule({
   imports: [
+    PortalModule,
     OverlayModule,
     CommonModule,
     MatRippleModule,
     MatCommonModule,
   ],
-  exports: [MatMenu, MatMenuItem, MatMenuTrigger, MatCommonModule],
-  declarations: [MatMenu, MatMenuItem, MatMenuTrigger],
+  exports: [MatMenu, MatMenuItem, MatMenuTrigger, MatMenuContent, MatCommonModule],
+  declarations: [MatMenu, MatMenuItem, MatMenuTrigger, MatMenuContent],
   providers: [
     MAT_MENU_SCROLL_STRATEGY_PROVIDER,
     {

src/lib/menu/menu-panel.ts

@@ -9,6 +9,7 @@
 import {EventEmitter, TemplateRef} from '@angular/core';
 import {MenuPositionX, MenuPositionY} from './menu-positions';
 import {Direction} from '@angular/cdk/bidi';
+import {MatMenuContent} from './menu-content';
 
 /**
  * Interface for a custom menu panel that can be used with `matMenuTriggerFor`.
@@ -26,4 +27,5 @@ export interface MatMenuPanel {
   resetActiveItem: () => void;
   setPositionClasses: (x: MenuPositionX, y: MenuPositionY) => void;
   setElevation?(depth: number): void;
+  lazyContent?: MatMenuContent;
 }

src/lib/menu/menu-trigger.ts

@@ -106,6 +106,9 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
   /** References the menu instance that the trigger is associated with. */
   @Input('matMenuTriggerFor') menu: MatMenuPanel;
 
+  /** Data to be passed along to any lazily-rendered content. */
+  @Input('matMenuTriggerData') menuData: any;
+
   /** Event emitted when the associated menu is opened. */
   @Output() menuOpened = new EventEmitter<void>();
 
@@ -191,14 +194,21 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
 
   /** Opens the menu. */
   openMenu(): void {
-    if (!this._menuOpen) {
-      this._createOverlay().attach(this._portal);
-      this._closeSubscription = this._menuClosingActions().subscribe(() => this.closeMenu());
-      this._initMenu();
+    if (this._menuOpen) {
+      return;
+    }
 
-      if (this.menu instanceof MatMenu) {
-        this.menu._startAnimation();
-      }
+    this._createOverlay().attach(this._portal);
+
+    if (this.menu.lazyContent) {
+      this.menu.lazyContent.attach(this.menuData);
+    }
+
+    this._closeSubscription = this._menuClosingActions().subscribe(() => this.closeMenu());
+    this._initMenu();
+
+    if (this.menu instanceof MatMenu) {
+      this.menu._startAnimation();
     }
   }
 

src/lib/menu/menu.md

@@ -6,12 +6,12 @@ By itself, the `<mat-menu>` element does not render anything. The menu is attach
 via application of the `matMenuTriggerFor` directive:
 ```html
 <mat-menu #appMenu="matMenu">
-  <button mat-menu-item> Settings </button>
-  <button mat-menu-item> Help </button>
+  <button mat-menu-item>Settings</button>
+  <button mat-menu-item>Help</button>
 </mat-menu>
 
 <button mat-icon-button [matMenuTriggerFor]="appMenu">
-   <mat-icon>more_vert</mat-icon>
+  <mat-icon>more_vert</mat-icon>
 </button>
 ```
 
@@ -36,16 +36,16 @@ Menus support displaying `mat-icon` elements before the menu item text.
 ```html
 <mat-menu #menu="matMenu">
   <button mat-menu-item>
-    <mat-icon> dialpad </mat-icon>
-    <span> Redial </span>
+    <mat-icon>dialpad</mat-icon>
+    <span>Redial</span>
   </button>
   <button mat-menu-item disabled>
-    <mat-icon> voicemail </mat-icon>
-    <span> Check voicemail </span>
+    <mat-icon>voicemail</mat-icon>
+    <span>Check voicemail</span>
   </button>
   <button mat-menu-item>
-    <mat-icon> notifications_off </mat-icon>
-    <span> Disable alerts </span>
+    <mat-icon>notifications_off</mat-icon>
+    <span>Disable alerts</span>
   </button>
 </mat-menu>
 ```
@@ -59,8 +59,8 @@ The position can be changed using the `xPosition` (`before | after`) and `yPosit
 
 ```html
 <mat-menu #appMenu="matMenu" yPosition="above">
-  <button mat-menu-item> Settings </button>
-  <button mat-menu-item> Help </button>
+  <button mat-menu-item>Settings</button>
+  <button mat-menu-item>Help</button>
 </mat-menu>
 
 <button mat-icon-button [matMenuTriggerFor]="appMenu">
@@ -93,6 +93,46 @@ that should trigger the sub-menu:
 
 <!-- example(nested-menu) -->
 
+### Lazy rendering
+By default, the menu content will be initialized even when the panel is closed. To defer
+initialization until the menu is open, the content can be provided as an `ng-template`
+with the `matMenuContent` attribute:
+
+```html
+<mat-menu #appMenu="matMenu">
+  <ng-template matMenuContent>
+    <button mat-menu-item>Settings</button>
+    <button mat-menu-item>Help</button>
+  </ng-template>
+</mat-menu>
+
+<button mat-icon-button [matMenuTriggerFor]="appMenu">
+  <mat-icon>more_vert</mat-icon>
+</button>
+```
+
+### Passing in data to a menu
+When using lazy rendering, additional context data can be passed to the menu panel via
+the `matMenuTriggerData` input. This allows for a single menu instance to be rendered
+with a different set of data, depending on the trigger that opened it:
+
+```html
+<mat-menu #appMenu="matMenu" let-user="user">
+  <ng-template matMenuContent>
+    <button mat-menu-item>Settings</button>
+    <button mat-menu-item>Log off {{name}}</button>
+  </ng-template>
+</mat-menu>
+
+<button mat-icon-button [matMenuTriggerFor]="appMenu" [matMenuTriggerData]="{name: 'Sally'}">
+  <mat-icon>more_vert</mat-icon>
+</button>
+
+<button mat-icon-button [matMenuTriggerFor]="appMenu" [matMenuTriggerData]="{name: 'Bob'}">
+  <mat-icon>more_vert</mat-icon>
+</button>
+```
+
 ### Keyboard interaction
 - <kbd>DOWN_ARROW</kbd>: Focuses the next menu item
 - <kbd>UP_ARROW</kbd>: Focuses previous menu item